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  • 1. Abolmasov, Pavel
    et al.
    Poutanen, Juri
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Gamma-ray opacity of the anisotropic stratified broad-line regions in blazars2017In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 464, no 1, p. 152-169Article in journal (Refereed)
    Abstract [en]

    The GeV-range spectra of blazars are shaped not only by non-thermal emission processes internal to the relativistic jet but also by external pair-production absorption on the thermal emission of the accretion disc and the broad-line region (BLR). For the first time, we compute here the pair-production opacities in the GeV range produced by a realistic BLR accounting for the radial stratification and radiation anisotropy. Using photoionization modelling with the CLOUDY code, we calculate a series of BLR models of different sizes, geometries, cloud densities, column densities and metallicities. The strongest emission features in the model BLR are Ly alpha and He II Ly alpha. Contribution of recombination continua is smaller, especially for hydrogen, because Ly continuum is efficiently trapped inside the large optical depth BLR clouds and converted to Lyman emission lines and higher order recombination continua. The largest effects on the gamma-ray opacity are produced by the BLR geometry and localization of the gamma-ray source. We show that when the gamma-ray source moves further from the central source, all the absorption details move to higher energies and the overall level of absorption drops because of decreasing incidence angles between the gamma-rays and BLR photons. The observed positions of the spectral breaks can be used to measure the geometry and the location of the gamma-ray emitting region relative to the BLR. Strong dependence on geometry means that the soft photons dominating the pair-production opacity may be actually produced by a different population of BLR clouds than the bulk of the observed broad line emission.

  • 2.
    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.

  • 3.
    Chashkina, Anna
    et al.
    Univ Turku, Dept Phys & Astron, Tuorla Observ, Turku 20014, Finland.;Lomonosov Moscow State Univ, Sternberg Astron Inst, Univ Sky Pr 13, Moscow 119992, Russia..
    Lipunova, Galina
    Lomonosov Moscow State Univ, Sternberg Astron Inst, Univ Sky Pr 13, Moscow 119992, Russia..
    Abolmasov, Pavel
    Univ Turku, Dept Phys & Astron, Tuorla Observ, Turku 20014, Finland.;Lomonosov Moscow State Univ, Sternberg Astron Inst, Univ Sky Pr 13, Moscow 119992, Russia..
    Poutanen, Juri
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Univ Turku, Dept Phys & Astron, Tuorla Observ, Turku 20014, Finland.;Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia.;KTH Royal Inst Technol, NORDITA, Roslagstullsbacken 23, S-10691 Stockholm, Sweden.;Stockholm Univ, Roslagstullsbacken 23, S-10691 Stockholm, Sweden..
    Super-Eddington accretion discs with advection and outflows around magnetized neutron stars2019In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 626, article id A18Article in journal (Refereed)
    Abstract [en]

    We present a model for a super-Eddington accretion disc around a magnetized neutron star taking into account advection of heat and the mass loss by the wind. The model is semi-analytical and predicts radial profiles of all the basic physical characteristics of the accretion disc. The magnetospheric radius is found as an eigenvalue of the problem. When the inner disc is in radiation-pressure-dominated regime but does not reach its local Eddington limit, advection is mild, and the radius of the magnetosphere depends weakly on the accretion rate. Once it approaches the local Eddington limit the disc becomes advection-dominated, and the scaling for the magnetospheric radius with the mass accretion rate is similar to the classical Alfven relation. Allowing for the mass loss in a wind leads to an increase in the magnetospheric radius. Our model can be applied to a wide variety of magnetized neutron stars accreting close to or above their Eddington limits: ultra-luminous X-ray pulsars, Be/X-ray binaries in outbursts, and other systems. In the context of our model we discuss the observational properties of NGC5907 X-1, the brightest ultra-luminous pulsar currently known, and NGC300 ULX1, which is apparently a Be/X-ray binary experiencing a very bright super-Eddington outburst.

  • 4. De Falco, V.
    et al.
    Kuiper, L.
    Bozzo, E.
    Ferrigno, C.
    Poutanen, Juri
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Stella, L.
    Falanga, M.
    The transitional millisecond pulsar IGR J18245-2452 during its 2013 outburst at X-rays and soft gamma-rays2017In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 603, article id A16Article in journal (Refereed)
    Abstract [en]

