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  • 301.
    Brandenburg, Axel
    et al.
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Lazarian, A.
    Astrophysical Hydromagnetic Turbulence2013In: Space Science Reviews, ISSN 0038-6308, E-ISSN 1572-9672, Vol. 178, no 2-4, p. 163-200Article, review/survey (Refereed)
    Abstract [en]

    Recent progress in astrophysical hydromagnetic turbulence is being reviewed. The physical ideas behind the now widely accepted Goldreich-Sridhar model and its extension to compressible magnetohydrodynamic turbulence are introduced. Implications for cosmic ray diffusion and acceleration is being discussed. Dynamo-generated magnetic fields with and without helicity are contrasted against each other. Certain turbulent transport processes are being modified and often suppressed by anisotropy and inhomogeneities of the turbulence, while others are being produced by such properties, which can lead to new large-scale instabilities of the turbulent medium. Applications of various such processes to astrophysical systems are being considered.

  • 302.
    Brandenburg, Axel
    et al.
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Mathur, Savita
    Metcalfe, Travis S.
    Evolution of Co-existing Long and Short Period Stellar Activity Cycles2017In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 845, no 1, article id 79Article in journal (Refereed)
    Abstract [en]

    The magnetic activity of the Sun becomes stronger and weaker over roughly an 11 year cycle, modulating the radiation and charged particle environment experienced by the Earth as "space weather." Decades of observations from the Mount Wilson Observatory have revealed that other stars also show regular activity cycles in their Ca II H+K line emission, and identified two different relationships between the length of the cycle and the rotation rate of the star. Recent observations at higher cadence have allowed the discovery of shorter cycles with periods between 1-3 years. Some of these shorter cycles coexist with longer cycle periods, suggesting that two underlying dynamos can operate simultaneously. We combine these new observations with previous data, and show that the longer and shorter cycle periods agree remarkably well with those expected from an earlier analysis based on the mean activity level and the rotation period. The relative turbulent length scales associated with the two branches of cyclic behavior suggest that a near-surface dynamo may be the dominant mechanism that drives cycles in more active stars, whereas a dynamo operating in deeper layers may dominate in less active stars. However, several examples of equally prominent long and short cycles have been found at all levels of activity of stars younger than 2.3 Gyr. Deviations from the expected cycle periods show no dependence on the depth of the convection zone or on the metallicity. For some stars that exhibit longer cycles, we compute the periods of shorter cycles that might be detected with future high-cadence observations.

  • 303.
    Brandenburg, Axel
    et al.
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Sokoloff, Dmitry
    Subramanian, Kandaswamy
    Current Status of Turbulent Dynamo Theory From Large Scale to Small-Scale Dynamos2012In: Space Science Reviews, ISSN 0038-6308, E-ISSN 1572-9672, Vol. 169, no 1-4, p. 123-157Article, review/survey (Refereed)
    Abstract [en]

    Several recent advances in turbulent dynamo theory are reviewed. High resolution simulations of small-scale and large-scale dynamo action in periodic domains are compared with each other and contrasted with similar results at low magnetic Prandtl numbers. It is argued that all the different cases show similarities at intermediate length scales. On the other hand, in the presence of helicity of the turbulence, power develops on large scales, which is not present in non-helical small-scale turbulent dynamos. At small length scales, differences occur in connection with the dissipation cutoff scales associated with the respective value of the magnetic Prandtl number. These differences are found to be independent of whether or not there is large-scale dynamo action. However, large-scale dynamos in homogeneous systems are shown to suffer from resistive slow-down even at intermediate length scales. The results from simulations are connected to mean field theory and its applications. Recent work on magnetic helicity fluxes to alleviate large-scale dynamo quenching, shear dynamos, nonlocal effects and magnetic structures from strong density stratification are highlighted. Several insights which arise from analytic considerations of small-scale dynamos are discussed.

  • 304.
    Breuillard, H.
    et al.
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Yordanova, Emiliya
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Vaivads, Andris
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Alexandrova, O.
    LESIA Observ Paris Meudon, Meudon, France..
    The Effects Of Kinetic Instabilities On Small-Scale Turbulence In Earth's Magnetosheath2016In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 829, no 1, article id 54Article in journal (Refereed)
    Abstract [en]

    The Earth's magnetosheath is the region delimited by the bow shock and the magnetopause. It is characterized by highly turbulent fluctuations covering all scales from MHD down to kinetic scales. Turbulence is thought to play a fundamental role in key processes such as energy transport and dissipation in plasma. In addition to turbulence, different plasma instabilities are generated in the magnetosheath because of the large anisotropies in plasma temperature introduced by its boundaries. In this study we use high-quality magnetic field measurements from Cluster spacecraft to investigate the effects of such instabilities on the small-scale turbulence (from ion down to electron scales). We show that the steepening of the power spectrum of magnetic field fluctuations in the magnetosheath occurs at the largest characteristic ion scale. However, the spectrum can be modified by the presence of waves/structures at ion scales, shifting the onset of the small-scale turbulent cascade toward the smallest ion scale. This cascade is therefore highly dependent on the presence of kinetic instabilities, waves, and local plasma parameters. Here we show that in the absence of strong waves the small-scale turbulence is quasi-isotropic and has a spectral index alpha approximate to 2.8. When transverse or compressive waves are present, we observe an anisotropy in the magnetic field components and a decrease in the absolute value of alpha. Slab/2D turbulence also develops in the presence of transverse/compressive waves, resulting in gyrotropy/non-gyrotropy of small-scale fluctuations. The presence of both types of waves reduces the anisotropy in the amplitude of fluctuations in the small-scale range.

  • 305.
    Bruno, A.
    et al.
    NASA, Goddard Space Flight Ctr, Heliophys Div, Greenbelt, MD 20771 USA.;INFN, Sez Bari, I-70126 Bari, Italy..
    Carlson, Per
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. AlbaNova Univ Ctr, Oskar Klein Ctr Cosmoparticle Phys, SE-10691 Stockholm, Sweden..
    Pearce, Mark
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. AlbaNova Univ Ctr, Oskar Klein Ctr Cosmoparticle Phys, SE-10691 Stockholm, Sweden..
    Zampa, N.
    INFN, Sez Trieste, I-34149 Trieste, Italy..
    et al.,
    Solar Energetic Particle Events Observed by the PAMELA Mission2018In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 862, no 2, article id 97Article in journal (Refereed)
    Abstract [en]

    Despite the significant progress achieved in recent years, the physical mechanisms underlying the origin of solar energetic particles (SEPs) are still a matter of debate. The complex nature of both particle acceleration and transport poses challenges to developing a universal picture of SEP events that encompasses both the low-energy (from tens of keV to a few hundreds of MeV) observations made by space-based instruments and the GeV particles detected by the worldwide network of neutron monitors in ground-level enhancements (GLEs). The high-precision data collected by the Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA) satellite experiment offer a unique opportunity to study the SEP fluxes between similar to 80 MeV and a few GeV, significantly improving the characterization of the most energetic events. In particular, PAMELA can measure for the first time with good accuracy the spectral features at moderate and high energies, providing important constraints for current SEP models. In addition, the PAMELA observations allow the relationship between low and high-energy particles to be investigated, enabling a clearer view of the SEP origin. No qualitative distinction between the spectral shapes of GLE, sub-GLE and non-GLE events is observed, suggesting that GLEs are not a separate class, but are the subset of a continuous distribution of SEP events that are more intense at high energies. While the spectral forms found are to be consistent with diffusive shock acceleration theory, which predicts spectral rollovers at high energies that are attributed to particles escaping the shock region during acceleration, further work is required to explore the relative influences of acceleration and transport processes on SEP spectra.

  • 306. Bruno, A.
    et al.
    Carlsson, Per
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. Oskar Klein Ctr Cosmoparticle Phys, Stockholm, Sweden.
    Pearce, Mark
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. Oskar Klein Ctr Cosmoparticle Phys, Stockholm, Sweden.
    Zverev, G.
    et al.,
    The May 17, 2012 solar event: back-tracing analysis and flux reconstruction with PAMELA2016In: INTERNATIONAL CONFERENCE ON PARTICLE PHYSICS AND ASTROPHYSICS (ICPPA-2015), PTS 1-4, Institute of Physics (IOP), 2016Conference paper (Refereed)
    Abstract [en]

    The PAMELA space experiment is providing first direct observations of Solar Energetic Particles (SEPs) with energies from about 80 MeV to several GeV in near-Earth orbit, bridging the low energy measurements by other spacecrafts and the Ground Level Enhancement (GLE) data by the worldwide network of neutron monitors. Its unique observational capabilities include the possibility of measuring the flux angular distribution and thus investigating possible anisotropies associated to SEP events. The analysis is supported by an accurate back-tracing simulation based on a realistic description of the Earth's magnetosphere, which is exploited to estimate the SEP energy spectra as a function of the asymptotic direction of arrival with respect to the Interplanetary Magnetic Field (IMF). In this work we report the results for the May 17, 2012 event.

  • 307.
    Burgess, J. Michael
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    On spectral evolution and temporal binning in gamma-ray bursts2014In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 445, no 3, p. 2589-2598Article in journal (Refereed)
    Abstract [en]

    The understanding of the prompt gamma-ray spectra of gamma-ray bursts (GRBs) is of great importance to correctly interpret the physical mechanisms that produce the underlying event as well as the structure of the relativistic jet from which the emission emanates. Time-resolved analysis of these spectra is the main method of extracting information from the data. In this work, several techniques for temporal binning of GRB spectra are examined to understand the systematics associated with each with the goal of finding the best method(s) to bin light curves for analysis. The following binning methods are examined: constant cadence (CC), Bayesian blocks (BBs), signal-to-noise (S/N) and Knuth bins (KB). I find that both the KB and BB methods reconstruct the intrinsic spectral evolution accurately while the S/N method fails in most cases. The CC method is accurate when the cadence is not too coarse but does not necessarily bin the data based on the true source variability. Additionally, the integrated pulse properties are investigated and compared to the time-resolved properties. If intrinsic spectral evolution is present, then the integrated properties are not useful in identifying physical and cosmological properties of GRBs without knowing the physical emission mechanism and its evolution.

