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  • 851.
    Kullen, Anita
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik.
    Fear, R. C.
    Milan, S. E.
    Carter, J. A.
    Karlsson, Tomas
    KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik.
    The statistical difference between bending arcs and regular polar arcs2015Ingår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 120, nr 12, s. 10443-10465Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In this work, the Polar UVI data set by Kullen et al. (2002) of 74 polar arcs is reinvestigated, focusing on bending arcs. Bending arcs are typically faint and form (depending on interplanetary magnetic field (IMF) B-y direction) on the dawnside or duskside oval with the tip of the arc splitting off the dayside oval. The tip subsequently moves into the polar cap in the antisunward direction, while the arc's nightside end remains attached to the oval, eventually becoming hook-shaped. Our investigation shows that bending arcs appear on the opposite oval side from and farther sunward than most regular polar arcs. They form during B-y-dominated IMF conditions: typically, the IMF clock angle increases from 60 to 90 degrees about 20min before the arc forms. Antisunward plasma flows from the oval into the polar cap just poleward of bending arcs are seen in Super Dual Auroral Radar Network data, indicating dayside reconnection. For regular polar arcs, recently reported characteristics are confirmed in contrast to bending arcs. This includes plasma flows along the nightside oval that originate close to the initial arc location and a significant delay in the correlation between IMF B-y and initial arc location. In our data set, the highest correlations are found with IMF B-y appearing at least 1-2 h before arc formation. In summary, bending arcs are distinctly different from regular arcs and cannot be explained by existing polar arc models. Instead, these results are consistent with the formation mechanism described in Carter et al. (2015), suggesting that bending arcs are caused by dayside reconnection.

  • 852.
    Kullen, Anita
    et al.
    KTH, Tidigare Institutioner (före 2005), Alfvénlaboratoriet.
    Karlsson, Tomas
    KTH, Tidigare Institutioner (före 2005), Alfvénlaboratoriet.
    On the relation between solar wind, pseudobreakups, and substorms2004Ingår i: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 109, nr A12Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A statistical study of pseudobreakups and substorms is performed using Polar UV images from a 3-month period in winter 1998-1999. Data from the ACE solar wind monitor are examined in order to determine the influence of solar wind parameters on the occurrence of different substorm and pseudobreakup types. The results confirm that the IMF clock angle and the amount of solar wind energy flux control the strength of a substorm. The majority of large substorms appear when the IMF is strongly southward and the solar wind energy flux is high. Most small substorms occur during weakly positive or zero IMF B-z and low solar wind energy flux values. Pseudobreakups are associated with even lower energy fluxes than small substorms and appear typically for weakly positive IMF B-z. These results are in agreement with the scenario that pseudobreakups essentially are very weak substorms. Pseudobreakups appear during quiet times and during the growth phase or the recovery phase of weak or medium strong substorms. Time periods of enhanced geomagnetic activity with recurrent substorms are devoid of pseudobreakups. A detailed analysis of the different pseudobreakup types reveals that quiet time pseudobreakups appear predominantly during northward IMF. At least 20 percent of these appear at the poleward oval boundary. Optically, they do not differ much from very weak substorms. Growth phase pseudobreakups develop typically at the end of a 1 to 2 hour long excursion from northward to weakly southward IMF and are followed by quite weak substorms. A large majority of recovery phase pseudobreakups occur at a strongly polewardly displaced oval boundary at the end of a very active recovery phase. A considerable decrease of the polar cap size during the preceding substorm is connected to a northward turning of the IMF.

  • 853.
    Kullen, Anita
    et al.
    KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik.
    Karlsson, Tomas
    KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik.
    Cumnock, Judy A.
    KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik.
    Sundberg, Torbjörn
    KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik.
    Occurrence and properties of substorms associated with pseudobreakups2010Ingår i: Journal of Geophysical Research, ISSN 0148-0227, E-ISSN 2156-2202, Vol. 115, s. A12310-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We investigate how substorms with and without growth-phase pseudobreakups are affected by solar wind and ionospheric conditions. The study is based on 874 events identified with Polar UVI. An AE index analysis shows that substorms with growth-phase pseudobreakups are typically weak and appear as isolated events after hours of low geomagnetic activity. During the hours before onset the average solar wind merging field E-m is weaker, and the length of time with enhanced values shorter than for regular substorms. Integrating E-m over the last southward IMF period before onset shows an upper limit above which these substorms do not occur. To estimate how much E-m reaches the ionosphere, polar cap potential drop and unified PC indices are examined. It is found that substorms with growth-phase pseudobreakups have on average lower PC index values than regular substorms. The temporal evolution of the PC indices is similar for both substorm groups; the summer index correlates better with E-m, the winter index with AE. Also the average polar cap potential drop curves for both types of substorms resemble one other; the dayside and nightside curves are mainly influenced by E-m and AE, respectively. Comparing growth-phase, isolated and recovery pseudobreakups shows that solar wind and ionospheric conditions around the first substorm after a pseudobreakup are similar, independent of whether the last pseudobreakup appeared hours (recovery and isolated pseudobreakups) or minutes before substorm onset (growth-phase pseudobreakups). Isolated and recovery pseudobreakups are less often associated with a northward IMF rotation than growth-phase pseudobreakups or substorms.

  • 854.
    Kullen, Anita
    et al.
    KTH, Skolan för elektroteknik och datavetenskap (EECS), Elektroteknik, Rymd- och plasmafysik.
    Thor, Simon
    KTH, Skolan för elektroteknik och datavetenskap (EECS), Elektroteknik, Rymd- och plasmafysik.
    Karlsson, Tomas
    KTH, Skolan för elektroteknik och datavetenskap (EECS), Elektroteknik, Rymd- och plasmafysik.
    The Difference Between Isolated Flux Transfer Events and Flux Transfer Event Cascades2019Ingår i: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 124, nr 10, s. 7850-7871Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    This flux transfer event (FTE) study is based on 984 FTEs originally identified by Wang et al. (2005, ) in Cluster data. Due to Cluster's orbit, the FTE list exclusively contains events detected at the high-latitude dayside magnetopause and low-latitude flanks. The focus of this study is on FTE separation time. The results show that FTEs appearing in cascades are mainly located at the northern dusk and southern dawn magnetopause, while isolated FTEs are equally spread over the region covered by Cluster. This difference may be explained by the different interplanetary magnetic field (IMF) conditions during which the subsets occur. For isolated FTEs, average IMF B-y is close to zero. During such conditions, FTEs are expected to form at arbitrary longitudes along an equatorial merging line. After formation, they propagate northward and southward, causing an equal distribution at higher latitudes. In contrast, FTE cascades typically occur during weakly southward IMF with a negative B-y component. Their asymmetric distribution at higher latitudes is consistent with both the component and the antiparallel merging model for nonzero B-y. In both scenarios, newly formed FTEs are expected to move to the northern dusk and southern dawn regions, as observed. Many FTE cascades appearing during northward IMF are located close to the low-latitude flanks, confirming previous reports. We discovered that such FTEs appear during large IMF values. Another new result is that 16% of all isolated FTEs appear during small IMF cone angles, suggesting that these may form as a result of magnetosheath jets impacting on the magnetopause.

  • 855.
    Kungel, Viktoria
    et al.
    Colorado School of Mines, Golden, CO, USA.
    Bolmgren, Karl
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
    Carlson, Per
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
    Fuglesang, Christer
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
    Zuccaro Marchi, A.
    RIKEN, Wako, Japan.
    EUSO-SPB2 Telescope Optics and Testing2022Ingår i: 37th International Cosmic Ray Conference, ICRC 2021, Sissa Medialab Srl , 2022, artikel-id 412Konferensbidrag (Refereegranskat)
    Abstract [en]

    The Extreme Universe Space Observatory - Super Pressure Balloon (EUSO-SPB2) mission will fly two custom telescopes that feature Schmidt optics to measure Čerenkov- and fluorescence-emission of extensive air-showers from cosmic rays at the PeV and EeV-scale, and search for τ-neutrinos. Both telescopes have 1-meter diameter apertures and UV/UV-visible sensitivity. The Čerenkov telescope uses a bifocal mirror segment alignment, to distinguish between a direct cosmic ray that hits the camera versus the Čerenkov light from outside the telescope. Telescope integration and laboratory calibration will be performed in Colorado. To estimate the point spread function and efficiency of the integrated telescopes, a test beam system that delivers a 1-meter diameter parallel beam of light is being fabricated. End-to-end tests of the fully integrated instruments will be carried out in a field campaign at dark sites in the Utah desert using cosmic rays, stars, and artificial light sources. Laser tracks have long been used to characterize the performance of fluorescence detectors in the field. For EUSO-SPB2 an improvement in the method that includes a correction for aerosol attenuation is anticipated by using a bi-dynamic Lidar configuration in which both the laser and the telescope are steerable. We plan to conduct these field tests in Fall 2021 and Spring 2022 to accommodate the scheduled launch of EUSO-SPB2 in 2023 from Wanaka, New Zealand.

  • 856.
    Kuramoto, H.
    et al.
    Department of Earth and Space Science, Osaka University, 1-1 Machikaneyama-cho, Toyonaka, Osaka 560-0043, Japan.
    Awaki, Hisamitsu
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    West, A.
    The Oskar Klein Centre for Cosmoparticle Physics, AlbaNova University Centre, 10691 Stockholm, Sweden.
    et al.,
    Characterization of the x-ray telescope after the first flight of XL-Calibur2023Ingår i: Optics for EUV, X-Ray, and Gamma-Ray Astronomy XI, SPIE , 2023, artikel-id 126791BKonferensbidrag (Refereegranskat)
    Abstract [en]

    X-ray polarization measurements can provide unique information that is complementary to that obtained through spectroscopic or imaging observations. However, there have been few cases where significant X-ray polarization has been observed. XL-Calibur, conducted in collaboration between Japan, the United States of America, and Sweden, is a balloon-borne mission that aims to conduct high-sensitivity polarimetric observations in the hard X-ray band from 15 to 80 keV. The Japanese group is in charge of developing the hard X-ray telescope (HXT) with high light-gathering power. Optical adjustments were completed in 2020, and the performance of the HXT was measured in June 2021 at the SPring-8 (synchrotron radiation facility in Hyogo, Japan). Subsequently, in July 2022, the first observation was conducted from Sweden to Canada. After the flight, the HXT was recovered, and we measured its performance again. By comparing the HXT performances before and after the flight, we found no significant changes that can affect the second flight scheduled in 2024.

  • 857. Kurita, K.
    et al.
    Kanai, Y.
    Ueno, M.
    Kataoka, J.
    Kawai, N.
    Umeki, Y.
    Yoshida, H.
    Tanaka, T.
    Takahashi, H.
    Mizuno, T.
    Fukazawa, Y.
    Tajima, H.
    Kamae, T.
    Madejski, G.
    Varner, G.
    Kiss, M.
    Carlson, P.
    Klamra, W.
    Pearce, Mark
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
    Ryde, F.
    Larsson, S.
    Florén, H. -G
    Olofsson, G.
    Bogaert, G.
    Takahashi, T.
    Gunji, S.
    Collaboration, The PoGOLite
    Recent Development Status of PoGOLite2009Ingår i: Astrophysics with All-Sky X-Ray Observations, 2009Konferensbidrag (Övrigt vetenskapligt)
  • 858.
    Kushwah, Rakhee
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
    Stana, T. A.
    Pearce, Mark
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
    The design and performance of CUBES - a CubeSat X-ray detector2021Ingår i: Journal of Instrumentation, ISSN 1748-0221, E-ISSN 1748-0221, Vol. 16, nr 8, artikel-id P08038Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    CUBES is a X-ray detector payload which will be installed on the KTH 3U CubeSat mission, MIST. The detector comprises cerium-doped Gd3Al2Ga3O12 (GAGG) scintillators read out with silicon photomultipliers through a Citiroc Application-Specific Integrated Circuit. The detector operates in the energy range similar to 35-800 keV. The aim of the CUBES mission is to provide experience in the operation of these relatively new technologies in a high-inclination low earth orbit, thereby providing confidence for component selection in more complex satellite missions. The design of the CUBES detector is described, and results from performance characterisation tests carried out on a prototype of CUBES, called Proto-CUBES, are reported. Proto-CUBES was flown on a stratospheric balloon platform from Timmins, Canada, in August 2019. During the similar to 12 hour long flight, the performance of Proto-CUBES was studied in the near-space environment. As well as measuring the X-ray counts spectra at different atmospheric depths, a 511 keV line from positron annihilation was observed.