    IGR J18245-2452/PSR J1824-2452I is one of the rare transitional accreting millisecond X-ray pulsars, showing direct evidence of switches between states of rotation-powered radio pulsations and accretion-powered X-ray pulsations, dubbed transitional pulsars. IGR J18245-2452 with a spin frequency of ∼ 254.3 Hz is the only transitional pulsar so far to have shown a full accretion episode, reaching an X-ray luminosity of ∼ 1037 erg s-1 permitting its discovery with INTEGRAL in 2013. In this paper, we report on a detailed analysis of the data collected with the IBIS/ISGRI and the two JEM-X monitors on-board INTEGRAL at the time of the 2013 outburst. We make use of some complementary data obtained with the instruments on-board XMM-Newton and Swift in order to perform the averaged broad-band spectral analysis of the source in the energy range 0.4-250 keV. We have found that this spectrum is the hardest among the accreting millisecond X-ray pulsars. We improved the ephemeris, now valid across its full outburst, and report the detection of pulsed emission up to ∼ 60 keV in both the ISGRI (10.9σ) and Fermi/GBM (5.9σ) bandpass. The alignment of the ISGRI and Fermi GBM 20-60 keV pulse profiles are consistent at a ∼ 25 μs level. We compared the pulse profiles obtained at soft X-rays with XMM-Newton with the soft γ-ray ones, and derived the pulsed fractions of the fundamental and first harmonic, as well as the time lag of the fundamental harmonic, up to 150 μs, as a function of energy. We report on a thermonuclear X-ray burst detected with INTEGRAL, and using the properties of the previously type-I X-ray burst, we show that all these events are powered primarily by helium ignited at a depth of yign ≈ 2.7 × 108 g cm-2. For such a helium burst the estimated recurrence time of Δtrec ≈ 5.6 d is in agreement with the observations. 

  • 5. Doroshenko, Victor
    et al.
    Tsygankov, Sergey S.
    Mushtukov, Alexander A.
    Lutovinov, Alexander A.
    Santangelo, Andrea
    Suleimanov, Valery F.
    Poutanen, Juri
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Luminosity dependence of the cyclotron line and evidence for the accretion regime transition in V 0332+532017In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 466, no 2, p. 2143-2150Article in journal (Refereed)
    Abstract [en]

    We report on the analysis of NuSTAR observations of the Be-transient X-ray pulsar V 0332+ 53 during the giant outburst in 2015 and another minor outburst in 2016. We confirm the cyclotronline energy-luminosity correlation previously reported in the source and the line energy decrease during the giant outburst. Based on 2016 observations, we find that a year later the line energy has increased again essentially reaching the pre-outburst values. We discuss this behaviour and conclude that it is likely caused by a change of the emission region geometry rather than previously suggested accretion-induced decay of the neutron stars magnetic field. At lower luminosities, we find for the first time a hint of departure from the anticorrelation of line energy with flux, which we interpret as a transition from super-to sub-critical accretion associated with the disappearance of the accretion column. Finally, we confirm and briefly discuss the orbital modulation observed in the outburst light curve of the source.

  • 6. Kajava, J. J. E.
    et al.
    Koljonen, K. I. I.
    Nattila, J.
    Suleimanov, V.
    Poutanen, Juri
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Univ Turku, Finland; Stockholm Univ, Sweden; Univ Calif Santa Barbara, USA.
    Variable spreading layer in 4U 1608-52 during thermonuclear X-ray bursts in the soft state2017In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 472, no 1, p. 78-89Article in journal (Refereed)
    Abstract [en]

    Thermonuclear (type-I) X-ray bursts, observed from neutron star (NS) low-massX-ray binaries (LMXB), provide constraints on NS masses and radii and consequently the equation of state of NS cores. In such analyses, various assumptions are made without knowing if they are justified. We have analysed X-ray burst spectra from the LMXB 4U 1608-52, with the aim of studying how the different persistent emission components react to the bursts. During some bursts in the soft spectral state we find that there are two variable components: one corresponding to the burst blackbody component and another optically thick Comptonized component. We interpret the latter as the spreading layer between the NS surface and the accretion disc, which is not present during the hard-state bursts. We propose that the spectral changes during the soft-state bursts are driven by the spreading layer that could cover almost the entire NS in the brightest phases due to the enhanced radiation pressure support provided by the burst, and that the layer subsequently returns to its original state during the burst decay. When deriving the NS mass and radius using the soft-state bursts two assumptions are therefore not met: the NS is not entirely visible and the burst emission is reprocessed in the spreading layer, causing distortions of the emitted spectrum. For these reasons, the NS mass and radius constraints using the soft-state bursts are different compared to the ones derived using the hard-state bursts.

  • 7. Kajava, J. J. E.
    et al.
    Nattila, J. N.
    KTH.
    Poutanen, Juri
    KTH. Univ Turku, Finland; Stockholm Univ, Sweden.
    Cumming, A.
    Suleimanov, V.
    Kuulkers, E.
    Detection of burning ashes from thermonuclear X-ray bursts2017In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 464, no 1, p. L6-L10Article in journal (Refereed)
    Abstract [en]

    When neutron stars (NS) accrete gas from low-mass binary companions, explosive nuclear burning reactions in the NS envelope fuse hydrogen and helium into heavier elements. The resulting thermonuclear (type-I) X-ray bursts produce energy spectra that are fit well with black bodies, but a significant number of burst observations show deviations from Planck spectra. Here we present our analysis of RXTE/ PCA observations of X-ray bursts from the NS low-mass X-ray binary HETE J1900.1-2455. We have discovered that the non-Planckian spectra are caused by photoionization edges. The anticorrelation between the strength of the edges and the colour temperature suggests that the edges are produced by the nuclear burning ashes that have been transported upwards by convection and become exposed at the photosphere. The atmosphere model fits show that occasionally the photosphere can consist entirely of metals, and that the peculiar changes in blackbody temperature and radius can be attributed to the emergence and disappearance of metals in the photosphere. As the metals are detected already in the Eddington-limited phase, it is possible that a radiatively driven wind ejects some of the burning ashes into the interstellar space.