  • 308.
    Burgess, J. Michael
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. Oskar Klein Ctr Cosmoparticle Phys, SE-10691 Stockholm, Sweden.
    The rest-frame Golenetskii correlation via a hierarchical Bayesian analysis2019In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 485, no 1, p. 1262-1277Article in journal (Refereed)
    Abstract [en]

    Gamma-ray bursts (GRBs) are characterized by a strong correlation between the instantaneous luminosity and the spectral peak energy within a burst. This correlation, which is known as the hardness-intensity correlation or the Golenetskii correlation, not only holds important clues to the physics of GRBs but is thought to have the potential to determine redshifts of bursts. In this paper, I use a hierarchical Bayesian model to study the universality of the rest-frame Golenetskii correlation and in particular I assess its use as a redshift estimator for GRBs. I find that, using a power-law prescription of the correlation, the power-law indices cluster near a common value, but have a broader variance than previously reported (similar to 1 - 2). Furthermore, I find evidence that there is a spread in intrinsic rest-frame correlation normalizations for the GRBs in our sample (similar to 10(51)-10(53) erg s(-1)). This points towards variable physical settings of the emission (magnetic field strength, number of emitting electrons, photospheric radius, viewing angle, etc.). Subsequently, these results eliminate the Golenetskii correlation as a useful tool for redshift determination and hence a cosmological probe in its current form. Though, modifications such as the introduction of correction terms similar to supernovae may alleviate these issues. Nevertheless, the Bayesian method introduced in this paper allows for a better determination of the rest-frame properties of the correlation, which in turn allows for more stringent limitations for physical models of the emission to be set.

  • 309.
    Burgess, J. Michael
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. Oskar Klein Centre for Cosmoparticle Physics, Sweden.
    Begue, Damien
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. Oskar Klein Centre for Cosmoparticle Physics, Sweden.
    Ryde, Felix
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. Oskar Klein Centre for Cosmoparticle Physics, Sweden.
    Omodei, Nicola
    Pe'er, Asaf
    Racusin, J. L.
    Cucchiara, A.
    AN EXTERNAL SHOCK ORIGIN OF GRB 141028A2016In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 822, no 2, article id 63Article in journal (Refereed)
    Abstract [en]

    The prompt emission of the long, smooth, and single-pulsed gamma-ray burst, GRB. 141028A, is analyzed under the guise of an external shock model. First, we fit the gamma-ray spectrum with a two-component photon model, namely, synchrotron+blackbody, and then fit the recovered evolution of the synchrotron nu F-nu peak to an analytic model derived considering the emission of a relativistic blast. wave expanding into an external medium. The prediction of the model for the nu F-nu peak evolution matches well with the observations. We observe the blast. wave transitioning into the deceleration phase. Furthermore, we assume the expansion of the blast. wave to be nearly adiabatic, motivated by the low magnetic field deduced from the observations. This allows us to recover within an order of magnitude the flux density at the nu F-nu peak, which is remarkable considering the simplicity of the analytic model. Under this scenario we argue that the distinction between prompt and afterglow emission is superfluous as both early-time emission and late-time emission emanate from the same source. While the external shock model is clearly not a universal solution, this analysis opens the possibility that at least some fraction of GRBs can be explained with an external shock origin of their prompt phase.

  • 310.
    Burgess, J Michael
    et al.
    Department of Physics, University of Alabama in Huntsville, United States.
    Preece, Robert D
    Baring, Matthew G
    Briggs, Michael S
    Connaughton, Valerie
    Guiriec, Sylvain
    Paciesas, William S
    Meegan, Charles A
    Bhat, P N
    Bissaldi, Elisabetta
    Chaplin, Vandiver
    Diehl, Roland
    Fishman, Gerald J
    Fitzpatrick, Gerard
    Foley, Suzanne
    Gibby, Melissa
    Giles, Misty
    Goldstein, Adam
    Greiner, Jochen
    Gruber, David
    van der Horst, Alexander J
    von Kienlin, Andreas
    Kippen, Marc
    Kouveliotou, Chryssa
    McBreen, Sheila
    Rau, Arne
    Tierney, Dave
    Wilson-Hodge, Colleen
    Constraints on the Synchrotron Shock Model for the Fermi GRB 090820A Observed by Gamma-Ray Burst Monitor2011In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 741, no 1Article in journal (Refereed)
    Abstract [en]

    Discerning the radiative dissipation mechanism for prompt emission in gamma-ray bursts (GRBs) requires detailed spectroscopic modeling that straddles the vF(v) peak in the 100 keV-1 MeV range. Historically, empirical fits such as the popular Band function have been employed with considerable success in interpreting the observations. While extrapolations of the Band parameters can provide some physical insight into the emission mechanisms responsible for GRBs, these inferences do not provide a unique way of discerning between models. By fitting physical models directly, this degeneracy can be broken, eliminating the need for empirical functions; our analysis here offers a first step in this direction. One of the oldest, and leading, theoretical ideas for the production of the prompt signal is the synchrotron shock model. Here we explore the applicability of this model to a bright Fermi gamma-ray burst monitor (GBM) burst with a simple temporal structure, GRB 090820A. Our investigation implements, for the first time, thermal and non-thermal synchrotron emissivities in the RMFIT forward-folding spectral analysis software often used in GBM burst studies. We find that these synchrotron emissivities, together with a blackbody shape, provide at least as good a match to the data as the Band GRB spectral fitting function. This success is achieved in both time-integrated and time-resolved spectral fits.

  • 311.
    Burgess, J. Michael
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. Oskar Klein Ctr Cosmoparticle Phys, Sweden.
    Ryde, Felix
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. Oskar Klein Ctr Cosmoparticle Phys, Sweden.
    Are GRB blackbodies an artefact of spectral evolution?2015In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 447, no 4, p. 3087-3094Article in journal (Refereed)
    Abstract [en]

    The analysis of gamma-ray burst (GRB) spectra with multicomponent emission models has become an important part of the field. In particular, multicomponent analysis where one component is a blackbody representing emission from a photosphere has enabled both a more detailed understanding of the energy content of the jet as well as the ability to examine the dynamic structure of the outflow. While the existence of a blackbody-like component has been shown to be significant and not a byproduct of background fluctuations, it is very possible that it can be an artefact of spectral evolution of a single component that is being poorly resolved in time. Herein, this possibility is tested by simulating a single component evolving in time and then folding the spectra through the Fermi detector response to generate time-tagged event Gamma-ray Burst Monitor (GBM) data. We then fit both the time-integrated and -resolved generated spectral data with a multicomponent model using standard tools. It is found that in time-integrated spectra, a blackbody can be falsely identified due to the spectral curvature introduced by the spectral evolution. However, in a time-resolved analysis defined by time bins that can resolve the evolution of the spectra, the significance of the falsely identified blackbody is very low. Additionally, the evolution of the artificial blackbody parameters does not match the recurring behaviour that has been identified in the actual observations. These results reinforce the existence of the blackbody found in time-resolved analysis of GRBs and stress the point that caution should be taken when using time-integrated spectral analysis for identifying physical properties of GRBs.

  • 312.
    Burgess, J. Michael
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. The Oskar Klein Centre for Cosmoparticle Physics, AlbaNova, Sweden.
    Ryde, Felix
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. The Oskar Klein Centre for Cosmoparticle Physics, AlbaNova, Sweden.
    Yu, Hoi-Fung
    Taking the band function too far: a tale of two alpha's2015In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 451, no 2, p. 1511-1521Article in journal (Refereed)
    Abstract [en]

    The long standing problem of identifying the emission mechanism operating in gamma-ray bursts (GRBs) has produced a myriad of possible models that have the potential of explaining the observations. Generally, the empirical Band function is fit to the observed gamma-ray data and the fit parameters that are used to infer which radiative mechanisms are at work in GRB outflows. In particular, the distribution of the Band function's low-energy power-law index, alpha, has led to the so-called synchrotron 'line-of-death' (LOD) which is a statement that the distribution cannot be explained by the simplest of synchrotron models alone. As an alternatively fitting model, a combination of a blackbody in addition to the Band function is used, which in many cases provide a better or equally good fit. It has been suggested that such fits would be able to alleviate the LOD problem for synchrotron emission in GRBs. However, these conclusions rely on the Band function's ability to fit a synchrotron spectrum within the observed energy band. In order to investigate if this is the case, we simulate synchrotron and synchrotron+blackbody spectra and fold them through the instrumental response of the Fermi Gamma-ray Burst Monitor (GBM). We then perform a standard data analysis by fitting the simulated data with both Band and Band+blackbody models. We find two important results: the synchrotron LOD is actually more severe than the original predictions: alpha(LOD) similar to -0.8. Moreover, we find that intrinsic synchrotron+blackbody emission is insufficient to account for the entire observed alpha distribution. This implies that some other emission mechanism(s) are required to explain a large fraction of observed GRBs.

  • 313. Bykov, A. M.
    et al.
    Brandenburg, Axel
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Department of Astronomy, Stockholm University, Sweden.
    Malkov, M. A.
    Osipov, S. M.
    Microphysics of Cosmic Ray Driven Plasma Instabilities2013In: Space Science Reviews, ISSN 0038-6308, E-ISSN 1572-9672, Vol. 178, no 2-4, p. 201-232Article, review/survey (Refereed)
    Abstract [en]

    Energetic nonthermal particles (cosmic rays, CRs) are accelerated in supernova remnants, relativistic jets and other astrophysical objects. The CR energy density is typically comparable with that of the thermal components and magnetic fields. In this review we discuss mechanisms of magnetic field amplification due to instabilities induced by CRs. We derive CR kinetic and magnetohydrodynamic equations that govern cosmic plasma systems comprising the thermal background plasma, comic rays and fluctuating magnetic fields to study CR-driven instabilities. Both resonant and non-resonant instabilities are reviewed, including the Bell short-wavelength instability, and the firehose instability. Special attention is paid to the longwavelength instabilities driven by the CR current and pressure gradient. The helicity production by the CR current-driven instabilities is discussed in connection with the dynamo mechanisms of cosmic magnetic field amplification.