  • 859.
    Kuuttila, J.
    et al.
    Univ Turku, Dept Phys & Astron, Tuorla Observ, Vaisalantie 20, Piikkio 21500, Finland..
    Kajava, J. J. E.
    Univ Turku, Dept Phys & Astron, Tuorla Observ, Vaisalantie 20, Piikkio 21500, Finland.;Univ Turku, Finnish Ctr Astron ESO FINCA, Vaisalantie 20, Piikkio 21500, Finland.;ESA, European Space Astron Ctr, Sci Operat Dept, Madrid 28691, Spain..
    Nättila, Joonas
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Univ Turku, Dept Phys & Astron, Tuorla Observ, Vaisalantie 20, Piikkio 21500, Finland; Stockholm Univ, Roslagstullsbacken 23, S-10691 Stockholm, Sweden..
    Motta, S. E.
    Univ Oxford, Dept Phys, Astrophys, Denys Wilkinson Bldg,Keble Rd, Oxford OX1 3RH, England..
    Sanchez-Fernandez, C.
    ESA, European Space Astron Ctr, Sci Operat Dept, Madrid 28691, Spain..
    Kuulkers, E.
    ESA, European Space Astron Ctr, Sci Operat Dept, Madrid 28691, Spain.;ESA, Estec, Keplerlaan 1, NL-2201 AZ Noordwijk, Netherlands..
    Cumming, A.
    McGill Univ, Dept Phys, 3550 Univ St, Montreal, PQ H3A 2T8, Canada.;McGill Univ, McGill Space Inst, 3550 Univ St, Montreal, PQ H3A 2T8, Canada..
    Poutanen, Juri
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Univ Turku, Dept Phys & Astron, Tuorla Observ, Vaisalantie 20, Piikkio 21500, Finland.;Stockholm Univ, Roslagstullsbacken 23, S-10691 Stockholm, Sweden.;Univ Calif Santa Barbara, Kavli Inst Theoret Phys, Santa Barbara, CA 93106 USA..
    Flux decay during thermonuclear X-ray bursts analysed with the dynamic power-law index method2017Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 604, artikel-id A77Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The cooling of type-I X-ray bursts can be used to probe the nuclear burning conditions in neutron star envelopes. The flux decay of the bursts has been traditionally modelled with an exponential, even if theoretical considerations predict power-law-like decays. We have analysed a total of 540 type-I X-ray bursts from five low-mass X-ray binaries observed with the Rossi X-ray Timing Explorer. We grouped the bursts according to the source spectral state during which they were observed (hard or soft), flagging those bursts that showed signs of photospheric radius expansion (PRE). The decay phase of all the bursts were then fitted with a dynamic power-law index method. This method provides a new way of probing the chemical composition of the accreted material. Our results show that in the hydrogen-rich sources the power-law decay index is variable during the burst tails and that simple cooling models qualitatively describe the cooling of presumably helium-rich sources 4U 1728-34 and 3A 1820-303. The cooling in the hydrogen-rich sources 4U 1608-52, 4U 1636-536, and GS 1826-24, instead, is clearly different and depends on the spectral states and whether PRE occurred or not. Especially the hard state bursts behave differently than the models predict, exhibiting a peculiar rise in the cooling index at low burst fluxes, which suggests that the cooling in the tail is much faster than expected. Our results indicate that the drivers of the bursting behaviour are not only the accretion rate and chemical composition of the accreted material, but also the cooling that is somehow linked to the spectral states. The latter suggests that the properties of the burning layers deep in the neutron star envelope might be impacted differently depending on the spectral state.

  • 860.
    Kuzanyan, K.
    et al.
    Chinese Acad Sci, Key Lab Solar Act, Natl Astron Observ, Beijing 100012, Peoples R China.;Russian Acad Sci, IZMIRAN, Troitsk 108840, Russia..
    Kleeorin, Nathan
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Ben Gurion Univ Negev, Dept Mech Engn, POB 653, IL-84105 Beer Sheva, Israel.;Stockholm Univ, Stockholm, Sweden..
    Rogachevskii, lgor
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Ben Gurion Univ Negev, Dept Mech Engn, POB 653, IL-84105 Beer Sheva, Israel..;Stockholm Univ, Stockholm, Sweden..
    Sokoloff, D.
    Moscow MV Lomonosov State Univ, Dept Phys, Moscow 119992, Russia.;Moscow Ctr Fundamental & Appl Math, Moscow 119991, Russia..
    Zhang, H.
    Chinese Acad Sci, Key Lab Solar Act, Natl Astron Observ, Beijing 100012, Peoples R China..
    Estimates of Current Helicity and Tilt of Solar Active Regions and Joy's Law2020Ingår i: Geomagnetism and Aeronomy, ISSN 0016-7932, E-ISSN 1555-645X, Vol. 60, nr 8, s. 1032-1037Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The tilt angle, current helicity and twist of solar magnetic fields can be observed in solar active regions. We carried out estimates of these parameters by two ways. Firstly, we consider the model of turbulent convective cells (super-granules) which have a loop floating structure towards the surface of the Sun. Their helical properties are attained during the rising process in the rotating stratified convective zone. The other estimate is obtained from a simple mean-field dynamo model that accounts magnetic helicity conservation. The both values are shown to be capable to give important contributions to the observable tilt, helicity and twist.

  • 861.
    Kyratzis, D.
    et al.
    Gran Sasso Science Institute (GSSI), Via Iacobucci 2, 67100, L'Aquila, Italy, Via Iacobucci 2; INFN, Laboratori Nazionali del Gran Sasso (LNGS), 67100 Assergi, L'Aquila, Italy, Assergi.
    Pearce, Mark
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
    Ryde, Felix
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
    Zhu, K. J.
    Institute of High Energy Physics, Chinese Academy of Sciences, Beijing, China.
    The Plastic Scintillator Detector of the HERD space mission2022Ingår i: 37th International Cosmic Ray Conference, ICRC 2021, Sissa Medialab Srl , 2022, artikel-id 054Konferensbidrag (Refereegranskat)
    Abstract [en]

    The High Energy cosmic-Radiation (HERD) detector is one of the prominent space-borne instruments to be installed on-board the Chinese Space Station (CSS), around 2027. Primary scientific goals regarding this initiative include: precise measurements of cosmic ray (CR) energy spectra and mass composition, at energies up to the PeV range; contributions to high energy gamma-ray astronomy and transient studies; as well as indirect searches for Dark Matter (DM) particles via their possible annihilation/decay to detectable products. HERD is configured to accept incident particles from both its top and four lateral sides. Owing to its pioneering design, an order of magnitude increase in acceptance is foreseen, with respect to previous and ongoing experiments. The Plastic Scintillator Detector (PSD) constitutes an important sub-detector of HERD, particularly aimed towards anti-coincidence (discriminating incident photons from charged particles), while providing precise charge measurement of incoming cosmic-ray nuclei in a range of Z = 1-26. Main requirements concerning its design, include: high detection efficiency, broad dynamic range and good energy resolution. In order to select the optimal layout, two geometries are currently under investigation: one based on long scintillator bars and the other on square tiles, with both layouts being readout by Silicon Photomultipliers (SiPMs). Ongoing activities and future plans regarding the HERD PSD will be presented in this work.

  • 862.
    Käpylä, Maarit
    et al.
    Aalto Univ, Dept Comp Sci, POB 15400, FI-00076 Aalto, Finland.;Max Planck Inst Solar Syst Res, Justus von Liebig Weg 3, D-37077 Gottingen, Germany.;Stockholm Univ, NORDITA, Hannes Alfvens Vag 12, SE-10691 Stockholm, Sweden..
    Rheinhardt, Matthias
    Aalto Univ, Dept Comp Sci, POB 15400, FI-00076 Aalto, Finland..
    Brandenburg, Axel
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Stockholm Univ, Hannes Alfvens Vag 12, SE-10691 Stockholm, Sweden.;Stockholm Univ, Dept Astron, Oskar Klein Ctr, AlbaNova, SE-10691 Stockholm, Sweden.;Carnegie Mellon Univ, McWilliams Ctr Cosmol, Pittsburgh, PA 15213 USA.;Carnegie Mellon Univ, Dept Phys, Pittsburgh, PA 15213 USA..
    Compressible Test-field Method and Its Application to Shear Dynamos2022Ingår i: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 932, nr 1, s. 8-, artikel-id 8Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    In this study, we present a compressible test-field method (CTFM) for computing alpha-effect and turbulent magnetic diffusivity tensors, as well as those relevant for the mean ponderomotive force and mass source, applied to the full MHD equations. We describe the theoretical background of the method and compare it to the quasi-kinematic test-field method and to the previously studied variant working in simplified MHD (SMHD). We present several test cases using velocity and magnetic fields of the Roberts geometry and also compare with the imposed-field method. We show that, for moderate imposed-field strengths, the nonlinear CTFM (nCTFM) gives results in agreement with the imposed-field method. A comparison of different flavors of the nCTFM in the shear dynamo case also yields agreement up to equipartition field strengths. Some deviations between the CTFM and SMHD variants exist. As a relevant physical application, we study nonhelically forced shear flows, which exhibit large-scale dynamo action, and present a reanalysis of low-Reynolds-number, moderate shear systems, where we previously ignored the pressure gradient in the momentum equation and found no coherent shear-current effect. Another key difference is that in the earlier study we used magnetic forcing to mimic small-scale dynamo action, while here it is self-consistently driven by purely kinetic forcing. The kinematic CTFM with general validity forms the core of our analysis. We still find no coherent shear-current effect, but do recover strong large-scale dynamo action that, according to our analysis, is driven by incoherent effects.

  • 863.
    Käpylä, P. J.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Georg August Univ Gottingen, Inst Astrophys, Friedrich Hund.
    Prandtl number dependence of stellar convection: Flow statistics and convective energy transport2021Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 655, s. A78-, artikel-id A78Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Context. The ratio of kinematic viscosity to thermal diffusivity, the Prandtl number, is much smaller than unity in stellar convection zones. Aims. The main goal of this work is to study the statistics of convective flows and energy transport as functions of the Prandtl number. Methods. Three-dimensional numerical simulations of compressible non-rotating hydrodynamic convection in Cartesian geometry are used. The convection zone (CZ) is embedded between two stably stratified layers. The dominant contribution to the diffusion of entropy fluctuations comes in most cases from a subgrid-scale diffusivity whereas the mean radiative energy flux is mediated by a diffusive flux employing Kramers opacity law. Here, we study the statistics and transport properties of up- and downflows separately. Results. The volume-averaged rms velocity increases with decreasing Prandtl number. At the same time, the filling factor of down-flows decreases and leads to, on average, stronger downflows at lower Prandtl numbers. This results in a strong dependence of convective overshooting on the Prandtl number. Velocity power spectra do not show marked changes as a function of Prandtl number except near the base of the convective layer where the dominance of vertical flows is more pronounced. At the highest Reynolds numbers, the velocity power spectra are more compatible with the Bolgiano-Obukhov k(-11/5 )than the Kolmogorov-Obukhov k(-5/3) scaling. The horizontally averaged convected energy flux ((F) over bar (conv)), which is the sum of the enthalpy ((F) over bar (enth)) and kinetic energy fluxes ((F) over bar (kin)), is independent of the Prandtl number within the CZ. However, the absolute values of (F) over bar (enth )and (F) over bar (kin) increase monotonically with decreasing Prandtl number. Furthermore, (F) over bar (enth) and (F) over bar (kin) have opposite signs for downflows and their sum (F) over bar (down arrow)(conv) diminishes with Prandtl number. Thus, the upflows (downflows) are the dominant contribution to the convected flux at low (high) Prandtl numbers. These results are similar to those from Rayleigh-Benard convection in the low Prandtl number regime where convection is vigorously turbulent but inefficient at transporting energy. Conclusions. The current results indicate a strong dependence of convective overshooting and energy flux on the Prandtl number. Numerical simulations of astrophysical convection often use a Prandtl number of unity because it is numerically convenient. The current results suggest that this can lead to misleading results and that the astrophysically relevant low Prandtl number regime is qualitatively different from the parameter regimes explored in typical contemporary simulations.

  • 864.
    Käpylä, P. J.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Georg August Univ Gottingen, Inst Astrophys, Friedrich Hund Pl 1, D-37077 Gottingen, Germany.;KTH Royal Inst Technol, NORDITA, S-10691 Stockholm, Sweden.;Stockholm Univ, S-10691 Stockholm, Sweden..
    Star-in-a-box simulations of fully convective stars2021Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 651, artikel-id A66Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Context. Main-sequence late-type stars with masses of less than 0.35 M-circle dot are fully convective.Aims. The goal is to study convection, differential rotation, and dynamos as functions of rotation in fully convective stars.Methods. Three-dimensional hydrodynamic and magnetohydrodynamic numerical simulations with a star-in-a-box model, in which a spherical star is immersed inside of a Cartesian cube, are used. The model corresponds to a 0.2 M-circle dot main-sequence M5 dwarf. A range of rotation periods (P-rot) between 4.3 and 430 d is explored.Results. The slowly rotating model with P-rot=430 days produces anti-solar differential rotation with a slow equator and fast poles, along with predominantly axisymmetric quasi-steady large-scale magnetic fields. For intermediate rotation (P-rot=144 and 43 days) the differential rotation is solar-like (fast equator, slow poles), and the large-scale magnetic fields are mostly axisymmetric and either quasi-stationary or cyclic. The latter occurs in a similar parameter regime as in other numerical studies in spherical shells, and the cycle period is similar to observed cycles in fully convective stars with rotation periods of roughly 100 days. In the rapid rotation regime the differential rotation is weak and the large-scale magnetic fields are increasingly non-axisymmetric with a dominating m=1 mode. This large-scale non-axisymmetric field also exhibits azimuthal dynamo waves.Conclusions. The results of the star-in-a-box models agree with simulations of partially convective late-type stars in spherical shells in that the transitions in differential rotation and dynamo regimes occur at similar rotational regimes in terms of the Coriolis (inverse Rossby) number. This similarity between partially and fully convective stars suggests that the processes generating differential rotation and large-scale magnetism are insensitive to the geometry of the star.