  • 314.
    Bégué, Hüsne Dereli
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Burgess, J Michael
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    The anatomy of a long gamma-ray burst: a simple classification scheme for the emission mechanism(s)2016In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 820, no 1, article id 68Article in journal (Refereed)
    Abstract [en]

    Ultra-relativistic motion and efficient conversion of kinetic energy to radiation are required by gamma-ray burst (GRB) observations, yet they are difficult to simultaneously achieve. Three leading mechanisms have been proposed to explain the observed emission emanating from GRB outflows: radiation from either relativistic internal or external shocks, or thermal emission from a photosphere. Previous works were dedicated to independently treating these three mechanisms and arguing for a sole, unique origin of the prompt emission of GRBs. In contrast, herein, we first explain why all three models are valid mechanisms and that a contribution from each of them is expected in the prompt phase. Additionally, we show that a single parameter, the dimensionless entropy of the GRB outflow, determines which mechanism contributes the most to the emission. More specifically, internal shocks dominate for low values of the dimensionless entropy, external shocks for intermediate values, and finally, photospheric emission for large values. We present a unified framework for the emission mechanisms of GRBs with easily testable predictions for each process.

  • 315.
    Campana, D.
    et al.
    Ist Nazl Fis Nucl, Sez Naples, I-80126 Naples, Italy..
    Carlson, Per
    KTH, School of Engineering Sciences (SCI), Physics.
    Munini, Riccardo
    KTH, School of Engineering Sciences (SCI), Physics.
    Pearce, Mark
    KTH, School of Engineering Sciences (SCI), Physics.
    Rossetto, Laura
    KTH, School of Engineering Sciences (SCI), Physics.
    Wu, Juan
    KTH, School of Engineering Sciences (SCI), Physics.
    Zverev, V. G.
    Moscow Engn Phys Inst, Moscow RU-11540, Russia..
    Search for cosmic ray electron-positron anisotropies with the Pamela data2013In: 23RD EUROPEAN COSMIC RAY SYMPOSIUM (AND 32ND RUSSIAN COSMIC RAY CONFERENCE), IOP PUBLISHING LTD , 2013, article id 012055Conference paper (Refereed)
    Abstract [en]

    Using data taken by the Pamela experiment during 5 years of operation we studied the anisotropy in the arrival direction of cosmic ray electrons and positrons with energy above 40 GeV. We report on a study of anisotropy in the e(+/-) flux at different angular scales extending from 30 degrees up to 90 degrees, further more a directional analysis has been performed around the Sun direction. The observed distribution of arrival directions is consistent with the isotropic expectation at any angular scale used in this study and no significant evidence of strong anisotropies has been observed, also the analysis around the Sun direction did not show any significant excess.

  • 316.
    Candelaresi, Simon
    et al.
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Brandenburg, Axel
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Topological constraints on magnetic field relaxation2012In: Proc. Int. Astron. Union, 2012, no S294, p. 353-357Conference paper (Refereed)
    Abstract [en]

    Magnetic field relaxation is determined by both the field's geometry and its topology. For relaxation processes, however, it turns out that its topology is a much more stringent constraint. As quantifier for the topology we use magnetic helicity and test whether it is a stronger condition than the linking of field lines. Further, we search for evidence of other topological invariants, which give rise to further restrictions in the field's relaxation. We find that magnetic helicity is the sole determinant in most cases. Nevertheless, we see evidence for restrictions not captured through magnetic helicity.

  • 317.
    Candelaresi, Simon
    et al.
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Stockholm University, Sweden .
    Hillier, A.
    Maehara, H.
    Brandenburg, Axel
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Stockholm University, Sweden .
    Shibata, K.
    Superflare occurrence and energies on G-, K-, and M-type dwarfs2014In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 792, no 1, p. 67-Article in journal (Refereed)
    Abstract [en]

    Kepler data from G-, K-, and M-type stars are used to study conditions that lead to superflares with energies above 10(34) erg. From the 117,661 stars included, 380 show superflares with a total of 1690 such events. We study whether parameters, like effective temperature or rotation rate, have any effect on the superflare occurrence rate or energy. With increasing effective temperature we observe a decrease in the superflare rate, which is analogous to the previous findings of a decrease in dynamo activity with increasing effective temperature. For slowly rotating stars, we find a quadratic increase of the mean occurrence rate with the rotation rate up to a critical point, after which the rate decreases linearly. Motivated by standard dynamo theory, we study the behavior of the relative starspot coverage, approximated as the relative brightness variation. For faster rotating stars, an increased fraction of stars shows higher spot coverage, which leads to higher superflare rates. A turbulent dynamo is used to study the dependence of the Ohmic dissipation as a proxy of the flare energy on the differential rotation or shear rate. The resulting statistics of the dissipation energy as a function of dynamo number is similar to the observed flare statistics as a function of the inverse Rossby number and shows similarly strong fluctuations. This supports the idea that superflares might well be possible for solar-type G stars.

  • 318.
    Capel, Francesca
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Belov, A.
    Bertaina, M.
    Fausti, F.
    Miyamoto, H.
    Collaboration, J E M-E U S O
    The mini-EUSO multi-level trigger algorithm and its performance2017In: Proceedings of Science, Sissa Medialab Srl , 2017Conference paper (Refereed)
    Abstract [en]

    The Mini-EUSO telescope is designed by the JEM-EUSO Collaboration to observe the UV emission of the Earth from the vantage point of the International Space Station in low Earth orbit. The main goal of the mission is to map the Earth in the UV, thus increasing the technological readiness level of future EUSO experiments and to lay the groundwork for the detection of Extreme Energy Cosmic Rays (EECRs) from space. Due to its high time resolution of 2.5 μs, Mini-EUSO is capable of detecting a wide range of UV phenomena in the Earth's atmosphere. In order to maximise the scientific return of the mission, it is necessary to implement a multi-level trigger logic for data selection on various different timescales. This logic is key to the success of the mission and thus must be thoroughly tested and integrated into the data processing system prior to launch. This article introduces the motivation behind the trigger design and details the testing of the logic through simulations and data taken at the TurLab facility. 

  • 319.
    Capel, Francesca
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. The Oskar Klein Centre for Cosmoparticle Physics, Stockholm.
    Belov, A.
    Casolino, M.
    Klimov, P.
    Mini-EUSO: A high resolution detector for the study of terrestrial and cosmic UV emission from the International Space Station2017In: Advances in Space Research, ISSN 0273-1177, E-ISSN 1879-1948Article in journal (Refereed)
    Abstract [en]

    The Mini-EUSO instrument is a UV telescope to be placed inside the International Space Station (ISS), looking down on the Earth from a nadir-facing window in the Russian Zvezda module. Mini-EUSO will map the earth in the UV range (300-400. nm) with a spatial resolution of 6.11. km and a temporal resolution of 2.5. μs, offering the opportunity to study a variety of atmospheric events such as transient luminous events (TLEs) and meteors, as well as searching for strange quark matter and bioluminescence. Furthermore, Mini-EUSO will be used to detect space debris to verify the possibility of using a EUSO-class telescope in combination with a high energy laser for space debris remediation. The high-resolution mapping of the UV emissions from Earth orbit allows Mini-EUSO to serve as a pathfinder for the study of Extreme Energy Cosmic Rays (EECRs) from space by the JEM-EUSO collaboration. 

  • 320.
    Capel, Francesca
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Fuglesang, Christer
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Casolino, M.
    Piotrowski, L.
    Collaboration, J E M-E U S O
    Mini-EUSO flight software and operations on ISS2017In: Proceedings of Science, Sissa Medialab Srl , 2017Conference paper (Refereed)
    Abstract [en]

    The Mini-EUSO instrument is designed by the JEM-EUSO collaboration to pave the way for space-based observations of Extreme Energy Cosmic Rays (EECRs). To be placed inside the International Space Station (ISS) in early 2018, it is a small UV (300-400 nm) telescope which will observe the Earth's atmosphere with a spatial resolution of 6.11 km. Mini-EUSO is capable of detecting a wide variety of UV events such as cosmic ray signals, transient luminous events and meteors with a minimum time resolution of 2.5 μs. It will also be possible to detect space debris during twilight periods. The flight software is fully automated and takes advantage of the frequent day/night cycles of the ISS orbit and ancillary instruments with which Mini-EUSO is equipped in order to optimise the mission's scientific output. The flight operations of Mini-EUSO are presented including the data acquisition, storage and transfer, astronaut interaction and predicted instrument duty cycle. 

  • 321.
    Caputa, Pawel
    et al.
    KTH. Stockholm Univ, Sweden.
    Veliz-Osorio, Alvaro
    Entanglement constant for conformal families2015In: Physical Review D, ISSN 1550-7998, E-ISSN 1550-2368, Vol. 92, no 6, article id 065010Article in journal (Refereed)
    Abstract [en]

    We show that in 1 + 1 dimensional conformal field theories, exciting a state with a local operator increases the Renyi entanglement entropies by a constant which is the same for every member of the conformal family. Hence, it is an intrinsic parameter that characterizes local operators from the perspective of quantum entanglement. In rational conformal field theories this constant corresponds to the logarithm of the quantum dimension of the primary operator. We provide several detailed examples for the second Renyi entropies and a general derivation.

  • 322. Carloni, Sante
    et al.
    Koivisto, Tomi Sebastian
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Lobo, Francisco S. N.
    Dynamical system analysis of hybrid metric-Palatini cosmologies2015In: Physical Review D, ISSN 1550-7998, E-ISSN 1550-2368, Vol. 92, no 6, article id 064035Article in journal (Refereed)
    Abstract [en]

    The so-called f(X) hybrid metric-Palatini gravity, where X = R + T, with T the stress-energy trace and R the Ricci scalar, presents a unique viable generalization of the f(R) theories within the metric-affine formalism. In this paper, the cosmology of the f(X) theories is studied using the dynamical system approach. The method consists of formulating the propagation equation in terms of suitable (expansion-normalized) variables as an autonomous system. The fixed points of the system then represent exact cosmological solutions described by power law or de Sitter expansion. The formalism is applied to two classes of f(X) models, revealing both standard cosmological fixed points and new accelerating solutions that can be attractors in the phase space. In addition, the fixed point with vanishing expansion rate is considered with special care in order to characterize the stability of Einstein static spaces and bouncing solutions.