  • 865.
    Käpylä, P. J.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Georg August Univ Gottingen, Inst Astrophys & Geophys, Friedrich Hund Pl 1, D-37077 Gottingen, Germany.;Stockholm Univ, Hannes Alfvens Vag 12, S-10691 Stockholm, Sweden..
    Transition from anti-solar to solar-like differential rotation: Dependence on Prandtl number2023Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 669, s. A98-, artikel-id A98Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Context. Late-type stars such as the Sun rotate differentially due to the interaction of turbulent convection and rotation.Aims. The aim of the study is to investigate the effects of the effective thermal Prandtl number, which is the ratio of kinematic viscosity to thermal diffusivity, on the transition from anti-solar (slow equator, fast poles) to solar-like (fast equator, slow poles) differential rotation.Methods. Three-dimensional hydrodynamic and magnetohydrodynamic simulations in semi-global spherical wedge geometry were used to model the convection zones of solar-like stars.Results. The overall convective velocity amplitude increases as the Prandtl number decreases, in accordance with earlier studies. The transition from anti-solar to solar-like differential rotation is insensitive to the Prandtl number for Prandtl numbers below unity, but for Prandtl numbers greater than unity, solar-like differential rotation becomes significantly harder to excite. Magnetic fields and more turbulent regimes with higher fluid and magnetic Reynolds numbers help to achieve solar-like differential rotation in near-transition cases where anti-solar rotation is found in more laminar simulations. Solar-like differential rotation occurs only in cases with radially outward turbulent angular momentum transport due to the Reynolds stress at the equator. The dominant contribution to this outward transport near the equator is due to prograde propagating thermal Rossby waves.Conclusions. The differential rotation is sensitive to the Prandtl number only for large Prandtl numbers in the parameter regime explored in this study. Magnetic fields have a greater effect on the differential rotation, although the inferred presence of a small-scale dynamo did not lead to drastically different results. The dominance of the thermal Rossby waves in the simulations is puzzling because they are not detected in the Sun. The current simulations are shown to be incompatible with the currently prevailing mean-field theory of differential rotation.

  • 866. Käpylä, P. J.
    et al.
    Rheinhardt, M.
    Brandenburg, Axel
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Käpylä, M. J.
    Turbulent viscosity and magnetic Prandtl number from simulations of isotropically forced turbulence2020Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 636, artikel-id A93Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Context. Turbulent diffusion of large-scale flows and magnetic fields plays a major role in many astrophysical systems, such as stellar convection zones and accretion discs. Aims. Our goal is to compute turbulent viscosity and magnetic diffusivity which are relevant for diffusing large-scale flows and magnetic fields, respectively. We also aim to compute their ratio, which is the turbulent magnetic Prandtl number, Pmt, for isotropically forced homogeneous turbulence. Methods. We used simulations of forced turbulence in fully periodic cubes composed of isothermal gas with an imposed large-scale sinusoidal shear flow. Turbulent viscosity was computed either from the resulting Reynolds stress or from the decay rate of the large-scale flow. Turbulent magnetic diffusivity was computed using the test-field method for a microphysical magnetic Prandtl number of unity. The scale dependence of the coefficients was studied by varying the wavenumber of the imposed sinusoidal shear and test fields. Results. We find that turbulent viscosity and magnetic diffusivity are in general of the same order of magnitude. Furthermore, the turbulent viscosity depends on the fluid Reynolds number (Re) and scale separation ratio of turbulence. The scale dependence of the turbulent viscosity is found to be well approximated by a Lorentzian. These results are similar to those obtained earlier for the turbulent magnetic diffusivity. The results for the turbulent transport coefficients appear to converge at sufficiently high values of Re and the scale separation ratio. However, a weak trend is found even at the largest values of Re, suggesting that the turbulence is not in the fully developed regime. The turbulent magnetic Prandtl number converges to a value that is slightly below unity for large Re. For small Re we find values between 0.5 and 0.6 but the data are insufficient to draw conclusions regarding asymptotics. We demonstrate that our results are independent of the correlation time of the forcing function. Conclusions. The turbulent magnetic diffusivity is, in general, consistently higher than the turbulent viscosity, which is in qualitative agreement with analytic theories. However, the actual value of Pmt found from the simulations (≈0.9-0.95) at large Re and large scale separation ratio is higher than any of the analytic predictions (0.4-0.8).

  • 867.
    Käpylä, Petri J.
    et al.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Käpylä, M. J.
    Olspert, N.
    Warnecke, J.
    Brandenburg, Axel
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Convection-driven spherical shell dynamos at varying Prandtl numbers2017Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 599, artikel-id A4Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Context. Stellar convection zones are characterized by vigorous high-Reynolds number turbulence at low Prandtl numbers. Aims. We study the dynamo and differential rotation regimes at varying levels of viscous, thermal, and magnetic diffusion. Methods. We perform three-dimensional simulations of stratified fully compressible magnetohydrodynamic convection in rotating spherical wedges at various thermal and magnetic Prandtl numbers (from 0.25 to 2 and from 0.25 to 5, respectively). Differential rotation and large-scale magnetic fields are produced self-consistently. Results. We find that for high thermal diffusivity, the rotation profiles show a monotonically increasing angular velocity from the bottom of the convection zone to the top and from the poles toward the equator. For sufficiently rapid rotation, a region of negative radial shear develops at mid-latitudes as the thermal diffusivity is decreased, corresponding to an increase of the Prandtl number. This coincides with and results in a change of the dynamo mode from poleward propagating activity belts to equatorward propagating ones. Furthermore, the clearly cyclic solutions disappear at the highest magnetic Reynolds numbers and give way to irregular sign changes or quasi-stationary states. The total (mean and fluctuating) magnetic energy increases as a function of the magnetic Reynolds number in the range studied here (5-151), but the energies of the mean magnetic fields level off at high magnetic Reynolds numbers. The differential rotation is strongly affected by the magnetic fields and almost vanishes at the highest magnetic Reynolds numbers. In some of our most turbulent cases, however, we find that two regimes are possible, where either differential rotation is strong and mean magnetic fields are relatively weak, or vice versa. Conclusions. Our simulations indicate a strong nonlinear feedback of magnetic fields on differential rotation, leading to qualitative changes in the behaviors of large-scale dynamos at high magnetic Reynolds numbers. Furthermore, we do not find indications of the simulations approaching an asymptotic regime where the results would be independent of diffusion coefficients in the parameter range studied here.

  • 868. Käpylä, Petri J.
    et al.
    Mantere, Maarit J.
    Brandenburg, Axel
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Cyclic magnetic activity due to turbulent convection in spherical wedge geometry2012Ingår i: Astrophysical Journal Letters, ISSN 2041-8205, Vol. 755, nr 1, s. L22-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We report on simulations of turbulent, rotating, stratified, magnetohydrodynamic convection in spherical wedge geometry. An initially small-scale, random, weak-amplitude magnetic field is amplified by several orders of magnitude in the course of the simulation to form oscillatory large-scale fields in the saturated state of the dynamo. The differential rotation is solar-like (fast equator), but neither coherent meridional poleward circulation nor near-surface shear layer develop in these runs. In addition to a poleward branch of magnetic activity beyond 50 degrees latitude, we find for the first time a pronounced equatorward branch at around 20 degrees latitude, reminiscent of the solar cycle.

  • 869.
    Käpylä, Petri J.
    et al.
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Mantere, Maarit J.
    Cole, Elizabeth
    Warnecke, Jörn
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Brandenburg, Axel
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA.
    Effects of enhanced stratification on equatorward dynamo wave propagation2013Ingår i: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 778, nr 1, s. 41-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present results from simulations of rotating magnetized turbulent convection in spherical wedge geometry representing parts of the latitudinal and longitudinal extents of a star. Here we consider a set of runs for which the density stratification is varied, keeping the Reynolds and Coriolis numbers at similar values. In the case of weak stratification, we find quasi-steady dynamo solutions for moderate rotation and oscillatory ones with poleward migration of activity belts for more rapid rotation. For stronger stratification, the growth rate tends to become smaller. Furthermore, a transition from quasi-steady to oscillatory dynamos is found as the Coriolis number is increased, but now there is an equatorward migrating branch near the equator. The breakpoint where this happens corresponds to a rotation rate that is about three to seven times the solar value. The phase relation of the magnetic field is such that the toroidal field lags behind the radial field by about pi/2, which can be explained by an oscillatory alpha(2) dynamo caused by the sign change of the alpha-effect about the equator. We test the domain size dependence of our results for a rapidly rotating run with equatorward migration by varying the longitudinal extent of our wedge. The energy of the axisymmetric mean magnetic field decreases as the domain size increases and we find that an m = 1 mode is excited for a full 2 pi azimuthal extent, reminiscent of the field configurations deduced from observations of rapidly rotating late-type stars.

  • 870.
    Käpylä, Petri J.
    et al.
    Department of Physics, PO BOX 64 (Gustaf Hällströmin katu 2a), FI-00014 University of Helsinki, Finland Nordita SU.
    Mantere, Marit
    Department of Physics, PO BOX 64 (Gustaf Hällströmin katu 2a), FI-00014 University of Helsinki, Finland.
    Brandenburg, Axel
    Department of Physics, PO BOX 64 (Gustaf Hällströmin katu 2a), FI-00014 University of Helsinki, Finland Nordita SU .
    Effects of stratification in spherical shell convection2011Ingår i: Astronomical Notes - Astronomische Nachrichten, ISSN 0004-6337, E-ISSN 1521-3994, Vol. 332, nr 9-10, s. 883-890Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We report on simulations of mildly turbulent convection in spherical wedge geometry with varying density stratification. We vary the density contrast within the convection zone by a factor of 20 and study the influence of rotation on the solutions. We demonstrate that the size of convective cells decreases and the anisotropy of turbulence increases as the stratification is increased. Differential rotation is found to change from anti-solar (slow equator) to solar-like (fast equator) at roughly the same Coriolis number for all stratifications. The largest stratification runs, however, are sensitive to changes of the Reynolds number. Evidence for a near-surface shear layer is found in runs with strong stratification and large Reynolds numbers.

  • 871.
    Könnig, F.
    KTH. Ruprecht-Karls-Universität Heidelberg, Germany.
    Higuchi ghosts and gradient instabilities in bimetric gravity2015Ingår i: Physical Review D, ISSN 1550-7998, E-ISSN 1550-2368, Vol. 91, nr 10, artikel-id 104019Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Bimetric gravity theories allow for many different types of cosmological solutions, but not all of them are theoretically allowed. In this work we discuss the conditions to satisfy the Higuchi bound and to avoid gradient instabilities in the scalar sector at the linear level. We find that in expanding universes the ratio of the scale factors of the reference and observable metric has to increase at all times. This automatically implies a ghost-free helicity-2 and helicity-0 sector and enforces a phantom dark energy. Furthermore, the condition for the absence of gradient instabilities in the scalar sector will be analyzed. Finally, we discuss whether cosmological solutions can exist, including exotic evolutions like bouncing cosmologies, in which both the Higuchi ghost and scalar instabilities are absent at all times.

  • 872.
    La Monaca, Fabio
    et al.
    INAF Istituto di Astrofisica e Planetologia Spaziali, Via del Fosso del Cavaliere 100, I-00133 Roma, Italy;, Via del Fosso del Cavaliere 100; Dipartimento di Fisica, Università degli Studi di Roma “Tor Vergata,” Via della Ricerca Scientifica 1, I-00133 Roma, Italy, Via della Ricerca Scientifica 1; Dipartimento di Fisica, Università degli Studi di Roma “La Sapienza,” Piazzale Aldo Moro 5, I-00185 Roma, Italy, Piazzale Aldo Moro 5.
    Veledina, Alexandra
    Nordita SU; Department of Physics and Astronomy, 20014 University of Turku, Finland.
    Wu, Kinwah
    Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, Surrey RH5 6NT, UK, Holmbury St Mary, Dorking.
    et al.,
    Highly Significant Detection of X-Ray Polarization from the Brightest Accreting Neutron Star Sco X-12024Ingår i: Astrophysical Journal Letters, ISSN 2041-8205, E-ISSN 2041-8213, Vol. 960, nr 2, artikel-id L11Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The Imaging X-ray Polarimetry Explorer measured with high significance the X-ray polarization of the brightest Z-source, Sco X-1, resulting in the nominal 2-8 keV energy band in a polarization degree of 1.0% ± 0.2% and a polarization angle of 8° ± 6° at a 90% confidence level. This observation was strictly simultaneous with observations performed by NICER, NuSTAR, and Insight-HXMT, which allowed for a precise characterization of its broadband spectrum from soft to hard X-rays. The source has been observed mainly in its soft state, with short periods of flaring. We also observed low-frequency quasiperiodic oscillations. From a spectropolarimetric analysis, we associate a polarization to the accretion disk at <3.2% at 90% confidence level, compatible with expectations for an electron scattering dominated optically thick atmosphere at the Sco X-1 inclination of ∼44°; for the higher-energy Comptonized component, we obtain a polarization of 1.3% ± 0.4%, in agreement with expectations for a slab of Thomson optical depth of ∼7 and an electron temperature of ∼3 keV. A polarization rotation with respect to previous observations by OSO-8 and PolarLight, and also with respect to the radio-jet position angle, is observed. This result may indicate a variation of the polarization with the source state that can be related to relativistic precession or a change in the corona geometry with the accretion flow.

  • 873. Lapenta, Giovanni
    et al.
    Pierrard, Viviane
    Keppens, Rony
    Markidis, Stefano
    KTH, Skolan för datavetenskap och kommunikation (CSC), High Performance Computing and Visualization (HPCViz). KTH, Skolan för datavetenskap och kommunikation (CSC), Centra, Parallelldatorcentrum, PDC.
    Poedts, Stefaan
    Sebek, Ondrej
    Travnicek, Pavel M.
    Henri, Pierre
    Califano, Francesco
    Pegoraro, Francesco
    Faganello, Matteo
    Olshevsky, Vyacheslav
    Restante, Anna Lisa
    Nordlund, Åke
    Frederiksen, Jacob Trier
    Mackay, Duncan H.
    Parnell, Clare E.
    Bemporad, Alessandro
    Susino, Roberto
    Borremans, Kris
    SWIFF: Space weather integrated forecasting framework2013Ingår i: Journal of Space Weather and Space Climate, E-ISSN 2115-7251, Vol. 3, s. A05-Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    SWIFF is a project funded by the Seventh Framework Programme of the European Commission to study the mathematical-physics models that form the basis for space weather forecasting. The phenomena of space weather span a tremendous scale of densities and temperature with scales ranging 10 orders of magnitude in space and time. Additionally even in local regions there are concurrent processes developing at the electron, ion and global scales strongly interacting with each other. The fundamental challenge in modelling space weather is the need to address multiple physics and multiple scales. Here we present our approach to take existing expertise in fluid and kinetic models to produce an integrated mathematical approach and software infrastructure that allows fluid and kinetic processes to be modelled together. SWIFF aims also at using this new infrastructure to model specific coupled processes at the Solar Corona, in the interplanetary space and in the interaction at the Earth magnetosphere.

  • 874. Larsson, J.
    et al.
    Ryde, Felix
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
    Lundman, Christoffer
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
    McGlynn, Sinead
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
    Larsson, Stefan
    Ohno, M.
    Yamaoka, K.
    Spectral components in the bright, long GRB 061007: properties of the photosphere and the nature of the outflow2011Ingår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 414, nr 3, s. 2642-2649Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present a time-resolved spectral analysis of the bright, long GRB 061007 (z = 1.261) using Swift and Suzaku data. We find that the prompt emission of the burst can be equally well explained by a photospheric component together with a power law as by a Band function, and we explore the implications of the former model. The photospheric component, which we model with a multicolour blackbody, dominates the spectra and has a very stable shape throughout the burst. This component provides a natural explanation for the hardness-intensity correlation seen within the burst and also allows us to estimate the bulk Lorentz factor and the radius of the photosphere. The power-law component dominates the fit at high energies and has a nearly constant slope of -1.5. We discuss the possibility that this component is of the same origin as the high-energy power laws recently observed in some Fermi bursts.

  • 875.
    Larsson, Josefin
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
    D’Ammando, F.
    Falocco, Serena
    KTH.
    Giroletti, M.
    Orienti, M.
    Piconcelli, E.
    Righini, S.
    A close look at the γ-ray emitting narrow-line seyfert 1 FBQS J1644+26192018Ingår i: Proceedings of Science, Sissa Medialab Srl , 2018Konferensbidrag (Refereegranskat)
    Abstract [en]

    FBQS J1644+2619 is one of the most recently discovered γ-ray emitting Narrow-Line Seyfert 1s (NLSy1s). Here we present a multiwavelength analysis of this source, focussing on a recent 80 ks X-ray observation with XMM-Newton. The spectral energy distribution of the source is similar to the other γ-ray NLSy1s, confirming its blazar-like nature. The X-ray spectrum is characterised by a hard photon index (Γ = 1.66) above 2 keV and a soft excess at lower energies. The hard photon index provides clear evidence that inverse Compton emission from the jet dominates the spectrum, while the soft excess can be explained by a contribution from the underlying Seyfert emission. This contribution can be fitted by reflection of emission from the base of the jet, as well as by Comptonisation in a warm, optically thick corona. We also compare these results with X-ray observations of other γ-ray NLSy1s. The majority of the sources have similar X-ray spectra, with properties intermediate between blazars and radio-quiet NLSy1s.

  • 876.
    Larsson, Josefin
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
    D'Ammando, F.
    Univ Bologna, Dipartimento Fis & Astron, Via Gobetti 93-2, I-40129 Bologna, Italy.;Ist Radio Astron Bologna, INAF, Via P Gobetti 101, I-40129 Bologna, Italy..
    Falocco, Serena
    KTH, Skolan för teknikvetenskap (SCI), Fysik.
    Giroletti, M.
    Ist Radio Astron Bologna, INAF, Via P Gobetti 101, I-40129 Bologna, Italy..
    Orienti, M.
    Ist Radio Astron Bologna, INAF, Via P Gobetti 101, I-40129 Bologna, Italy..
    Piconcelli, E.
    Osserv Astron Roma, INAF, Via Frascati 33, I-00040 Rome, Italy..
    Righini, S.
    Ist Radio Astron Bologna, INAF, Via P Gobetti 101, I-40129 Bologna, Italy..
    FBQS J1644+2619: multiwavelength properties and its place in the class of gamma-ray emitting Narrow Line Seyfert 1s2018Ingår i: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 476, nr 1, s. 43-55Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A small fraction of Narrow Line Seyfert 1s (NLSy1s) are observed to be gamma-ray emitters. Understanding the properties of these sources is of interest since the majority of NLSy1s are very different from typical blazars. Here, we present a multifrequency analysis of FBQS J1644+2619, one of the most recently discovered gamma-ray emitting NLSy1s. We analyse an similar to 80 ks XMM-Newton observation obtained in 2017, as well as quasi-simultaneous multi-wavelength observations covering the radio-gamma-ray range. The spectral energy distribution of the source is similar to the other gamma-ray NLSy1s, confirming its blazar-like nature. The X-ray spectrum is characterized by a hard photon index (Gamma = 1.66) above 2 keV and a soft excess at lower energies. The hard photon index provides clear evidence that inverse Compton emission from the jet dominates the spectrum, while the soft excess can be explained by a contribution from the underlying Seyfert emission. This contribution can be fitted by reflection of emission from the base of the jet, as well as by Comptonization in a warm, optically thick corona. We discuss our results in the context of the other gamma-ray NLSy1s and note that the majority of them have similar X-ray spectra, with properties intermediate between blazars and radio-quiet NLSy1s.

  • 877.
    Larsson, Josefin
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. The Oskar Klein Centre, AlbaNova, SE-106 91 Stockholm, Sweden.
    Fransson, C.
    Department of Astronomy, Stockholm University, The Oskar Klein Centre, AlbaNova, SE-106 91 Stockholm, Sweden.
    Sargent, B.
    Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA; Center for Astrophysical Sciences, The William H. Miller III Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA.
    Jones, O. C.
    UK Astronomy Technology Centre, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK.
    Barlow, M. J.
    Department of Physics and Astronomy, University College London (UCL), Gower Street, London WC1E 6BT, UK.
    Bouchet, P.
    Laboratoire AIM Paris-Saclay, CNRS, Universite Paris Diderot, F-91191 Gif-sur-Yvette, France.
    Meixner, M.
    Stratospheric Observatory for Infrared Astronomy, NASA Ames Research Center, Mail Stop 204-14, Moffett Field, CA 94035, USA; Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA.
    Blommaert, J. A.D.L.
    Astronomy and Astrophysics Research Group, Department of Physics and Astrophysics, Vrije Universiteit Brussel, Pleinlaan 2, B-1050 Brussels, Belgium.
    Coulais, A.
    LERMA, Observatoire de Paris, Université PSL, Sorbonne Universié, CNRS, Paris, France; Astrophysics Department CEA-Saclay, France.
    Fox, O. D.
    Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA.
    Gastaud, R.
    Laboratoire AIM Paris-Saclay, CEA-IRFU/SAp, CNRS, Universite Paris Diderot, F-91191 Gif-sur-Yvette, France.
    Glasse, A.
    UK Astronomy Technology Centre, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK.
    Habel, N.
    Stratospheric Observatory for Infrared Astronomy, NASA Ames Research Center, Mail Stop 204-14, Moffett Field, CA 94035, USA; Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, CA 91109, USA.
    Hirschauer, A. S.
    Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA.
    Hjorth, J.
    DARK, Niels Bohr Institute, University of Copenhagen, Jagtvej 128, DK-2200 Copenhagen, Denmark.
    Jaspers, J.
    Dublin Institute for Advanced Studies, School of Cosmic Physics, Astronomy & Astrophysics Section, 31 Fitzwilliam Place, Dublin 2, Ireland; Department of Experimental Physics, Maynooth University, Maynooth, Co Kildare, Ireland.
    Kavanagh, P. J.
    Dublin Institute for Advanced Studies, School of Cosmic Physics, Astronomy & Astrophysics Section, 31 Fitzwilliam Place, Dublin 2, Ireland; Department of Experimental Physics, Maynooth University, Maynooth, Co Kildare, Ireland.
    Krause, O.
    Max-Planck-Institut fuer Astronomie, Koenigstuhl 17, D-69117 Heidelberg, Germany.
    Lau, R. M.
    NSF's NOIR Lab 950 N. Cherry Avenue, Tucson, AZ 85721, USA.
    Lenkić, L.
    Stratospheric Observatory for Infrared Astronomy, NASA Ames Research Center, Mail Stop 204-14, Moffett Field, CA 94035, USA.
    Nayak, O.
    Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA.
    Rest, A.
    Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA; Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA.
    Temim, T.
    Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA.
    Tikkanen, T.
    School of Physics & Astronomy, Space Research Centre, University of Leicester, Space Park Leicester, 92 Corporation Road, Leicester LE4 5SP, UK.
    Wesson, R.
    School of Physics and Astronomy, Cardiff Univerity, Queen's Buildings, The Parade, Cardiff CF24 3AA, UK.
    Wright, G. S.
    UK Astronomy Technology Centre, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK.
    JWST NIRSpec Observations of Supernova 1987A: From the Inner Ejecta to the Reverse Shock2023Ingår i: Astrophysical Journal Letters, ISSN 2041-8205, E-ISSN 2041-8213, Vol. 949, nr 2, artikel-id L27Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present initial results from JWST NIRSpec integral field unit observations of the nearby supernova SN 1987A. The observations provide the first spatially resolved spectroscopy of the ejecta and equatorial ring (ER) over the 1-5 μm range. We construct 3D emissivity maps of the [Fe i] 1.443 μm line from the inner ejecta and the He i 1.083 μm line from the reverse shock (RS), where the former probes the explosion geometry and the latter traces the structure of the circumstellar medium. We also present a model for the integrated spectrum of the ejecta. The [Fe i] 3D map reveals a highly asymmetric morphology resembling a broken dipole, dominated by two large clumps with velocities of ∼2300 km s−1. We also find evidence that the Fe-rich inner ejecta have started to interact with the RS. The RS surface traced by the He i line extends from just inside the ER to higher latitudes on both sides of the ER with a half-opening angle ∼45°, forming a bubble-like structure. The spectral model for the ejecta allows us to identify the many emission lines, including numerous H2 lines. We find that the H2 is most likely excited by far-UV emission, while the metal-line ratios are consistent with a combination of collisional excitation and recombination in the low-temperature ejecta. We also find several high-ionization coronal lines from the ER, requiring a temperature ≳2 × 106 K.

  • 878.
    Larsson, Josefin
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
    Racusin, J. L.
    Burgess, J. Michael
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
    Evidence for jet launching close to the black hole in GRB 101219b: A Fermi grb dominated by thermal emission2015Ingår i: Astrophysical Journal Letters, ISSN 2041-8205, E-ISSN 2041-8213, Vol. 800, nr 2, artikel-id L34Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We present observations by the Fermi Gamma-Ray Space Telescope Gamma-Ray Burst Monitor (GBM) of the nearby (z = 0.55) GRB 101219B. This burst is a long GRB, with an associated supernova and with a blackbody (BB) component detected in the early afterglow observed by the Swift X-ray Telescope (XRT). Here we show that the prompt gamma-ray emission has a BB spectrum, making this the second such burst observed by Fermi GBM. The properties of the BB, together with the redshift and our estimate of the radiative efficiency makes it possible to calculate the absolute values of the properties of the outflow. We obtain an initial Lorentz factor G = 138 +/- 8, a photospheric radius r(phot) = 4.4 +/- 1.9 x 10(11) cm, and a launch radius r(0) = 2.7 +/- 1.6 x 10(7) cm. The latter value is close to the black hole and suggests that the jet has a relatively unobstructed path through the star. There is no smooth connection between the BB components seen by GBM and XRT, ruling out the scenario that the late emission is due to high-latitude effects. In the interpretation that the XRT BB is prompt emission due to late central engine activity, the jet either has to be very wide or have a clumpy structure where the emission originates from a small patch. Other explanations for this component, such as emission from a cocoon surrounding the jet, are also possible.