  • 323.
    Carlqvist, Per
    KTH, School of Electrical Engineering (EES), Centres, Alfvén Laboratory Centre for Space and Fusion Plasma Physics.
    A remarkable double helix in the V838 Mon nebula2005In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 436, no 1, p. 231-239Article in journal (Refereed)
    Abstract [en]

    In the beginning of 2002 the previously unnoted star V838 Mon had a powerful outburst. The star is surrounded by pre-existing, dusty clouds which are illuminated by the star in an expanding, parabolic layer. Spectacular images captured by the Hubble Space Telescope Advanced Camera for Surveys show that the clouds are to a great extent built up by filaments and concentric shells. One of the most remarkable features is a filamentary structure forming a double helix. The structure, which has a projected length and width of similar to 9 '' and similar to 1.'' 4, respectively, points almost radially towards V838 Mon. In order to reveal the geometry of the double helix in some more detail, a three-dimensional computer model of the structure has been constructed. The model also assists in determining the expansion rate of the light echoes along the double helix. By means of the expansion rate and the tilt of the double helix the distance to V838 Mon is determined to be 2.4 +/- 0.5 kpc. A theory of the double helix, based on a magnetized and twisted filament, is presented. Dynamic and magnetic forces play an essential role in the shaping of the double helix. The theory is supported by a mechanical analogy model. Double helices in other cosmic environments are also discussed.

  • 324.
    Carlqvist, Per
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Dark mammoth trunks in the merging galaxy NGC 1316 and a mechanism of cosmic double helices2010In: Astrophysics and Space Science, ISSN 0004-640X, E-ISSN 1572-946X, Vol. 327, no 2, p. 267-278Article in journal (Refereed)
    Abstract [en]

    NGC 1316 is a giant, elliptical galaxy containing a complex network of dark, dust features. The morphology of these features has been examined in some detail using a Hubble Space Telescope, Advanced Camera for Surveys image. It is found that most of the features are constituted of long filaments. There also exist a great number of dark structures protruding inwards from the filaments. Many of these structures are strikingly similar to elephant trunks in H ii regions in the Milky Way Galaxy, although much larger. The structures, termed mammoth trunks, generally are filamentary and often have shapes resembling the letters V or Y. In some of the mammoth trunks the stem of the Y can be resolved into two or more filaments, many of which showing signs of being intertwined. A model of the mammoth trunks, related to a recent theory of elephant trunks, is proposed. Based on magnetized filaments, the model is capable of giving an account of the various shapes of the mammoth trunks observed, including the twined structures.

  • 325.
    Carlqvist, Per
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Twisted Crab fingers revisited2015In: Astrophysics and Space Science, ISSN 0004-640X, E-ISSN 1572-946X, Vol. 357, no 1, article id 47Article in journal (Refereed)
    Abstract [en]

    Narrowband images of the Crab Nebula captured by the Hubble Space Telescope have earlier shown that the nebula does not only present a network of broad, bright filaments crossing the nebula but also numerous so-called fingers mostly pointing inwards. Using archival Hubble images we have in some detail studied the morphology of a great number of such fingers. This scrutiny has revealed that practically all the fingers are made up of filaments. Most of the larger fingers show overall shapes that are similar to either of the two letters V and Y. In many of these fingers it is also possible to see internal details. Interestingly, a number of the larger, Y-shaped fingers turn out to have a stem that consists of intertwined filaments. By contrast with this, the smaller fingers usually appear only as diffuse and sometimes incomplete pegs. In none of the smaller fingers is it possible to find any plain, internal structure. The observational results obtained are compared with the properties of a previously proposed model of the fingers. The model suggests that the fingers have evolved out of magnetized filaments. The evolution should lead to fingers with overall shapes that are similar to either a V or a Y, very much in agreement with the observations. In addition to this, the model prescribes that the stems of the Y-shaped fingers should be made up of intertwined filaments. From all these points of agreement we conclude that the properties of the fingers observed lend strong support to the model.

  • 326.
    Carlqvist, Per
    et al.
    KTH, Superseded Departments, Alfvén Laboratory.
    Gahm, G. F.
    Kristen, H.
    Formation of Twisted Elephant Trunks in the Rosette Nebula2002In: Astrophysics and Space Science, ISSN 0004-640X, E-ISSN 1572-946X, Vol. 280, no 4, p. 405-412Article in journal (Refereed)
    Abstract [en]

    New observations show that dark elephant trunks in the Rosette nebula are often built up by thin filaments. In several of the trunks the filaments seem to form a twisted pattern. This pattern is hard to reconcile with current theory. We propose a new model for the formation of twisted elephant trunks in which electromagnetic forces play an important role. The model considers the behaviour of a twisted magnetic filament in a molecular cloud, where a cluster of hot stars has been recently born. As a result of stellar winds, and radiation pressure, electromagnetic forces, and inertia forces part of the filament can develop into a double helix pointing towards the stars. The double helix represents the twisted elephant trunk. A simple analogy experiment visualizes and supports the trunk model.

  • 327.
    Carlqvist, Per
    et al.
    KTH, Superseded Departments, Alfvén Laboratory.
    Gahm, G. F.
    Kristen, H.
    Theory of Twisted Trunks2003In: Astronomy and Astrophysics, ISSN 0004-6361, Vol. 403, p. 399-412Article in journal (Refereed)
    Abstract [en]

    Using the 2.6 m Nordic Optical Telescope we have observed a large number of elephant trunks in several regions. Here, we present a small selection of this material consisting of a few large, well-developed trunks, and some smaller ones. We find that: (i) the well-developed trunks are made up of dark filaments and knots which show evidence of twisted structures, (ii) the trunks are connected with essentially two filamentary legs running in V-shape, and (iii) all trunks have the maximum extinction in their heads. We advance a theory of twisted elephant trunks which is based on the presence of magnetic flux ropes in molecular clouds where hot OB stars are formed. If the rope contains a local condensation it may adopt a V-shape as the region around the hot stars expands. If, in addition, the magnetic field in the rope is sufficiently twisted, the rope may form a double helix at the apex of the V. The double helix is identified with the twisted elephant trunks. In order to illustrate the mechanisms behind the double helix we have constructed a mechanical analogy model of the magnetic flux rope in which the rope has been replaced by a bundle of elastic strings loaded by a weight. Experiments with the model clearly show that part of the bundle will transform into a double helix when the twist of the bundle is sufficiently large. We have also worked out a simple theoretical model of a mass-loaded magnetic flux rope. Numerical calculations show that a double helix will indeed form when the twist of the rope exceeds a certain critical limit. Numerical model calculations are applied to both the analogy model experiments and one of the well-developed elephant trunks. On the basis of our model we also suggest a new interpretation of the so called EGGs.

    The double helix mechanism is quite general, and should be active also in other suitable environments. One such environment may be the shell of supernova remnants. Another example is the expanding bubble outlined by the North Celestial Pole Loop.

  • 328.
    Carlson, Per
    KTH, School of Engineering Sciences (SCI), Physics.
    Astrophysics and cosmology with balloons2005In: 17th ESA Symposium on European Rocket and Balloon Programmes and Related Research, 2005, Vol. 590, p. 47-52Conference paper (Refereed)
    Abstract [en]

    In a pioneering balloon experiment 1912 Victor Hess discovered cosmic rays. Over the soon 100 years since that time enormous progress in fundamental physics, astrophysics and cosmology has been made with balloon experiments. New elementary particles were discovered in the period from 1930 to 1950. Later balloon experiments searched for primordial antimatter. Supernova properties have been studied in balloon borne experiments measuring radioactive nuclei. Cosmology is today very much an experimental science with many experiments giving new data. One of the most important discoveries was the observation of the first acoustic peak in the cosmic microwave background in balloon experiments in 2000. The improved balloon technique of today makes experiments with very long duration possible and opens new science windows.

  • 329.
    Carlson, Per
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Discovery of Cosmic Rays2013In: Centenary Symposium 2012: Discovery Of Cosmic Rays / [ed] Ormes, JF, American Institute of Physics (AIP), 2013, p. 9-16Conference paper (Refereed)
    Abstract [en]

    The mysterious invisible radiation that ionized air was studied a century ago by many scientists. Finally, on 7 August 1912, Victor Hess in his seventh balloon flight that year, reached an altitude of about 5000 m. With his electroscopes on board the hydrogen-filled balloon he observed that the ionization instead of decreasing with altitude increased significantly. Hess had discovered cosmic rays, a discovery that gave him the 1936 Nobel Prize in physics. When research resumed after World War I focus was on understanding the nature of the cosmic radiation. Particles or radiation? Positive or negative? Electrons, positrons or protons? Progress came using new instruments like the Geiger-Muller tube and around 1940 it was clear that cosmic rays were mostly protons.

  • 330. Casolino, M.
    et al.
    Adriani, O.
    Barbarino, G. C.
    Bazilevskaya, G. A.
    Bellotti, R.
    Boezio, M.
    Bogomolov, E. A.
    Bongi, M.
    Bonvicini, V.
    Bottai, S.
    Bruno, A.
    Cafagna, F.
    Campana, D.
    Carbone, R.
    Carlson, Per
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Castellini, G.
    Danilchenko, I. A.
    Donato, C. D. E.
    Santis, C. D. E.
    Simone, N. D. E.
    Felice, V. D. I.
    Formato, V.
    Galper, A. M.
    Karelin, A. V.
    Koldashov, S. V.
    Koldobskiy, S.
    Krutkov, S. Y.
    Kvashnin, A. N.
    Leonov, A.
    Malakhov, V.
    Marcelli, L.
    Martucci, M.
    Mayorov, A. G.
    Menn, W.
    Merge, M.
    Mikhailov, V. V.
    Mocchiutti, E.
    Monaco, A.
    Mori, N.
    Munini, R.
    Osteria, G.
    Palma, F.
    Papini, P.
    Pearce, M.
    Picozza, P.
    Pizzolotto, C.
    Ricci, M.
    Ricciarini, S. B.
    Sarkar, R.
    Simon, M.
    Scotti, V.
    Sparvoli, R.
    Spillantini, P.
    Stozhkov, Y. I.
    Vacchi, A.
    Vannuccini, E.
    Vasilyev, G.
    Voronov, S. A.
    Yurkin, Y. T.
    Zampa, G.
    Zampa, N.
    Zverev, V. G.
    New upper limit on strange quark matter flux with the PAMELA experiment2013In: Proceedings of the 33rd International Cosmic Rays Conference, ICRC 2013, Sociedade Brasileira de Fisica , 2013Conference paper (Refereed)
    Abstract [en]

    In this work we present a new upper limit for anomalous charge / mass (Z/A) particles with PAMELA experiment. These particles would exhibit a low velocity in the Time-of-Flight system and an high rigidity in the tracker. The redundant nature of the PAMELA detectors make it particularly suited to search for these particles in a mass number (10 ≤ A ≤ 105), charge (1≤ Z ≤ 8) and rigidity (0.4 ≤ R ≤ 1200 GV) range complementary to those of ground-based experiments. 

  • 331. Casolino, M.
    et al.
    Belov, A.
    Bertaina, M.
    Cambie, G.
    Capel, Francesca
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Ebisuzaki, T.
    Klimov, P.
    Panasyuk, M.
    Picozza, P.
    Ricci, M.
    collaboration, JEM-EUSO
    Science of Mini-EUSO detector on board the International Space Station2017In: Proceedings of Science, Sissa Medialab Srl , 2017Conference paper (Refereed)
    Abstract [en]

    The Mini-EUSO space experiment ("UV atmosphere" in Russian Space Program) is a telescope designed to perform observations of night-time Earth in the UV spectrum. The instrument comprises a compact telescope with a large field of view (44°x44°), based on an optical system employing two 25 cm diameter Fresnel lenses (focal length ∼ 30 cm) for increased light collection. Mini-EUSO will study different scientific phenomena ranging from strange quark matter and Ultra High Energy Cosmic Rays (UHECRs) to bioluminescence and atmospheric physics. It will also create the first night-time map of the Earth in UV light. The mission will raise the technology readiness level (TRL) of the future JEM-EUSO missions to observe UHECRs from space. The Mini-EUSO measurements will be performed from the ISS through a UV transparent window in the Russian Zvezda Service Module. Launch is foreseen between Autumn 2017 and beginning 2018 in the framework of the next manned ASI (Italian Space Agency) flight and observations are supposed to continue with Russian cosmonauts for several years. 

  • 332. Casolino, M.
    et al.
    Picozza, P.
    Altamura, F.
    Basili, A.
    De Simone, N.
    Di Felice, V.
    De Pascale, M. P.
    Marcelli, L.
    Minori, M.
    Nagni, M.
    Sparvoli, R.
    Galper, A. M.
    Mikhailov, V. V.
    Runtso, M. F.
    Voronov, S. A.
    Yurkin, Y. T.
    Zverev, V. G.
    Castellini, G.
    Adriani, O.
    Bonechi, L.
    Bongi, M.
    Taddei, E.
    Vannuccini, E.
    Fedele, D.
    Papini, P.
    Ricciarini, S. B.
    Spillantini, P.
    Ambriola, M.
    Cafagna, F.
    De Marzo, C.
    Barbarino, G. C.
    Campana, D.
    De Rosa, G.
    Osteria, G.
    Russo, S.
    Bazilevskaja, G. A.
    Kvashnin, A. N.
    Maksumov, O.
    Misin, S.
    Stozhkov, Yu. I.
    Bogomolov, E. A.
    Krutkov, S. Yu.
    Nikonov, N. N.
    Bonvicini, V.
    Boezio, M.
    Lundquist, J.
    Mocchiutti, E.
    Vacchi, A.
    Zampa, G.
    Zampa, N.
    Bongiorno, L.
    Ricci, M.
    Carlson, Per
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Hofverberg, Petter
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Lund, Jens
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Orsi, Silvio
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Pearce, Mark
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Menn, W.
    Simon, M.
    Launch of the space experiment PAMELA2008In: Advances in Space Research, ISSN 0273-1177, E-ISSN 1879-1948, Vol. 42, no 3, p. 455-466Article in journal (Refereed)
    Abstract [en]

    PAMELA is a satellite borne experiment designed to study with great accuracy cosmic rays of galactic, solar, and trapped nature in a wide energy range (protons 80 MeV-700 GeV, electrons 50 MeV-400 GeV). Main objective is the study of the antimatter component: antiprotons (80 MeV-190 GeV), positrons (50 MeV-270 GeV) and search for antimatter with a precision of the order of 10-8. The experiment, housed on board the Russian Resurs-DK I satellite, was launched on June 15th, 2006 in a 350 x 600 km orbit with all inclination of 70'. The detector is composed of a series of scintillator counters arranged at the extremities of a permanent magnet spectrometer to provide charge, time-of-flight, and rigidity information. Lepton/hadron identification is performed by a silicon-tungsten calorimeter and a neutron detector placed at the bottom of the device. An anticounter system is used offline to reject false triggers coming from the satellite. In self-trigger mode the calorimeter, the neutron detector, and a shower tail catcher are capable of an independent measure of the lepton component up to 2 TeV. In this work we describe the experiment, its scientific objectives, and the performance in the first months after launch.

  • 333. Cataldi, Gianni
    et al.
    Brandeker, Alexis
    Thebault, Philippe
    Singer, Kelsi
    Ahmed, Engy
    KTH, Centres, Science for Life Laboratory, SciLifeLab. Stockholm University, Sweden.
    de Vries, Bernard L.
    Neubeck, Anna
    Olofsson, Goeran
    Searching for Biosignatures in Exoplanetary Impact Ejecta2017In: Astrobiology, ISSN 1531-1074, E-ISSN 1557-8070, Vol. 17, no 8, p. 721-746Article in journal (Refereed)
    Abstract [en]

    With the number of confirmed rocky exoplanets increasing steadily, their characterization and the search for exoplanetary biospheres are becoming increasingly urgent issues in astrobiology. To date, most efforts have concentrated on the study of exoplanetary atmospheres. Instead, we aim to investigate the possibility of characterizing an exoplanet (in terms of habitability, geology, presence of life, etc.) by studying material ejected from the surface during an impact event. For a number of impact scenarios, we estimate the escaping mass and assess its subsequent collisional evolution in a circumstellar orbit, assuming a Sun-like host star. We calculate the fractional luminosity of the dust as a function of time after the impact event and study its detectability with current and future instrumentation. We consider the possibility to constrain the dust composition, giving information on the geology or the presence of a biosphere. As examples, we investigate whether calcite, silica, or ejected microorganisms could be detected. For a 20km diameter impactor, we find that the dust mass escaping the exoplanet is roughly comparable to the zodiacal dust, depending on the exoplanet's size. The collisional evolution is best modeled by considering two independent dust populations, a spalled population consisting of nonmelted ejecta evolving on timescales of millions of years, and dust recondensed from melt or vapor evolving on much shorter timescales. While the presence of dust can potentially be inferred with current telescopes, studying its composition requires advanced instrumentation not yet available. The direct detection of biological matter turns out to be extremely challenging. Despite considerable difficulties (small dust masses, noise such as exozodiacal dust, etc.), studying dusty material ejected from an exoplanetary surface might become an interesting complement to atmospheric studies in the future.

  • 334.
    Chan, Chi-Kwan
    et al.
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Department of Astronomy, University of Arizona, United States .
    Psaltis, Dimitrios
    Özel, Feryal
    GRay: A massively parallel gpu-based code for ray tracing in relativistic spacetimes2013In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 777, no 1, p. 13-Article in journal (Refereed)
    Abstract [en]

    We introduce GRay, a massively parallel integrator designed to trace the trajectories of billions of photons in a curved spacetime. This graphics-processing-unit (GPU)-based integrator employs the stream processing paradigm, is implemented in CUDA C/C++, and runs on nVidia graphics cards. The peak performance of GRay using single-precision floating-point arithmetic on a single GPU exceeds 300 GFLOP (or 1 ns per photon per time step). For a realistic problem, where the peak performance cannot be reached, GRay is two orders of magnitude faster than existing central-processing-unit-based ray-tracing codes. This performance enhancement allows more effective searches of large parameter spaces when comparing theoretical predictions of images, spectra, and light curves from the vicinities of compact objects to observations. GRay can also perform on-the-fly ray tracing within general relativistic magnetohydrodynamic algorithms that simulate accretion flows around compact objects. Making use of this algorithm, we calculate the properties of the shadows of Kerr black holes and the photon rings that surround them. We also provide accurate fitting formulae of their dependencies on black hole spin and observer inclination, which can be used to interpret upcoming observations of the black holes at the center of the Milky Way, as well as M87, with the Event Horizon Telescope.