  • 879.
    Larsson, Josefin
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
    Sollerman, J.
    Stockholm Univ, Oskar Klein Ctr, Dept Astron, AlbaNova, SE-10691 Stockholm, Sweden..
    Lyman, J. D.
    Univ Warwick, Dept Phys, Gibbet Hill Rd, Coventry CV4 7AL, W Midlands, England..
    Spyromilio, J.
    European Southern Observ, Karl Schwarzschild Str 2, D-85748 Garching, Germany..
    Tenhu, Linda
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
    Fransson, C.
    Stockholm Univ, Oskar Klein Ctr, Dept Astron, AlbaNova, SE-10691 Stockholm, Sweden..
    Lundqvist, P.
    Stockholm Univ, Oskar Klein Ctr, Dept Astron, AlbaNova, SE-10691 Stockholm, Sweden..
    Clumps and Rings of Ejecta in SNR 0540-69.3 as Seen in 3D2021Ingår i: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 922, nr 2, s. 265-, artikel-id 265Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The distribution of ejecta in young supernova remnants offers a powerful observational probe of their explosions and progenitors. Here we present a 3D reconstruction of the ejecta in SNR 0540-69.3, which is an O-rich remnant with a pulsar wind nebula located in the LMC. We use observations from the Very Large Telescope (VLT)/MUSE to study H beta, [O iii] lambda lambda 4959, 5007, H alpha, [S ii] lambda lambda 6717, 6731, [Ar iii] lambda 7136, and [S iii] lambda 9069. This is complemented by 2D spectra from VLT/X-shooter, which also cover [O ii] lambda lambda 3726, 3729, and [Fe ii] lambda 12567. We identify three main emission components: (i) clumpy rings in the inner nebula (less than or similar to 1000 km s(-1)) with similar morphologies in all lines; (ii) faint extended [O iii] emission dominated by an irregular ring-like structure with radius similar to 1600 km s(-1) and inclination similar to 40 degrees, but with maximal velocities reaching similar to 3000 km s(-1); and (iii) a blob of H alpha and H beta located southeast of the pulsar at velocities similar to 1500-3500 km s(-1). We analyze the geometry using a clump-finding algorithm and use the clumps in the [O iii] ring to estimate an age of 1146 +/- 116 yr. The observations favor an interpretation of the [O iii] ring as ejecta, while the origin of the H-blob is more uncertain. An alternative explanation is that it is the blown-off envelope of a binary companion. From the detection of Balmer lines in the innermost ejecta we confirm that SNR 0540 was a Type II supernova and that hydrogen was mixed down to low velocities in the explosion.

  • 880.
    Larsson, Josefin
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
    Spyromilio, J.
    European Southern Observ, Karl Schwarzschild Str 2, D-85748 Garching, Germany..
    Fransson, C.
    Stockholm Univ, Dept Astron, Oskar Klein Ctr, AlbaNova, SE-10691 Stockholm, Sweden..
    Indebetouw, R.
    Natl Radio Astron Observ, 520 Edgemont Rd, Charlottesville, VA 22903 USA.;Univ Virginia, 520 Edgemont Rd, Charlottesville, VA 22903 USA..
    Matsuura, M.
    Cardiff Univ, Sch Phys & Astrophys, Queens Bldg, Cardiff CF24 3AA, S Glam, Wales..
    Abellan, F. J.
    Univ Valencia, Dept Astron & Astrofis, C Dr Moliner 50, E-46100 Burjassot, Spain..
    Cigan, P.
    Cardiff Univ, Sch Phys & Astrophys, Queens Bldg, Cardiff CF24 3AA, S Glam, Wales..
    Gomez, H.
    Cardiff Univ, Sch Phys & Astrophys, Queens Bldg, Cardiff CF24 3AA, S Glam, Wales..
    Leibundgut, B.
    European Southern Observ, Karl Schwarzschild Str 2, D-85748 Garching, Germany..
    A Three-dimensional View of Molecular Hydrogen in SN 1987A2019Ingår i: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 873, nr 1, artikel-id 15Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    SN 1987A is the only young supernova (SN) in which H-2 has been detected in the ejecta. The properties of H-2 are important for understanding the explosion and the ejecta chemistry. Here we present new Very Large Telescope/SINFONI observations of H-2 in SN 1987A, focusing on the 2.12 mu m (1,0)S(1) line. We find that the 3D emissivity is dominated by a single clump in the southern ejecta, with weaker emission being present in the north along the plane of the circumstellar ring. The lowest observed velocities are in the range of 400-800 km s(-1), in agreement with previous limits on inward mixing of H. The brightest regions of H-2 coincide with faint regions of H alpha, which can be explained by H alpha being powered by X-ray emission from the ring, while the H-2 is powered by Ti-44. A comparison with ALMA observations of other molecules and dust shows that the brightest regions of H-2, CO, and SiO occupy different parts of the inner ejecta and that the brightest H-2 clump coincides with a region of very weak dust emission. The latter is consistent with theoretical predictions that the H-2 should form in the gas phase rather than on dust grains.

  • 881. Larsson, S.
    et al.
    Ryde, Felix
    Stockholm Observatory, AlbaNova.
    Borgonovo, L.
    Bagoly, Z.
    Mészáros, A.
    Pearce, Mark
    KTH, Tidigare Institutioner (före 2005), Fysik.
    Lund, N.
    von Kienlin, A.
    Lichti, G.
    The Background of the INTEGRAL SPI Anticoincidence Shield and the Observations of GRBs2004Ingår i: PROCEEDINGS OF THE 5TH INTEGRAL WORKSHOP ON THE INTEGRAL UNIVERSE / [ed] Battrick, B, European Space Agency, 2004, Vol. 552, s. 649-652Konferensbidrag (Refereegranskat)
    Abstract [en]

    The anti-coincidence shield, ACS, of the spectrometer, SPI (Vedrenne et al., 2003), on INTEGRAL provides the possibility to study temporal properties of gamma-ray bursts (GRBs) with high resolution (Rau et al. 2004). To correctly interpret the results of the analysis, a good understanding of the back,ground and noise levels of the instrument is required. The back-round noise of the ACS, oil timescales up to the length of a GRB, has a Gaussian distribution and its power is independent of frequency, that is, it is white noise. However, it does not follow a Poisson statistic since on average the variance is similar to 1.6 times larger than the mean, and varies significantly. We find a systematic relation between the mean count rate and its variance. The ratio, variance/mean, decreases as the mean count rate increases. As well as helping to understand the cause of this effect, this characterisation is useful when modelling the time variability of GRBs. To illustrate the discussion we present some GRB light curves detected by the SPI/ACS and shortly discuss the analysis that has been made of them.

  • 882. Larsson, S.
    et al.
    Ryde, Felix
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
    Ivchenko, Nickolay
    KTH, Skolan för elektro- och systemteknik (EES), Rymd- och plasmafysik.
    Pearce, Mark
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
    Studies of auroral X-ray backgrounds for high latitude balloon astrophysical experiments2007Ingår i: 18th ESA Symposium on European Rocket and Balloon Programmes and Related Research, European Space Agency, 2007, nr 647 SP, s. 513-516Konferensbidrag (Refereegranskat)
    Abstract [en]

    Auroral X-ray emission was discovered in the 1950s and has been studied with instruments on balloons, rockets and satellites. While this radiation is of prime interest for studies of space plasma in the Earth's magnetosphere the same radiation is also a background for astrophysical observations made during balloon flights at high latitudes. For such observations it is necessary to monitor and understand the properties of this radiation. This is particularly true for hard X-ray polarimetry which is still an observationally unexplored field. Instruments to measure X-ray polarizations are being developed and will probably first be flown on balloons. We discuss how the auroral X-ray emission and in particular its polarization properties may affect these observations and whether these instruments also can provide information about the high energy electrons producing the X-ray aurora. Optical monitoring of the auroral emissions to allow unambiguous relation of X-ray background to aurora is also discussed.

  • 883.
    Launila, Olli
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Tillämpad fysik.
    Banerjee, D. P. K.
    Rotational spectroscopy of AlO Low-N transitions of astronomical interest in the X-2 Sigma(+) state2009Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 508, nr 2, s. 1067-1072Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Aims. The detection of rotational transitions of the AlO radical at millimeter wavelengths from an astronomical source has recently been reported. In view of this, rotational transitions in the ground X-2 Sigma(+) state of AlO have been reinvestigated. Methods. Comparisons between Fourier transform and microwave data indicate a discrepancy regarding the derived value of gamma(D) in the v = 0 level of the ground state. This discrepancy is discussed in the light of comparisons between experimental data and synthesized rotational spectra in the v = 0, 1 and 2 levels of X-2 Sigma(+). Results. A list of calculated rotational lines in v = 0, 1 and 2 of the ground state up to N' = 11 is presented which should aid astronomers in analysis and interpretation of observed AlO data and also facilitate future searches for this radical.

  • 884.
    Lavraud, B.
    et al.
    Univ Bordeaux, Lab Astrophys Bordeaux, CNRS, Pessac, France.;Inst Rech Astrophys & Planetol, CNES, UPS, CNRS, Toulouse, France..
    Vaivads, Andris
    KTH, Skolan för elektroteknik och datavetenskap (EECS), Elektroteknik, Rymd- och plasmafysik.
    O'Brien, H.
    Imperial Coll London, Blackett Lab, Space & Atmospher Phys, London, England..
    Magnetic reconnection as a mechanism to produce multiple thermal proton populations and beams locally in the solar wind2021Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 656, artikel-id A37Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Context. Spacecraft data revealed early on the frequent observation of multiple near-thermal proton populations in the solar wind. Decades of research on their origin have focused on processes such as magnetic reconnection in the low corona and wave-particle interactions in the corona and locally in the solar wind. Aims. This study aims to highlight the fact that such multiple thermal proton populations and beams are also produced by magnetic reconnection occurring locally in the solar wind. Methods. We used high-resolution Solar Orbiter proton velocity distribution function measurements, complemented by electron and magnetic field data, to analyze the association of multiple thermal proton populations and beams with magnetic reconnection during a period of slow Alfvenic solar wind on 16 July 2020. Results. At least six reconnecting current sheets with associated multiple thermal proton populations and beams, including a case of magnetic reconnection at a switchback boundary, were found on this day. This represents 2% of the measured distribution functions. We discuss how this proportion may be underestimated, and how it may depend on solar wind type and distance from the Sun. Conclusions. Although suggesting a likely small contribution, but which remains to be quantitatively assessed, Solar Orbiter observations show that magnetic reconnection must be considered as one of the mechanisms that produce multiple thermal proton populations and beams locally in the solar wind.

  • 885.
    Leblanc, F.
    et al.
    Sorbonne Univ, LATMOS, CNRS, UVSQ, Paris, France..
    Roth, Lorenz
    KTH, Skolan för elektroteknik och datavetenskap (EECS), Elektroteknik, Rymd- och plasmafysik.
    Chaufray, J. Y.
    Sorbonne Univ, LATMOS, CNRS, UVSQ, Paris, France..
    Modolo, R.
    Sorbonne Univ, LATMOS, CNRS, UVSQ, Paris, France..
    Galand, M.
    Imperial Coll London, Dept Phys, London, England..
    Ivchenko, Natalya
    KTH, Skolan för elektroteknik och datavetenskap (EECS), Elektroteknik, Rymd- och plasmafysik.
    Carnielli, G.
    Imperial Coll London, Dept Phys, London, England..
    Baskevitch, C.
    Sorbonne Univ, LATMOS, CNRS, UVSQ, Paris, France..
    Oza, A.
    CALTECH, Jet Prop Lab, Pasadena, CA USA..
    Werner, A. L. E.
    Sorbonne Univ, LATMOS, CNRS, UVSQ, Paris, France.;Swedish Inst Space Phys, Uppsala, Sweden..
    Ganymede's atmosphere as constrained by HST/STIS observations2023Ingår i: Icarus, ISSN 0019-1035, E-ISSN 1090-2643, Vol. 399, artikel-id 115557Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    A new analysis of aurora observations of Ganymede's atmosphere on the orbital leading and trailing hemispheres has been recently published by Roth et al. (2021), suggesting that water is its main constituent near noon. Here, we present two additional aurora observations of Ganymede's sub-Jovian and anti-Jovian hemispheres, which suggest a modulation of the atmospheric H2O/O-2 ratio on the moon's orbital period, and analyze the orbital evolution of the atmosphere. For this, we propose a reconstruction of aurora observations based on a physical modelling of the exosphere taking into account its orbital variability (the Exospheric Global Model; Leblanc et al., 2017). The solution described in this paper agrees with Roth et al. (2021) that Ganymede's exosphere should be dominantly composed of water molecules. From Ganymede's position when its leading hemisphere is illuminated to when it is its trailing hemisphere, the column density of O-2 may vary between 4.3 x 10(14) and 3.6 x 10(14) cm(-2) whereas the H2O column density should vary between 5.6 x 10(14) and 1.3 x 10(15) cm(-2). The water content of Ganymede's atmosphere is essentially constrained by its sublimation rate whereas the O-2 component of Ganymede's atmosphere is controlled by the radiolytic yield. The other species, products of the water molecules, vary in a more complex way depending on their sources, either as ejecta from the surface and/or as product of the dissociation of the other atmospheric constituents. Electron impact on H2O and H-2 molecules is shown to likely produce H Lyman-alpha emissions close to Ganymede, in addition to the observed extended Lyman-alpha corona from H resonant scattering. All these conclusions being highly dependent on our capability to accurately model the origins of the observed Ganymede auroral emissions, modelling these emissions remains poorly constrained without an accurate knowledge of the Jovian magnetospheric and Ganymede ionospheric electron populations.