  • 335.
    Chand, Vikas
    et al.
    Tata Inst Fundamental Res, Bombay, Maharashtra, India..
    Chattopadhyay, Tanmoy
    Penn State Univ, State Coll, PA 16804 USA..
    Iyyani, S.
    Interuniv Ctr Astron & Astrophys, Pune, Maharashtra, India..
    Basak, Rupal
    KTH, School of Engineering Sciences (SCI), Physics. AlbaNova, Oskar Klein Ctr Cosmoparticle Phys, SE-10691 Stockholm, Sweden.
    Aarthy, E.
    Phys Res Lab, Ahmadabad, Gujarat, India..
    Rao, A. R.
    Tata Inst Fundamental Res, Bombay, Maharashtra, India..
    Vadawale, Santosh V.
    Phys Res Lab, Ahmadabad, Gujarat, India..
    Bhattacharya, Dipankar
    Interuniv Ctr Astron & Astrophys, Pune, Maharashtra, India..
    Bhalerao, V. B.
    Indian Inst Technol, Bombay, Maharashtra, India..
    Violation of Synchrotron Line of Death by the Highly Polarized GRB 160802A2018In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 862, no 2, article id 154Article in journal (Refereed)
    Abstract [en]

    GRB 160802A is one of the brightest gamma-ray bursts (GRBs) observed by the Fermi Gamma-ray Burst Monitor (GBM) in the energy range of 10-1000 keV, while at the same time it is surprisingly faint at energies greater than or similar to 2 MeV. An observation with the AstroSat/CZT Imager also provides the polarization that helps in constraining different prompt emission models using the novel joint spectra-polarimetric data. We analyze the Fermi/GBM data, and find two main bursting episodes that are clearly separated in time, one of which is particularly faint in higher energies and having certain differences in their spectra. The spectrum in general shows a hard-to-soft evolution in both the episodes. Only the later part of the first episode shows intensity tracking behavior corresponding to multiple pulses. The photon index of the spectrum is hard, and in over 90% cases, crosses even the slow cooling limit (alpha = -2/3) of an optically thin synchrotron shock model. Though such hard values are generally associated with a sub-dominant thermal emission, such a component is not statistically required in our analysis. In addition, the measured polarization in 100-300 keV is too high, pi = 85 +/- 29%, to be accommodated in such a scenario. Jitter radiation, which allows a much harder index up to alpha = + 0.5, in principle can produce high polarization, but only beyond the spectral peak, which in our case lies close to 200-300 keV during the time when most of the polarization signal is obtained. The spectro-polarimetric data seems to be consistent with a subphotospheric dissipation process occurring within a narrow jet with a sharp drop in emissivity beyond the jet edge, and viewed along its boundary.

  • 336. Chasapis, A.
    et al.
    Retino, A.
    Sahraoui, F.
    Vaivads, Andris
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Khotyaintsev, Yuri V.
    Uppsala universitet, Institutet för rymdfysik, Uppsalaavdelningen.
    Sundkvist, D.
    Greco, A.
    Sorriso-Valvo, L.
    Canu, P.
    Thin Current Sheets and Associated Electron Heating in Turbulent Space Plasma2015In: Astrophysical Journal Letters, ISSN 2041-8205, E-ISSN 2041-8213, Vol. 804, no 1, article id L1Article in journal (Refereed)
    Abstract [en]

    Intermittent structures, such as thin current sheets, are abundant in turbulent plasmas. Numerical simulations indicate that such current sheets are important sites of energy dissipation and particle heating occurring at kinetic scales. However, direct evidence of dissipation and associated heating within current sheets is scarce. Here, we show a new statistical study of local electron heating within proton-scale current sheets by using high-resolution spacecraft data. Current sheets are detected using the Partial Variance of Increments (PVI) method which identifies regions of strong intermittency. We find that strong electron heating occurs in high PVI (>3) current sheets while no significant heating occurs in low PVI cases (<3), indicating that the former are dominant for energy dissipation. Current sheets corresponding to very high PVI (>5) show the strongest heating and most of the time are consistent with ongoing magnetic reconnection. This suggests that reconnection is important for electron heating and dissipation at kinetic scales in turbulent plasmas.

  • 337.
    Chashkina, Anna
    et al.
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. University of Turku, Finland.
    Abolmasov, Pavel
    Poutanen, Juri
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. University of Turku, Finland.
    Super-Eddington accretion on to a magnetized neutron star2017In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 470, no 3, p. 2799-2813Article in journal (Refereed)
    Abstract [en]

    Most of ultraluminous X-ray sources are thought to be objects accreting above their Eddington limits. In the recently identified class of ultraluminous X-ray pulsars, accretor is a neutron star and thus has a fairly small mass with a small Eddington limit. The accretion disc structure around such an object affects important observables such as equilibrium period, period derivative and the size of the magnetosphere. We propose a model of a nearly standard accretion disc interacting with the magnetosphere only in a thin layer near the inner disc rim. Our calculations show that the size of the magnetosphere may be represented as the classical Alfven radius times a dimensionless factor. which depends only on the disc thickness. In the case of radiation-pressure-dominated disc, the size of the magnetosphere does not depend on the mass accretion rate. In general, increasing the disc thickness leads to a larger magnetosphere size in units of the Alfven radius. For large enough mass accretion rates and magnetic moments, it is important to take into account not only the pressure of the magnetic field and the radiation pressure inside the disc, but also the pressure of the radiation produced close to the surface of the neutron star in accretion column. The magnetospheric size may increase by up to factor of 2 as a result of the effects related to the disc thickness and the irradiation from the central source. Accounting for these effects reduces the estimate of the neutron star magnetic moment by a factor of several orders.

  • 338.
    Chauvin, Maxime
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. AlbaNova Univ Ctr, Oskar Klein Ctr Cosmoparticle Phys, SE-10691 Stockholm, Sweden.
    Floren, H. -G
    Stockholm Univ, Dept Astron, SE-10691 Stockholm, Sweden.
    Friis, Mette
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. AlbaNova Univ Ctr, Oskar Klein Ctr Cosmoparticle Phys, SE-10691 Stockholm, Sweden..
    Jackson, Miranda
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Kamae, T.
    Univ Tokyo, Dept Phys, Tokyo 1130033, Japan..
    Kataoka, J.
    Waseda Univ, Res Inst Sci & Engn, Tokyo 1698555, Japan..
    Kawano, T.
    Hiroshima Univ, Dept Phys Sci, Hiroshima 7398526, Japan..
    Kiss, Mózsi
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. AlbaNova Univ Ctr, Oskar Klein Ctr Cosmoparticle Phys, SE-10691 Stockholm, Sweden..
    Mikhalev, Victor
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. AlbaNova Univ Ctr, Oskar Klein Ctr Cosmoparticle Phys, SE-10691 Stockholm, Sweden..
    Mizuno, T.
    Hiroshima Univ, Dept Phys Sci, Hiroshima 7398526, Japan..
    Tajima, H.
    Nagoya Univ, Inst Space Earth Environm Res, Nagoya, Aichi 4648601, Japan..
    Takahashi, H.
    Hiroshima Univ, Dept Phys Sci, Hiroshima 7398526, Japan..
    Uchida, N.
    Hiroshima Univ, Dept Phys Sci, Hiroshima 7398526, Japan..
    Pearce, Mark
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. AlbaNova Univ Ctr, Oskar Klein Ctr Cosmoparticle Phys, SE-10691 Stockholm, Sweden..
    The PoGO plus view on Crab off-pulse hard X-ray polarization2018In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 477, no 1, p. L45-L49Article in journal (Refereed)
    Abstract [en]

    The linear polarization fraction (PF) and angle of the hard X-ray emission from the Crab provide unique insight into high-energy radiation mechanisms, complementing the usual imaging, timing, and spectroscopic approaches. Results have recently been presented by two missions operating in partially overlapping energy bands, PoGO+ (18-160 keV) and AstroSat CZTI (100-380 keV). We previously reported PoGO+ results on the polarization parameters integrated across the light curve and for the entire nebula-dominated off-pulse region. We now introduce finer phase binning, in light of the AstroSat CZTI claim that the PF varies across the off-pulse region. Since both missions are operating in a regime where errors on the reconstructed polarization parameters are non-Gaussian, we adopt a Bayesian approach to compare results from each mission. We find no statistically significant variation in off-pulse polarization parameters, neither when considering the mission data separately nor when they are combined. This supports expectations from standard high-energy emission models.

  • 339.
    Chauvin, Maxime
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Floren, H. -G
    Jackson, Miranda
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Kamae, T.
    Kawano, T.
    Kiss, Mózsi
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Kole, Merlin
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Mikhalev, Victor
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Moretti, Elena
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. Max Planck Institute for Astrophysics, Germany.
    Olofsson, G.
    Rydström, Stefan
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Takahashi, H.
    Iyudin, A.
    Arimoto, M.
    Fukazawa, Y.
    Kataoka, J.
    Kawai, N.
    Mizuno, T.
    Ryde, Felix
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Tajima, H.
    Takahashi, T.
    Pearce, Mark
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Observation of polarized hard X-ray emission from the Crab by the PoGOLite Pathfinder2016In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 456, no 1, p. L84-L88Article in journal (Refereed)
    Abstract [en]

    We have measured the linear polarization of hard X-ray emission from the Crab in a previously unexplored energy interval, 20-120 keV. The introduction of two new observational parameters, the polarization fraction and angle stands to disentangle geometrical and physical effects, thereby providing information on the pulsar wind geometry and magnetic field environment. Measurements are conducted using the PoGOLite Pathfinder - a balloon-borne polarimeter. Polarization is determined by measuring the azimuthal Compton scattering angle of incident X-rays in an array of plastic scintillators housed in an anticoincidence well. The polarimetric response has been characterized prior to flight using both polarized and unpolarized calibration sources. We address possible systematic effects through observations of a background field. The measured polarization fraction for the integrated Crab light curve is 18.4(-10.6)(+9.8) per cent, corresponding to an upper limit (99 per cent credibility) of 42.4 per cent, for a polarization angle of (149.2 +/- 16.0)degrees.