  • 886.
    Lehnert, Bo
    KTH, Skolan för elektro- och systemteknik (EES), Fusionsplasmafysik. KTH, Skolan för elektro- och systemteknik (EES), Centra, Alfvénlaboratoriet.
    Cosmic equilibrium of a photon gas in its gravitational field2008Ingår i: Advanced Studies in Theoretical Physics, ISSN 1313-1311, E-ISSN 1314-7609, Vol. 2, nr 17, s. 837-843Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Recent research indicates that the universe is expanding at an accelerated rate, and that dark energy and dark matter are underlying such a behaviour. In this paper the proposal is made that dark energy and matter are due to the zero point energy of a photon gas and its related Casimir force. For cosmical dimensions an equilibrium is shown to exist within such a gas, in the form of a balance between its pressure and gravitational forces. With commonly accepted values of the parameters involved, there is further an indication that the universe should be somewhat off such an equilibrium, and be governed by a pressure-dominated accelerated state. The obtained numerical results depend critically on the effective spectral contribution to the zero point pressure. Expansion at a constant rate is only possible at special conditions. This analysis can also be modified to apply to the hot photon gas prevailing during the earliest stages of the expansion, thereby leading to a strongly reduced characteristic radius within the parameter range of an equilibrium.

  • 887.
    Lehtinen, J. J.
    et al.
    Univ Turku, Finnish Ctr Astron ESO FINCA, Vesilinnantie 5, Turku 20014, Finland.;Univ Helsinki, Dept Phys, POB 64, Helsinki 00014, Finland..
    Käpylä, Maarit
    Aalto Univ, Dept Comp Sci, POB 15400, Aalto 00076, Finland.;Max Planck Inst Solar Syst Res, Justus von Liebig Weg 3, D-37077 Gottingen, Germany.; Nordita SU.
    Hackman, T.
    Univ Helsinki, Dept Phys, POB 64, Helsinki 00014, Finland..
    Kochukhov, O.
    Uppsala Univ, Dept Phys & Astron, Box 516, S-75120 Uppsala, Sweden..
    Willamo, T.
    Univ Helsinki, Dept Phys, POB 64, Helsinki 00014, Finland..
    Marsden, S. C.
    Univ Southern Queensland, Ctr Astrophys, Toowoomba, Qld 4350, Australia..
    Jeffers, S. , V
    Henry, G. W.
    Tennessee State Univ, Ctr Excellence Informat Syst, Nashville, TN 37209 USA..
    Jetsu, L.
    Univ Helsinki, Dept Phys, POB 64, Helsinki 00014, Finland..
    Topological changes in the magnetic field of LQ Hya during an activity minimum2022Ingår i: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 660, artikel-id A141Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Aims. Previous studies have related surface temperature maps, obtained with the Doppler imaging (DI) technique, of LQ Hya with long-term photometry. Here, we compare surface magnetic field maps, obtained with the Zeeman Doppler imaging (ZDI) technique, with contemporaneous photometry, with the aim of quantifying the star's magnetic cycle characteristics. Methods. We inverted Stokes IV spectropolarimetry, obtained with the HARPSpol and ESPaDOnS instruments, into magnetic field and surface brightness maps using a tomographic inversion code that models high signal-to-noise ratio mean line profiles produced by the least squares deconvolution (LSD) technique. The maps were compared against long-term ground-based photometry acquired with the T3 0.40 m Automatic Photoelectric Telescope (APT) at Fairborn Observatory, which offers a proxy for the spot cycle of the star, as well as with chromospheric Ca II H&K activity derived from the observed spectra. Results. The magnetic field and surface brightness maps reveal similar patterns relative to previous DI and ZDI studies: nonaxisymmetric polar magnetic field structure, void of fields at mid-latitudes, and a complex structure in the equatorial regions. There is a weak but clear tendency of the polar structures to be linked with a strong radial field and the equatorial ones with the azimuthal field. We find a polarity reversal in the radial field between 2016 and 2017 that is coincident with a spot minimum seen in the long-term photometry, although the precise relation of chromospheric activity to the spot activity remains complex and unclear. The inverted field strengths cannot be easily related with the observed spottedness, but we find that they are partially connected to the retrieved field complexity. Conclusions. This field topology and the dominance of the poloidal field component, when compared to global magnetoconvection models for rapidly rotating young suns, could be explained by a turbulent dynamo, where differential rotation does not play a major role (so-called alpha(2)Omega(2) or alpha(2) dynamos) and axi- and non-axisymmetric modes are excited simultaneously. The complex equatorial magnetic field structure could arise from the twisted (helical) wreaths often seen in these simulations, while the polar feature would be connected to the mostly poloidal non-axisymmetric component that has a smooth spatial structure.

  • 888.
    Lenni, A.
    et al.
    INFN, Section of Trieste, I-34127 Trieste, Italy; IFPU - Institute of Fundamental Physics of the Universe, I-34151 Trieste, Italy.
    Carlson, Per
    KTH, Skolan för teknikvetenskap (SCI), Fysik. Oskar Klein Centre for Cosmoparticle Physics, AlbaNova University Centre, SE-10691 Stockholm, Sweden.
    Pearce, Mark
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. the Oskar Klein Centre for Cosmoparticle Physics, AlbaNova University Centre, SE-10691 Stockholm, Sweden.
    Zampa, N.
    INFN, Sezione di Trieste, I-34149 Trieste, Italy.
    et al.,
    Studies of cosmic-ray solar modulation with the PAMELA experiment2023Ingår i: 27th European Cosmic Ray Symposium, ECRS 2022, Sissa Medialab Srl , 2023, artikel-id 044Konferensbidrag (Refereegranskat)
    Abstract [en]

    The launch of the satellite-borne PAMELA instrument on the 15th June 2006 opened a new era of high-precision studies of cosmic rays. Due to its low detection energy threshold and its long operation, PAMELA was able to accurately measure the fluxes of several cosmic-ray species over a large energy range and study their time variations below a few tens of GeVs. In this presentation we will review PAMELA results on the time-dependent proton, helium and electron fluxes measured between a few tens of MeV/n and few tens of GeV/n from 2006 to 2014. Moreover, preliminary results of yearly energy spectra of deuterons, helium-3 and helium-4 nuclei below 1 GeV/n will be discussed. These measurements covered a time including the minimum phase of the 23rd solar cycle and the 24th solar maximum including the polarity reversal of the solar magnetic field. The PAMELA measurements have allowed to significantly improve the understanding of the charged-particle propagation through the Heliosphere, the charge-sign effect due to the drift motions of these particles and to calibrate state-of-the-art models of cosmic-ray transport in the Heliosphere.

  • 889.
    Lenni, Alex
    et al.
    INFN, Sezione di Trieste, I-34149 Trieste, Italy.; IFPU - Institute of Fundamental Physics of the Universe, I-34151 Trieste, Italy.; University of Trieste, Department of Physics, I-34127 Trieste, Italy..
    Carlson, Per
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
    Pearce, Mark
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
    Zampa, N.
    INFN, Sezione di Trieste, I-34149 Trieste, Italy..
    Study of the solar modulation for the cosmic ray isotopes with the PAMELA experiment2022Ingår i: 37th International Cosmic Ray Conference, ICRC 2021, Sissa Medialab Srl , 2022, artikel-id 1310Konferensbidrag (Refereegranskat)
    Abstract [en]

    The space-borne PAMELA experiment was launched on the 15th June 2006 on board the Russian satellite Resurs-DK1 from the Baikonur cosmodrome. PAMELA performed high-precision measurements of cosmic rays over a wide energy range until January 2016. Owing to its long-duration operation, PAMELA had turned out to be an optimal detecting apparatus for studies of the solar modulation of cosmic rays over time. The PAMELA collaboration has already published time-dependent proton, helium and electron spectra as well as the positron to electron ratio spanning almost an entire solar cycle. These results are fundamentally important in the fine-tuning of propagation and modulation models of cosmic rays through the Heliosphere. In this talk, the yearly average spectra for protons, deuterons, Helium-3 and Helium-4 nuclei are presented for the 23rd solar minimum (July 2006 - January 2009) and the first part of the 24th solar maximum (until September 2014). The isotopic composition was measured between 0.1 and 1.1 GeV/n using two different detector systems. As expected, the measured spectra display a rising trend towards the solar minimum followed by a decreasing trend as the solar maximum approaches. The time-dependent ratio of these isotopes is also presented. According to solar modulation studies, a non-constant ratio is expected due to the different charge-to-mass ratios and the different shapes of the respective local interstellar spectra. Additionally, it is of interest to analyze the observed spectra and ratios with state-of-the-art solar modulation models to obtain a deeper understanding of the relative importance of the mechanisms responsible for the propagation of cosmic rays in the Heliosphere over time.

  • 890. Leonov, A. A.
    et al.
    Galper, A. M.
    Adriaini, O.
    Aptekar, R. L.
    Arkhangelskaja, I. V.
    Arkhangelskiy, A. I.
    Bergstrom, L.
    Berti, E.
    Bigongiari, G.
    Bobkov, S. G.
    Boezio, M.
    Bogomolov, E. A.
    Bonechi, S.
    Bongi, M.
    Bonvicini, V.
    Bottai, S.
    Boyarchuk, K. A.
    Castellini, G.
    Cattaneo, P. W.
    Cumani, P.
    Dedenko, G. L.
    De Donato, C.
    Dogiel, V. A.
    Fuglesang, Ch.
    Gorbunov, M. S.
    Gusakov, Yu.V.
    Hnatyk, B. I.
    Kadilin, V. V.
    Kaplin, V. A.
    Kaplun, A. A.
    Kheymits, M. D.
    Korepanov, V. E.
    Larsson, Josefin
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
    Loginov, V. A.
    Longo, F.
    Maestro, P.
    Marrocchesi, P. S.
    Mikhailov, V. V.
    Mocchiutti, E.
    Moiseev, A. A.
    Mori, N.
    Moskalenko, I. V.
    Naumov, P.Yu.
    Papini, P.
    Pearce, Mark
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
    Picozza, P.
    Popov, A. V.
    Rappoldi, A.
    Ricciarini, S.
    Runtso, M. F.
    Ryde, Felix
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
    Serdin, O. V.
    Sparvoli, R.
    Spillantini, P.
    Suchkov, S. I.
    Taraskin, A. A.
    Tavani, M.
    Tiberio, A.
    Topchiev, N. P.
    Tyurin, E. M.
    Ulanov, M. V.
    Vacchi, A.
    Vannuccini, E.
    Vasilyev, G. I.
    Yurkin, Yu.T.
    Zampa, N.
    Zarikashvili, V. N.
    Zverev, V. G.
    The GAMMA-400 gamma-ray telescope characteristics. Angular resolution and electrons/protons separation2014Ingår i: Proceedings of Science, Sissa Medialab Srl , 2014Konferensbidrag (Refereegranskat)
    Abstract [en]

    The measurements of gamma-ray fluxes and cosmic-ray electrons and positrons in the energy range from 100 MeV to several TeV, which will be realized by the specially designed GAMMA-400 gamma-ray telescope, concern with the following broad range of scientific topics. Search for signatures of dark matter, surveying the celestial sphere in order to study point and extended sources of gamma-rays, measuring the energy spectra of Galactic and extragalactic diffuse gamma-ray emission, study of gamma-ray bursts and gamma-ray emission from the Sun, as well as high precision measurements of spectra of high-energy electrons and positrons, protons and nuclei up to the knee. To clarify these scientific problems with the new experimental data the GAMMA-400 gamma-ray telescope possesses unique physical characteristics comparing with previous and present experiments. For gamma-ray energies more than 100 GeV GAMMA-400 provides the energy resolution ~1% and angular resolution better than 0.02 deg. The methods, developed to reconstruct the direction of incident gamma photon, are presented in this paper, as well as, the capability of the GAMMA-400 gamma-ray telescope to distinguish electrons and positrons from protons in cosmic rays is investigated. The first point concerns with the space topology of high-energy gamma photon interaction in the matter of GAMMA-400. Multiple secondary particles, generated inside gamma-ray telescope, produce significant problems to restore the direction of initial gamma photon. Also back-splash particles, i.e., charged particles and gamma photons generated in calorimeter and moved upward, mask the initial tracks of electron/positron pair from conversion of incident gamma photon. The processed methods allow us to reconstruct the direction of electromagnetic shower axis and extract the electron/positron trace. As a result, the direction of incident gamma photon with the energy of 100 GeV is calculated with an accuracy of better than 0.02 deg. The main components of cosmic rays are protons and helium nuclei, whereas the part of lepton component in the total flux is ~10 -3 for high energies. The separate contribution in proton rejection is studied for each detector system of the GAMMA-400 gamma-ray telescope. Using combined information from all detector systems allow us to provide the rejection from protons with a factor of ~4 10 5 for vertical incident particles and ~3 10 5 for particle with initial inclination of 30 deg. Science with the New Generation of High Energy Gamma-ray experiments, 10th Workshop (Scineghe2014) 04-06 June 2014 Lisbon - Portugal. 

  • 891. Leonov, A. A.
    et al.
    Larsson, Josefin
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
    Pearce, Mark
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
    Ryde, Felix
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
    Zverev, V. G.
    et al.,
    Separation of electrons and protons in the GAMMA-400 gamma-ray telescope2015Ingår i: Advances in Space Research, ISSN 0273-1177, E-ISSN 1879-1948, Vol. 56, nr 7, s. 1538-1545Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The GAMMA-400 telescope will measure the fluxes of gamma rays and cosmic-ray electrons and positrons in the energy range from 100 MeV to several TeV. These measurements will allow it to achieve the following scientific objectives: search for signatures of dark matter, investigation of gamma-ray point-like and extended sources, study of the energy spectrum of the Galactic and extragalactic diffuse emission, study of gamma-ray bursts and gamma-ray emission from the active Sun, together with high-precision measurements of the high-energy electrons and positrons spectra, protons and nuclei up to the knee. The bulk of cosmic rays are protons and helium nuclei, whereas the lepton component in the total flux is similar to 10(-3) at high energy. In the present paper, the simulated capability of the GAMMA-400 telescope to distinguish electrons and positrons from protons in cosmic rays is addressed. The individual contribution to the proton rejection from each detector system of GAMMA-400 is studied separately. The use of the combined information from all detectors allows us to reach a proton rejection of the order of similar to 4 x 10(5) for vertical incident particles and similar to 3 x 10(5) for particles with initial inclination of 30 degrees in the electron energy range from 50 GeV to 1 TeV. (C) 2015 COSPAR. Published by Elsevier Ltd. All rights reserved.

  • 892.
    Lesage, S.
    et al.
    Univ Alabama Huntsville, Dept Space Sci, 320 Sparkman Dr, Huntsville, AL 35899 USA.;Univ Alabama Huntsville, Ctr Space Plasma & Aeron Res, Huntsville, AL 35899 USA..
    Ryde, Felix
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
    Zaharijas, G.
    Univ Nova Gorica, Ctr Astrophys & Cosmol, Nova Gorica, Slovenia..
    Fermi-GBM Discovery of GRB 221009A: An Extraordinarily Bright GRB from Onset to Afterglow2023Ingår i: Astrophysical Journal Letters, ISSN 2041-8205, E-ISSN 2041-8213, Vol. 952, nr 2, artikel-id L42Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We report the discovery of GRB 221009A, the highest flux gamma-ray burst (GRB) ever observed by the Fermi Gamma-ray Burst Monitor (Fermi-GBM). This GRB has continuous prompt emission lasting more than 600 s, which smoothly transitions to afterglow emission visible in the Fermi-GBM energy range (8 keV-40 MeV), and total energetics higher than any other burst in the Fermi-GBM sample. By using a variety of new and existing analysis techniques we probe the spectral and temporal evolution of GRB 221009A. We find no emission prior to the Fermi-GBM trigger time (t (0); 2022 October 9 at 13:16:59.99 UTC), indicating that this is the time of prompt emission onset. The triggering pulse exhibits distinct spectral and temporal properties suggestive of the thermal, photospheric emission of shock breakout, with significant emission up to & SIM;15 MeV. We characterize the onset of external shock at t (0) + 600 s and find evidence of a plateau region in the early-afterglow phase, which transitions to a slope consistent with Swift-XRT afterglow measurements. We place the total energetics of GRB 221009A in context with the rest of the Fermi-GBM sample and find that this GRB has the highest total isotropic-equivalent energy (E ( & gamma;,iso) = 1.0 x 10(55) erg) and second highest isotropic-equivalent luminosity (L ( & gamma;,iso) = 9.9 x 10(53) erg s(-1)) based on its redshift of z = 0.151. These extreme energetics are what allowed us to observe the continuously emitting central engine of Fermi-GBM from the beginning of the prompt emission phase through the onset of early afterglow.

  • 893.
    Lewis, Harry C.
    et al.
    Department of Physics, Imperial College London, London SW7 2AZ, United Kingdom.
    Stawarz, Julia E.
    Department of Physics, Imperial College London, London SW7 2AZ, United Kingdom; Department of Mathematics, Physics and Electrical Engineering, Northumbria University, Newcastle upon Tyne NE1 8ST, United Kingdom.
    Franci, Luca
    Department of Physics, Imperial College London, London SW7 2AZ, United Kingdom.
    Matteini, Lorenzo
    Department of Physics, Imperial College London, London SW7 2AZ, United Kingdom.
    Klein, Kristopher
    Lunar and Planetary Laboratory and Department of Planetary Sciences, University of Arizona, Tucson, Arizona 85721, USA.
    Salem, Chadi S.
    Space Sciences Laboratory, University of California, Berkeley, California 94720, USA.
    Burch, James L.
    Southwest Research Institute, San Antonio, Texas 78238, USA.
    Ergun, Robert E.
    Department of Astrophysical and Planetary Sciences, University of Colorado, Boulder, Colorado 80305, USA; Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, Colorado 80303, USA.
    Giles, Barbara L.
    NASA Goddard Space Flight Center, Greenbelt, Maryland 20771, USA.
    Russell, Christopher T.
    Department of Earth, Planetary, and Space Sciences, University of California, Los Angeles, California 90095, USA.
    Lindqvist, Per-Arne
    KTH, Skolan för elektroteknik och datavetenskap (EECS), Elektroteknik, Rymd- och plasmafysik.
    Magnetospheric Multiscale measurements of turbulent electric fields in earth's magnetosheath: How do plasma conditions influence the balance of terms in generalized Ohm's law?2023Ingår i: Physics of Plasmas, ISSN 1070-664X, E-ISSN 1089-7674, Vol. 30, nr 8, artikel-id 082901Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Turbulence is ubiquitous within space plasmas, where it is associated with numerous nonlinear interactions. Magnetospheric Multiscale (MMS) provides the unique opportunity to decompose the electric field (E) dynamics into contributions from different linear and nonlinear processes via direct measurements of the terms in generalized Ohm's law. Using high-resolution multipoint measurements, we compute the magnetohydrodynamic ( E MHD ), Hall ( E Hall ), electron pressure ( E P e ), and electron inertia ( E inertia ) terms for 60 turbulent magnetosheath intervals, to uncover the varying contributions to the dynamics as a function of scale for different plasma conditions. We identify key spectral characteristics of the Ohm's law terms: the Hall scale, k Hall , where E Hall becomes dominant over E MHD ; the relative amplitude of E P e to E Hall , which is constant in the sub-ion range; and the relative scaling of the nonlinear and linear components of E MHD and of E Hall , which are independent of scale. We find expressions for the characteristics as a function of plasma conditions. The underlying relationship between turbulent fluctuation amplitudes and ambient plasma conditions is discussed. Depending on the interval, we observe that E MHD and E Hall can be dominated by either nonlinear or linear dynamics. We find that E P e is dominated by its linear contributions, with a tendency for electron temperature fluctuations to dominate at small scales. The findings are not consistent with existing linear kinetic Alfvén wave theory for isothermal fluctuations. Our work shows how contributions to turbulent dynamics change in different plasma conditions, which may provide insight into other turbulent plasma environments.

  • 894. Li, K. -A
    et al.
    Qi, Chong
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Kärnfysik.
    Lugaro, M.
    Yagüe López, A.
    Karakas, A. I.
    Den Hartogh, J.
    Gao, B. -S
    Tang, X. -D
    The Stellar β-decay Rate of 134Cs and Its Impact on the Barium Nucleosynthesis in the s-process2021Ingår i: Astrophysical Journal Letters, ISSN 2041-8205, E-ISSN 2041-8213, Vol. 919, nr 2, s. L19-, artikel-id L19Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We have calculated the stellar β-decay rate of the important s-process branching point 134Cs based on the state-of-the-art shell model calculations. At typical s-process temperatures (T ∼ 0.2-0.3 GK), our new rate is one order of magnitude lower than the widely used rate from Takahashi and Yokoi (hereafter TY87). The impact on the nucleosynthesis in AGB stars is investigated with various masses and metallicities. Our new decay rate leads to an overall decrease in the 134Ba/136Ba ratio, and well explains the measured ratio in meteorites without introducing the i-process. We also derive the elapsed time from the last AGB nucleosynthetic event that polluted the early solar system to be >28 Myr based on the 135Cs/133Cs ratio, which is consistent with the elapsed times derived from 107Pd and 182Hf. The s-process abundance sum of 135Ba and 135Cs is found to increase, resulting in a smaller r-process contribution of 135Ba in the solar system.

  • 895. Li, K.
    et al.
    Lam, Y. H.
    Qi, Chong
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Kärnfysik.
    Tang, X.
    Zhang, N.
    The beta-decay rates of Fe-59 isotopes in shell burning environments and their influences on the production of Fe-60 in massive star2016Ingår i: 13TH INTERNATIONAL SYMPOSIUM ON ORIGIN OF MATTER AND EVOLUTION OF GALAXIES (OMEG2015), EDP Sciences, 2016, artikel-id 04006Konferensbidrag (Refereegranskat)
    Abstract [en]

    The experimental B(GT) strengths of the Fe-59 excited states were employed to determine the transition strengths which greatly contribute Fe-59 stellar beta-decay at typical carbon shell burning temperature. The result has been compared with the theoretical rates FFN (Fuller-Fowler-Newman) and LMP (Langanke&Martinez-Pinedo). Impact of the newly determined rate on the synthesis of cosmic gamma-emitter 60Fe has also been studied using one-zone model calculation. Our results show Fe-59 stellar beta-decay rate plays an important role in the Fe-60 nucleosynthesis. However the uncertainty of the decay rate is rather large due to the error of B(GT) strength that requires further studies.

  • 896.
    Li, Liang
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik. ICRANet, Piazza Repubbl 10, I-65122 Pescara, Italy.;INAF Osservatorio Astron Abruzzo, Via M Maggini Snc, I-64100 Teramo, Italy.;Sapienza Univ Roma, Dipartimento Fis, ICRA, Ple Aldo Moro 5, I-00185 Rome, Italy.;KTH Royal Inst Technol, Dept Phys, SE-10691 Stockholm, Sweden.;Oskar Klein Ctr Cosmoparticle Phys, SE-10691 Stockholm, Sweden.;Stockholm Univ, Dept Phys, AlbaNova, SE-10691 Stockholm, Sweden..
    Ryde, Felix
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. KTH Royal Inst Technol, Dept Phys, SE-10691 Stockholm, Sweden.;Oskar Klein Ctr Cosmoparticle Phys, SE-10691 Stockholm, Sweden..
    Pe'er, Asaf
    Bar Ilan Univ, Dept Phys, IL-52900 Ramat Gan, Israel..
    Yu, Hoi-Fung
    Univ Hong Kong, Fac Sci, Pokfulam, Hong Kong, Peoples R China..
    Acuner, Zeynep
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
    Bayesian Time-resolved Spectroscopy of Multipulse GRBs: Variations of Emission Properties among Pulses2021Ingår i: Astrophysical Journal Supplement Series, ISSN 0067-0049, E-ISSN 1538-4365, Vol. 254, nr 2, artikel-id 35Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Gamma-ray bursts (GRBs) are highly variable and exhibit strong spectral evolution. In particular, the emission properties vary from pulse to pulse in multipulse bursts. Here we present a time-resolved Bayesian spectral analysis of a compilation of GRB pulses observed by the Fermi/Gamma-ray Burst Monitor. The pulses are selected to have at least four time bins with a high statistical significance, which ensures that the spectral fits are well determined and spectral correlations can be established. The sample consists of 39 bursts, 117 pulses, and 1228 spectra. We confirm the general trend that pulses become softer over time, with mainly the low-energy power-law index alpha becoming smaller. A few exceptions to this trend exist, with the hardest pulse occurring at late times. The first pulse in a burst is clearly different from the later pulses; three-fourths of them violate the synchrotron line of death, while around half of them significantly prefer photospheric emission. These fractions decrease for subsequent pulses. We also find that in two-thirds of the pulses, the spectral parameters (alpha and peak energy) track the light-curve variations. This is a larger fraction compared to what is found in previous samples. In conclusion, emission compatible with the GRB photosphere is typically found close to the trigger time, while the chance of detecting synchrotron emission is greatest at late times. This allows for the coexistence of emission mechanisms at late times.