  • 340.
    Chauvin, Maxime
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. The Oskar Klein Centre for Cosmoparticle Physics, AlbaNova University Centre, Stockholm, Sweden.
    Florén, Hans-Gustav
    Friis, Mette
    KTH, School of Engineering Sciences (SCI), Physics. The Oskar Klein Centre for Cosmoparticle Physics, AlbaNova University Centre, Stockholm, SwedenThe Oskar Klein Centre for Cosmoparticle Physics, AlbaNova University Centre, Stockholm, Sweden.
    Jackson, Miranda
    KTH, School of Engineering Sciences (SCI), Physics.
    Kamae, Tuneyoshi
    Kataoka, Jun
    Kawano, Takafumi
    Kiss, Mózsi
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Mikhalev, Victor
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Mizuno, Tsunefumi
    Ohashi, Norie
    Stana, Theodor
    KTH, School of Engineering Sciences (SCI), Physics.
    Tajima, Hiro
    Takahashi, Hiromitsu
    Uchida, Nagomi
    Pearce, Mark
    KTH, School of Engineering Sciences (SCI), Physics.
    Accretion geometry of the black-hole binary Cygnus X-1 from X-ray polarimetry2018In: Nature Astronomy, ISSN 2397-3366, Vol. 2, no 8, p. 652-655Article in journal (Refereed)
    Abstract [en]

    Black hole binary (BHB) systems comprise a stellar-mass black hole and a closely orbiting companion star. Matter is transferred from the companion to the black hole, forming an accretion disk, corona and jet structures. The resulting release of gravitational energy leads to the emission of X-rays1. The radiation is affected by special/general relativistic effects, and can serve as a probe for the properties of the black hole and surrounding environment, if the accretion geometry is properly identified. Two competing models describe the disk–corona geometry for the hard spectral state of BHBs, based on spectral and timing measurements2,3. Measuring the polarization of hard X-rays reflected from the disk allows the geometry to be determined. The extent of the corona differs between the two models, affecting the strength of the relativistic effects (such as enhancement of the polarization fraction and rotation of the polarization angle). Here, we report observational results on the linear polarization of hard X-ray emission (19–181 keV) from a BHB, Cygnus X-14, in the hard state. The low polarization fraction, <8.6% (upper limit at a 90% confidence level), and the alignment of the polarization angle with the jet axis show that the dominant emission is not influenced by strong gravity. When considered together with existing spectral and timing data, our result reveals that the accretion corona is either an extended structure, or is located far from the black hole in the hard state of Cygnus X-1.

  • 341.
    Chauvin, Maxime
    et al.
    KTH, School of Engineering Sciences (SCI), Physics. The Oskar Klein Centre for Cosmoparticle Physics, AlbaNova University Centre, 106 91, Stockholm, Sweden.
    Florén, H.-G.
    Friis, Mette
    KTH, School of Engineering Sciences (SCI), Physics. The Oskar Klein Centre for Cosmoparticle Physics, AlbaNova University Centre, 106 91, Stockholm, Sweden.
    Jackson, Miranda
    KTH, School of Engineering Sciences (SCI), Physics. School of Physics and Astronomy, Cardiff University, Cardiff, CF24 3AA, UK.
    Kamae, T.
    Kataoka, J.
    Kawano, T.
    Kiss, Mózsi
    KTH, School of Engineering Sciences (SCI), Physics. The Oskar Klein Centre for Cosmoparticle Physics, AlbaNova University Centre, 106 91, Stockholm, Sweden.
    Mikhalev, Victor
    KTH, School of Engineering Sciences (SCI), Physics. The Oskar Klein Centre for Cosmoparticle Physics, AlbaNova University Centre, 106 91, Stockholm, Sweden.
    Mizuno, T.
    Ohashi, N.
    Stana, Theodor-Adrian
    KTH, School of Engineering Sciences (SCI), Physics. The Oskar Klein Centre for Cosmoparticle Physics, AlbaNova University Centre, 106 91, Stockholm, Sweden.
    Tajima, H.
    Takahashi, H.
    Uchida, N.
    Pearce, Mark
    KTH, School of Engineering Sciences (SCI), Physics. The Oskar Klein Centre for Cosmoparticle Physics, AlbaNova University Centre, 106 91, Stockholm, Sweden.
    Shedding new light on the Crab with polarized X-rays2017In: Scientific Reports, ISSN 2045-2322, E-ISSN 2045-2322, Vol. 7, no 7816, p. 1-6Article in journal (Refereed)
    Abstract [en]

    Strong magnetic fields, synchrotron emission, and Compton scattering are omnipresent in compactcelestial X-ray sources. Emissions in the X-ray energy band are consequently expected to be linearlypolarized. X-ray polarimetry provides a unique diagnostic to study the location and fundamentalmechanisms behind emission processes. The polarization of emissions from a bright celestial X-raysource, the Crab, is reported here for the first time in the hard X-ray band (~20–160 keV). The Crab isa complex system consisting of a central pulsar, a diffuse pulsar wind nebula, as well as structures inthe inner nebula including a jet and torus. Measurements are made by a purpose-built and calibratedpolarimeter, PoGO+. The polarization vector is found to be aligned with the spin axis of the pulsar for apolarization fraction, PF = (20.9 ± 5.0)%. This is higher than that of the optical diffuse nebula, implyinga more compact emission site, though not as compact as, e.g., the synchrotron knot. Contrary tomeasurements at higher energies, no significant temporal evolution of phase-integrated polarisationparameters is observed. The polarization parameters for the pulsar itself are measured for the first timein the X-ray energy band and are consistent with observations at optical wavelengths.

  • 342.
    Chauvin, Maxime
    et al.
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. AlbaNova University Centre, Sweden.
    Jackson, Miranda
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. AlbaNova University Centre, Sweden.
    Kawano, T.
    Kiss, Mózsi
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. AlbaNova University Centre, Sweden.
    Kole, Merlin
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. AlbaNova University Centre, Sweden.
    Mikhalev, Victor
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. AlbaNova University Centre, Sweden.
    Moretti, Elena
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. AlbaNova University Centre, Sweden.
    Takahashi, H.
    Pearce, Mark
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics. AlbaNova University Centre, Sweden.
    Optimising a balloon-borne polarimeter in the hard X-ray domain: From the PoGOLite Pathfinder to PoGO2016In: Astroparticle physics, ISSN 0927-6505, E-ISSN 1873-2852, Vol. 82, p. 99-107Article in journal (Refereed)
    Abstract [en]

    PoGOLite is a balloon-borne hard X-ray polarimeter dedicated to the study of point sources. Compton scattered events are registered using an array of plastic scintillator units to determine the polarisation of incident X-rays in the energy range 20-240 keV. In 2013, a near circumpolar balloon flight of 14 days duration was completed after launch from Esrange, Sweden, resulting in a measurement of the linear polarisation of the Crab emission. Building on the experience gained from this Pathfinder flight, the polarimeter is being modified to improve performance for a second flight in 2016. Such optimisations, based on Geant4 Monte Carlo simulations, take into account the source characteristics, the instrument response and the background environment which is dominated by atmospheric neutrons. This paper describes the optimisation of the polarimeter and details the associated increase in performance. The resulting design, PoGO+, is expected to improve the Minimum Detectable Polarisation (MDP) for the Crab from 19.8% to 11.1% for a 5 day flight. Assuming the same Crab polarisation fraction as measured during the 2013 flight, this improvement in MDP will allow a 5 sigma constrained result. It will also allow the study of the nebula emission only (Crab off-pulse) and Cygnus X-1 if in the hard state.

  • 343. Chen, L. -J
    et al.
    Hesse, M.
    Wang, S.
    Gershman, D.
    Ergun, R. E.
    Burch, J.
    Bessho, N.
    Torbert, R. B.
    Giles, B.
    Webster, J.
    Pollock, C.
    Dorelli, J.
    Moore, T.
    Paterson, W.
    Lavraud, B.
    Strangeway, R.
    Russell, C.
    Khotyaintsev, Y.
    Lindqvist, Per-Arne
    KTH, School of Electrical Engineering (EES), Space and Plasma Physics.
    Avanov, L.
    Electron diffusion region during magnetopause reconnection with an intermediate guide field: Magnetospheric multiscale observations2017In: Journal of Geophysical Research - Space Physics, ISSN 2169-9380, E-ISSN 2169-9402, Vol. 122, no 5, p. 5235-5246Article in journal (Refereed)
    Abstract [en]

    An electron diffusion region (EDR) in magnetic reconnection with a guide magnetic field approximately 0.2 times the reconnecting component is encountered by the four Magnetospheric Multiscale spacecraft at the Earth's magnetopause. The distinct substructures in the EDR on both sides of the reconnecting current sheet are visualized with electron distribution functions that are 2 orders of magnitude higher cadence than ever achieved to enable the following new findings: (1) Motion of the demagnetized electrons plays an important role to sustain the reconnection current and contributes to the dissipation due to the nonideal electric field, (2) the finite guide field dominates over the Hall magnetic field in an electron-scale region in the exhaust and modifies the electron flow dynamics in the EDR, (3) the reconnection current is in part carried by inflowing field-aligned electrons in the magnetosphere part of the EDR, and (4) the reconnection electric field measured by multiple spacecraft is uniform over at least eight electron skin depths and corresponds to a reconnection rate of approximately 0.1. The observations establish the first look at the structure of the EDR under a weak but not negligible guide field.

  • 344.
    Chen, Tao
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Theoretical Chemistry and Biology. Leiden University, Leiden Observatory, Niels Bohrweg 2, NL-2333 CA Leiden, Netherlands.
    Formation of Covalently Bonded Polycyclic Aromatic Hydrocarbons in the Interstellar Medium2018In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 866, no 113Article in journal (Refereed)
    Abstract [en]

    Photo-/ion-induced ionization and dissociation processes are commonly observed for polycyclic aromatic hydrocarbon (PAH) molecules. This work performs theoretical studies of PAHs and their fragments. Molecular dynamics simulations in combination with static quantum chemical calculations reveal that following a single hydrogen atom loss, the fragments, PAH-H, are extremely reactive. They catch a neighbor molecule within picoseconds to form a covalently bonded large molecule regardless of orientations/angles and temperatures. We calculate the infrared spectra of the covalently bonded molecules, which indicate that such species could be the carrier of unidentified infrared emission bands. It also implies that regular PAHs might be less abundant in space than what is expected. 