  • 897.
    Li, Liang
    et al.
    ICRANet, Piazza Repubbl 10, I-65122 Pescara, Italy.;INAF Osservatorio Astron Abruzzo, Via M Maggini Snc, I-64100 Teramo, Italy.;Sapienza Univ Roma, Dept Fis, Piazzale Aldo Moro 5, I-00185 Rome, Italy.;Sapienza Univ Roma, ICRA, Piazzale Aldo Moro 5, I-00185 Rome, Italy..
    Wang, Yu
    ICRANet, Piazza Repubbl 10, I-65122 Pescara, Italy.;INAF Osservatorio Astron Abruzzo, Via M Maggini Snc, I-64100 Teramo, Italy.;Sapienza Univ Roma, Dept Fis, Piazzale Aldo Moro 5, I-00185 Rome, Italy.;Sapienza Univ Roma, ICRA, Piazzale Aldo Moro 5, I-00185 Rome, Italy..
    Ryde, Felix
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. Oskar Klein Ctr Cosmoparticle Phys, SE-10691 Stockholm, Sweden..
    Pe'er, Asaf
    Bar Ilan Univ, Dept Phys, IL-52900 Ramat Gan, Israel..
    Zhang, Bing
    Univ Nevada, Dept Phys & Astron, Las Vegas, NV 89154 USA..
    Guiriec, Sylvain
    George Washington Univ, Dept Phys, 725 21st St NW, Washington, DC 20052 USA.;NASA, Goddard Space Flight Ctr, Greenbelt, MD 20771 USA..
    Castro-Tirado, Alberto J. J.
    CSIC, Inst Astrofis Andalucia IAA, POB 03004, E-18008 Granada, AL, Spain.;Univ Malaga, Escuela Ingn, Dept Ingn Sistemas & Automat, Malaga, Spain..
    Kann, D. Alexander
    NASA, Goddard Space Flight Ctr, Greenbelt, MD 20771 USA.;CSIC, Inst Astrofis Andalucia IAA, POB 03004, E-18008 Granada, AL, Spain..
    Axelsson, Magnus
    Stockholm Univ, Dept Astron, SE-10691 Stockholm, Sweden..
    Page, Kim
    Univ Leicester, Sch Phys & Astron, Univ Rd, Leicester LE1 7RH, England..
    Veres, Peter
    Bhat, P. N.
    A Cosmological Fireball with 16% Gamma-Ray Radiative Efficiency2023Ingår i: Astrophysical Journal Letters, ISSN 2041-8205, E-ISSN 2041-8213, Vol. 944, nr 2, artikel-id L57Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    Gamma-ray bursts (GRBs) are the most powerful explosions in the universe. How efficiently the jet converts its energy to radiation is a long-standing problem, which is poorly constrained. The standard model invokes a relativistic fireball with a bright photosphere emission component. A definitive diagnosis of GRB radiation components and the measurement of GRB radiative efficiency require prompt emission and afterglow data, with high resolution and wide band coverage in time and energy. Here, we present a comprehensive temporal and spectral analysis of the TeV-emitting bright GRB 190114C. Its fluence is one of the highest for all the GRBs that have been detected so far, which allows us to perform a high-resolution study of the prompt emission spectral properties and their temporal evolutions, down to a timescale of about 0.1 s. We observe that each of the initial pulses has a thermal component contributing similar to 20% of the total energy and that the corresponding temperature and inferred Lorentz factor of the photosphere evolve following broken power-law shapes. From the observation of the nonthermal spectra and the light curve, the onset of the afterglow corresponding to the deceleration of the fireball is considered to start at similar to 6 s. By incorporating the thermal and nonthermal observations, as well as the photosphere and synchrotron radiative mechanisms, we can directly derive the fireball energy budget with little dependence on hypothetical parameters, measuring a similar to 16% radiative efficiency for this GRB. With the fireball energy budget derived, the afterglow microphysics parameters can also be constrained directly from the data.

  • 898.
    Li, Liang
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik. Oskar Klein Centre - AlbaNova, Sverige.
    Wang, Yu
    Shao, Lang
    Wu, Xue-Feng
    Huang, Yong-Feng
    Zhang, Bing
    Ryde, Felix
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. Oskar Klein Centre - AlbaNova, Sverige.
    Yu, Hoi-Fung
    KTH, Skolan för teknikvetenskap (SCI), Fysik. Oskar Klein Centre - AlbaNova, Sverige.
    A Large Catalog of Multiwavelength GRB Afterglows. I. Color Evolution and Its Physical Implication2018Ingår i: Astrophysical Journal Supplement Series, ISSN 0067-0049, E-ISSN 1538-4365, Vol. 234, nr 2, artikel-id 26Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The spectrum of gamma-ray burst (GRB) afterglows can be studied with color indices. Here, we present a large comprehensive catalog of 70 GRBs with multiwavelength optical transient data on which we perform a systematic study to find the temporal evolution of color indices. We categorize them into two samples based on how well the color indices are evaluated. The Golden sample includes 25 bursts mostly observed by GROND, and the Silver sample includes 45 bursts observed by other telescopes. For the Golden sample, we find that 96% of the color indices do not vary over time. However, the color indices do vary during short periods in most bursts. The observed variations are consistent with effects of (i) the cooling frequency crossing the studied energy bands in a wind medium (43%) and in a constant-density medium (30%), (ii) early dust extinction (12%), (iii) transition from reverse-shock to forward-shock emission (5%), or (iv) an emergent SN emission (10%). We also study the evolutionary properties of the mean color indices for different emission episodes. We find that 86% of the color indices in the 70 bursts show constancy between consecutive ones. The color index variations occur mainly during the late GRB-SN bump, the flare, and early reverse-shock emission components. We further perform a statistical analysis of various observational properties and model parameters (spectral index beta(CI)(o), electron spectral indices p(CI), etc.) using color indices. Overall, we conclude that similar to 90% of colors are constant in time and can be accounted for by the simplest external forward-shock model, while the varying color indices call for more detailed modeling.

  • 899.
    Li, Liang
    et al.
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik. Chinese Acad Sci, Purple Mt Observ, Nanjing 210008, Jiangsu, Peoples R China.;Stockholm Univ, AlbaNova, Dept Phys, SE-10691 Stockholm, Sweden.
    Wu, Xue-Feng
    Chinese Acad Sci, Purple Mt Observ, Nanjing 210008, Jiangsu, Peoples R China..
    Lei, Wei-Hua
    Huazhong Univ Sci & Technol, Sch Phys, Wuhan 430074, Hubei, Peoples R China..
    Dai, Zi-Gao
    Nanjing Univ, Sch Astron & Space Sci, Nanjing 210093, Jiangsu, Peoples R China..
    Lian, En-Wei
    Guanxi Univ, GXU NAOC Ctr Astrophys & Space Sci, Dept Phys, Nanning 530004, Peoples R China..
    Ryde, Felix
    KTH, Skolan för teknikvetenskap (SCI), Fysik, Partikel- och astropartikelfysik.
    Constraining the Type of Central Engine of GRBs with Swift Data2018Ingår i: Astrophysical Journal Supplement Series, ISSN 0067-0049, E-ISSN 1538-4365, Vol. 236, nr 2, artikel-id 26Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    The central engine of gamma-ray bursts (GRBs) is poorly constrained. There exist two main candidates: a fast-rotating black hole and a rapidly spinning magnetar. Furthermore, X-ray plateaus are widely accepted to be the energy injection into the external shock. In this paper, we systematically analyze the Swift/XRT light curves of 101 GRBs having plateau phases and known redshifts (before 2017 May). Since a maximum energy budget (similar to 2 x 10(52) erg) exists for magnetars but not for black holes, this provides a good clue to identifying the type of GRB central engine. We calculate the isotropic kinetic energy E-K,(iso) and the isotropic X-ray energy release E-X,E-iso for individual GRBs. We identify three categories based on how likely a black hole harbors a central engine: "Gold" (9 out of 101; both E-X,E-iso and E-K,E-iso exceed the energy budget), "Silver" (69 out of 101; E-X,E-iso less than the limit but E-K,E-iso greater than the limit), and "Bronze" (23 out of 101; the energies are not above the limit). We then derive and test the black hole parameters with the Blandford-Znajek mechanism, and find that the observations of the black hole candidate ("Gold" + "Silver") samples are consistent with the expectations of the black hole model. Furthermore, we also test the magnetar candidate ("Bronze") sample with the magnetar model, and find that the magnetar surface magnetic field (B-p) and initial spin period (P-0) fall into reasonable ranges. Our analysis indicates that if the magnetar wind is isotropic, a magnetar central engine is possible for 20% of the analyzed GRBs. For most GRBs, a black hole is most likely operating.

  • 900.
    Li, Wenxiong
    et al.
    Tsinghua Univ, Phys Dept, Beijing 100084, Peoples R China.;Tsinghua Univ, Tsinghua Ctr Astrophys THCA, Beijing 100084, Peoples R China.;Tel Aviv Univ, Sch Phys & Astron, IL-69978 Tel Aviv, Israel..
    Wang, Xiaofeng
    Tsinghua Univ, Phys Dept, Beijing 100084, Peoples R China.;Tsinghua Univ, Tsinghua Ctr Astrophys THCA, Beijing 100084, Peoples R China.;Beijing Acad Sci & Technol, Beijing Planetarium, Beijing 100089, Peoples R China.;Chinese Acad Sci, Purple Mt Observ, Nanjing 210023, Peoples R China..
    Bulla, Mattia
    KTH, Centra, Nordic Institute for Theoretical Physics NORDITA. Stockholm Univ, Roslagstullsbacken 23, SE-10691 Stockholm, Sweden..
    Pan, Yen-Chen
    Natl Cent Univ, Grad Inst Astron, 300 Jhongda Rd, Taoyuan 32001, Taiwan..
    Wang, Lifan
    Texas A&M Univ, Dept Phys & Astron, George P & Cynthia Woods Mitchell Inst Fundamenta, 4242 TAMU, College Stn, TX 77843 USA..
    Mo, Jun
    Tsinghua Univ, Phys Dept, Beijing 100084, Peoples R China.;Tsinghua Univ, Tsinghua Ctr Astrophys THCA, Beijing 100084, Peoples R China..
    Zhang, Jujia
    Chinese Acad Sci, Yunnan Observ YNAO, Kunming 650216, Yunnan, Peoples R China.;Chinese Acad Sci, Key Lab Struct & Evolut Celestial Objects, Kunming 650216, Yunnan, Peoples R China..
    Wu, Chengyuan
    Tsinghua Univ, Phys Dept, Beijing 100084, Peoples R China.;Tsinghua Univ, Tsinghua Ctr Astrophys THCA, Beijing 100084, Peoples R China..
    Zhang, Jicheng
    Tsinghua Univ, Phys Dept, Beijing 100084, Peoples R China.;Tsinghua Univ, Tsinghua Ctr Astrophys THCA, Beijing 100084, Peoples R China..
    Zhang, Tianmeng
    Chinese Acad Sci, Natl Astron Observ, Key Lab Opt Astron, Beijing 10101, Peoples R China.;Univ Chinese Acad Sci, Sch Astron & Space Sci, Beijing 101408, Peoples R China..
    Xiang, Danfeng
    Tsinghua Univ, Phys Dept, Beijing 100084, Peoples R China.;Tsinghua Univ, Tsinghua Ctr Astrophys THCA, Beijing 100084, Peoples R China..
    Lin, Han
    Tsinghua Univ, Phys Dept, Beijing 100084, Peoples R China.;Tsinghua Univ, Tsinghua Ctr Astrophys THCA, Beijing 100084, Peoples R China..
    Sai, Hanna
    Tsinghua Univ, Phys Dept, Beijing 100084, Peoples R China.;Tsinghua Univ, Tsinghua Ctr Astrophys THCA, Beijing 100084, Peoples R China..
    Zhang, Xinghan
    Tsinghua Univ, Phys Dept, Beijing 100084, Peoples R China.;Tsinghua Univ, Tsinghua Ctr Astrophys THCA, Beijing 100084, Peoples R China..
    Chen, Zhihao
    Tsinghua Univ, Phys Dept, Beijing 100084, Peoples R China.;Tsinghua Univ, Tsinghua Ctr Astrophys THCA, Beijing 100084, Peoples R China..
    Yan, Shengyu
    Tsinghua Univ, Phys Dept, Beijing 100084, Peoples R China.;Tsinghua Univ, Tsinghua Ctr Astrophys THCA, Beijing 100084, Peoples R China..
    Can the Helium-detonation Model Explain the Observed Diversity of Type Ia Supernovae?2021Ingår i: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 906, nr 2, artikel-id 99Artikel i tidskrift (Refereegranskat)
    Abstract [en]

    We study a sample of 16 Type Ia supernovae (SNe Ia) having both spectroscopic and photometric observations within 2-3 days after the first light. The early B - V colors of such a sample tend to show a continuous distribution. For objects with normal ejecta velocity (NV), the C ii lambda 6580 feature is always visible in the early spectra, while it is absent or very weak in the high-velocity (HV) counterpart. Moreover, the velocities of the detached high-velocity features (HVFs) of the Ca II near-IR triplet (CaIR3) above the photosphere are found to be much higher in HV objects than in NV objects, with typical values exceeding 30,000 km s(-1) at 2-3 days. We further analyze the relation between the velocity shift of late-time [Fe II] lines (v([Fe II])) and host galaxy mass. We find that all HV objects have redshifted v([Fe II]), while NV objects have both blue- and redshifted v([Fe II]). It is interesting to point out that the objects with redshifted v([Fe II]) are all located in massive galaxies, implying that HV and a portion of NV objects may have similar progenitor metallicities and explosion mechanisms. We propose that, with a geometric/projected effect, the He-detonation model may account for the similarity in birthplace environment and the differences seen in some SNe Ia, including B - V colors, C II features, CaIR3 HVFs at early times, and v([Fe II]) in the nebular phase. Nevertheless, some features predicted by He-detonation simulation, such as the rapidly decreasing light curve, deviate from the observations, and some NV objects with blueshifted nebular v([Fe II]) may involve other explosion mechanisms.

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