  • 345.
    Chen, Tao
    et al.
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Theoretical Chemistry and Biology.
    Zhen, J.
    Wang, Yin
    KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Theoretical Chemistry and Biology.
    Linnartz, H.
    Tielens, A. G. G. M.
    Photodissociation processes of Bisanthenquinone cation2017In: Proceedings of the International Astronomical Union, ISSN 1743-9213, no S332, p. 353-359Article in journal (Refereed)
    Abstract [en]

    A systematic study, using ion trap time-of-flight mass spectrometry, is presented for the photo-dissociation processes of Bisanthenquinone (Bq) cations, C28H12O2+, a ketone substituted Polycyclic Aromatic Hydrocarbon (PAH). The Bq cation fragments through sequential loss of the two neutral carbonyl (CO) units upon laser (626nm) irradiation, resulting in a PAH-like derivative C26H12+. Upon further irradiation, C26H12+ exhibits both stepwise dehydrogenation and C2/C2H2 loss fragmentation channels. Quantum chemistry calculations reveal a detailed picture for the first CO-loss, which involves a transition state with a barrier of ∼ 3.4 eV, which is lower than the energy required for the lowest H-loss pathway (∼ 5.0 eV). The barrier for the second CO-loss is higher (∼ 4.9 eV). The subsequent loss of this unit changes the Bq geometry from a planar to a bent one. It is concluded that the photodissociation mechanism of the substituted PAH cations studied here is site selective in the substituted subunit. This work also shows that an acetone substituted PAH cation is not photo-stable upon irradiation. 

  • 346.
    Chen, Xinyi
    et al.
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Department of Physics and Astronomy, Uppsala University Uppsala, Sweden .
    Gordon, James
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Department of Physics and Astronomy, Uppsala University Uppsala, Sweden .
    Zarembo, Konstantin
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Department of Physics and Astronomy, Uppsala University Uppsala, Sweden .
    N=2*super-Yang-Mills theory at strong coupling2014In: Journal of High Energy Physics (JHEP), ISSN 1126-6708, E-ISSN 1029-8479, no 11, p. 057-Article in journal (Refereed)
    Abstract [en]

    The planar N = 2* Super-Yang-Mills (SYM) theory is solved at large 't Hooft coupling using localization on S-4. The solution permits detailed investigation of the resonance phenomena responsible for quantum phase transitions in infinite volume, and leads to quantitative predictions for the semiclassical string dual of the N = 2* theory.

  • 347. Chevalier, Roger A.
    et al.
    Fransson, Claes
    Nymark, Tanja K.
    Stockholm University.
    Radio and X-Ray Emission as Probes of Type IIP Supernovae and Red Supergiant Mass Loss2006In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 641, no 2, p. 1029-1038Article in journal (Refereed)
    Abstract [en]

    Type IIP ( plateau) supernovae are thought to come from stars with initial mass similar to 8 - 25 M-circle dot that end their lives as red supergiants. The expected stellar endpoints can be found from evolutionary calculations, and the corresponding mass-loss properties at these points can be estimated from typical values for Galactic stars. The mass-loss densities of observed supernovae can be estimated from observations of the thermal X-ray and radio synchrotron emission that result from the interaction of the supernova with the surrounding wind. Type IIP supernovae are expected to have energy-conserving interaction during typical times of observation. Because Type IIP supernovae have an extended period of high optical luminosity, Compton cooling could affect the radio-emitting electrons, giving rise to a relatively flat radio light curve in the optically thin regime. Alternatively, a high efficiency of magnetic field production results in synchrotron cooling of the radio-emitting electrons. Both the X-ray and radio luminosities are sensitive to the mass loss and initial masses of the progenitor stars, although the turn-on of radio emission is probably the best estimator of circumstellar density. Both the mass-loss density and the variation of density with stellar mass are consistent with expectations for the progenitor stars deduced from direct observations of recent supernovae. Current observations are consistent with mass being the only parameter; observations of supernovae in metal-poor regions could show how the mass loss depends on metallicity.

  • 348.
    Choudhury, Shouvik Roy
    et al.
    Harish Chandra Res Inst, Chhatnag Rd, Allahabad 211019, Uttar Pradesh, India.;Homi Bhabha Natl Inst, Training Sch Complex, Bombay 400094, Maharashtra, India..
    Choubey, Sandhya
    KTH, School of Engineering Sciences (SCI), Physics. Harish Chandra Res Inst, Chhatnag Rd, Allahabad 211019, Uttar Pradesh, India.;Homi Bhabha Natl Inst, Training Sch Complex, Bombay 400094, Maharashtra, India..
    Updated bounds on sum of neutrino masses in various cosmological scenarios2018In: Journal of Cosmology and Astroparticle Physics, ISSN 1475-7516, E-ISSN 1475-7516, no 9, article id 017Article in journal (Refereed)
    Abstract [en]

    We present strong bounds on the sum of three active neutrino masses ( Sigma m(v)) using selected cosmological datasets and priors in various cosmological models. We use the following baseline datasets: Cosmic Microwave Background (CMB) temperature data from Planck 2015, Baryon Acoustic Oscillations measurements from SDSS-III BOSS DR12, the newly released Type Ia supernovae (SNe Ia) dataset from Pantheon Sample, and a prior on the optical depth to reionization from 2016 Planck Intermediate results. We constrain cosmological parameters with these datasets with a Bayesian analysis in the background of Lambda CDM model with 3 massive active neutrinos. For this minimal Lambda CDM + Sigma m(v) model we find a upper bound of Sigma m(v) < 0.152 eV at 95% C.L. Adding the high-l polarization data from Planck strengthens this bound to Sigma m(v) < 0.118 eV, which is very close to the minimum required mass of Sigma m(v) similar or equal to 0.1 eV for inverted hierarchy. This bound is reduced to Sigma m(v) < 0.110 eV when we also vary r, the tensor to scalar ratio (Lambda CDM + r + Sigma m(v) model), and add an additional dataset, BK14, the latest data released from the Bicep-Keck collaboration (which we add only when r is varied). This bound is further reduced to Sigma m(v) < 0.101 eV in a cosmology with non-phantom dynamical dark energy (w(0)w(a)CDM + Sigma m(v) model with w(z) >= -1 for all z). Considering the w0waCDM + r + Sigma m(v) model and adding the BK14 data again, the bound can be even further reduced to Sigma m(v) < 0.093 eV. For the w0waCDM + Sigma m(v) model P without any constraint on w(z), the bounds however relax to Sigma m(v) < 0.276 eV. Adding a prior on the Hubble constant (H-0 = 73.24 +/- 1.74 km/sec/Mpc) from Hubble Space Telescope (HST), the above mentioned bounds further improve to Sigma m(v) < 0.117 eV, 0.091 eV, 0.085 eV, 0.082 eV, 0.078 eV and 0.247 eV respectively. This substantial improvement is mostly driven by a more than 3 sigma tension between Planck 2015 and HST measurements of H-0 and should be taken cautiously.

  • 349. Chulapakorn, T.
    et al.
    Sychugov, Ilya
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Suvanam, Sethu Saveda
    KTH, School of Information and Communication Technology (ICT), Integrated Devices and Circuits.
    Linnros, Jan T.
    KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics.
    Primetzhofer, D.
    Hallén, Anders
    KTH, School of Information and Communication Technology (ICT), Electronics, Integrated devices and circuits.
    MeV ion irradiation effects on the luminescence properties of Si-implanted SiO2-thin films2016In: Physica Status Solidi (C) Current Topics in Solid State Physics, ISSN 1862-6351, Vol. 13, no 10-12, p. 921-926Article in journal (Refereed)
    Abstract [en]

    The effects of MeV heavy ion irradiation at varying fluence and flux on excess Si, introduced in SiO2 by keV ion implantation, are investigated by photoluminescence (PL). From the PL peak wavelength (λ) and decay lifetime (τ), two PL sources are distinguished: i) quasi-direct recombination of excitons of Si-nanoparticles (SiNPs), appearing after thermal annealing (λ &gt; 720 nm, τ ∼ μs), and ii) fast-decay PL, possibly due to oxide-related defects (λ ∼ 575-690 nm, τ ∼ ns). The fast-decay PL (ii) observed before and after ion irradiation is induced by ion implantation. It is found that this fast-decay luminescence decreases for higher irradiation fluence of MeV heavy ions. After thermal annealing (forming SiNPs), the SiNP PL is reduced for samples irradiated by MeV heavy ions but found to stabilize at higher level for higher irradiation flux; the (ii) band vanishes as a result of annealing. The results are discussed in terms of the influence of electronic and nuclear stopping powers.

  • 350. Clark, C J
    et al.
    Larsson, Stefan
    KTH, School of Engineering Sciences (SCI), Physics, Particle and Astroparticle Physics.
    Li, L.
    KTH.
    Yassine, M.
    et al.,
    PSR J1906+0722: AN ELUSIVE GAMMA-RAY PULSAR2015In: Astrophysical Journal Letters, ISSN 2041-8205, E-ISSN 2041-8213, Vol. 809, no 1, article id L2Article in journal (Refereed)
    Abstract [en]

    We report the discovery of PSR J1906+0722, a gamma-ray pulsar detected as part of a blind survey of unidentified Fermi Large Area Telescope (LAT) sources being carried out on the volunteer distributed computing system, Einstein@Home. This newly discovered pulsar previously appeared as the most significant remaining unidentified gamma-ray source without a known association in the second Fermi-LAT source catalog (2FGL) and was among the top 10 most significant unassociated sources in the recent third catalog (3FGL). PSR J1906+0722 is a young, energetic, isolated pulsar, with a spin frequency of 8.9 Hz, a characteristic age of 49 kyr, and spin-down power 1.0 x 10(36) erg s(-1). In 2009 August it suffered one of the largest glitches detected from a gamma-ray pulsar (Delta f/f approximate to 4.5 x 10(-6)). Remaining undetected in dedicated radio follow-up observations, the pulsar is likely radio-quiet. An off-pulse analysis of the gamma-ray flux from the location of PSR J1906+0722 revealed the presence of an additional nearby source, which may be emission from the interaction between a neighboring supernova remnant and a molecular cloud. We discuss possible effects which may have hindered the detection of PSR J1906+0722 in previous searches and describe the methods by which these effects were mitigated in this survey. We also demonstrate the use of advanced timing methods for estimating the positional, spin and glitch parameters of difficult-to-time pulsars such as this.

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