  • 8. Kajava, J. J. E.
    et al.
    Sanchez-Fernandez, C.
    Kuulkers, E.
    Poutanen, Juri
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    X-ray burst-induced spectral variability in 4U 1728-342017In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 599, article id A89Article in journal (Refereed)
    Abstract [en]

    Aims. INTEGRAL has been monitoring the Galactic center region for more than a decade. Over this time it has detected hundreds of type-I X-ray bursts from the neutron star low-mass X-ray binary 4U 1728-34, also known as the slow burster. Our aim is to study the connection between the persistent X-ray spectra and the X-ray burst spectra in a broad spectral range. Methods. We performed spectral modeling of the persistent emission and the X-ray burst emission of 4U 1728-34 using data from the INTEGRAL JEM-X and IBIS/ISGRI instruments. Results. We constructed a hardness intensity diagram to track spectral state variations. In the soft state, the energy spectra are characterized by two thermal components likely coming from the accretion disc and the boundary/spreading layer, together with a weak hard X-ray tail that we detect in 4U 1728-34 for the first time in the similar to 40 to 80 keV range. In the hard state, the source is detected up to similar to 200 keV and the spectrum can be described by a thermal Comptonization model plus an additional component: either a powerlaw tail or reflection. By stacking 123 X-ray bursts in the hard state, we detect emission up to 80 keV during the X-ray bursts. We find that during the bursts the emission above 40 keV decreases by a factor of approximately three with respect to the persistent emission level. Conclusions. Our results suggest that the enhanced X-ray burst emission changes the spectral properties of the accretion disc in the hard state. The likely cause is an X-ray burst induced cooling of the electrons in the inner hot flow near the neutron star.

  • 9.
    Kosenkov, Ilia A.
    et al.
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Berdyugin, Andrei V.
    Piirola, Vilppu
    Tsygankov, Sergey S.
    Palle, Enric
    Miles-Paez, Paulo A.
    Poutanen, Juri
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    High-precision optical polarimetry of the accreting black hole V404 Cyg during the 2015 June outburst2017In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 468, no 4, p. 4362-4373Article in journal (Refereed)
    Abstract [en]

    Our simultaneous three-colour (BVR) polarimetric observations of the low-mass black hole X-ray binary V404 Cyg show a small but statistically significant change of polarization degree (Delta(p) similar to 1 per cent) between the outburst in 2015 June and the quiescence. The polarization of V404 Cyg in the quiescent state agrees within the errors with that of the visually close (1.4 arc-sec) companion (pR = 7.3 +/- 0.1 per cent), indicating that it is predominantly of interstellar origin. The polarization pattern of the surrounding field stars supports this conclusion. From the observed variable polarization during the outburst, we show that the polarization degree of the intrinsic component peaks in the V band, p(V) = 1.1 +/- 0.1 per cent, at the polarization position angle of theta(V) =-7 degrees+/- 2 degrees, which is consistent in all three passbands. We detect significant variations in the position angle of the intrinsic polarization in the R band from -30. to similar to 0 degrees during the outburst peak. The observed wavelength dependence of the intrinsic polarization does not support non-thermal synchrotron emission from a jet as a plausible mechanism, but it is in better agreement with the combined effect of electron (Thomson) scattering and absorption in a flattened plasma envelope or outflow surrounding the illuminating source. Alternatively, the polarization signal can be produced by scattering of the disc radiation in a mildly relativistic polar outflow. The position angle of the intrinsic polarization, nearly parallel to the jet direction (i. e. perpendicular to the accretion disc plane), is in agreement with these interpretations.

  • 10. Krivonos, Roman
    et al.
    Clavel, Maica
    Hong, JaeSub
    Mori, Kaya
    Ponti, Gabriele
    Poutanen, Juri
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Univ Turku, Finland.
    Rahoui, Farid
    Tomsick, John
    Tsygankov, Sergey
    NuSTAR and XMM-Newton observations of the Arches cluster in 2015: fading hard X-ray emission from the molecular cloud2017In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 468, no 3, p. 2822-2835Article in journal (Refereed)
    Abstract [en]

    We present results of long Nuclear Spectroscopic Telescope Array (NuSTAR; 200 ks) and XMM-Newton (100 ks) observations of the Arches stellar cluster, a source of bright thermal (kT similar to 2 keV) X-rays with prominent Fe XXV K alpha 6.7 keV line emission and a nearby molecular cloud, characterized by an extended non-thermal hard X-ray continuum and fluorescent Fe K alpha 6.4 keV line of a neutral or low-ionization state material around the cluster. Our analysis demonstrates that the non-thermal emission of the Arches cloud underwent a dramatic change, with its homogeneous morphology, traced by fluorescent Fe K alpha line emission, vanishing after 2012, revealing three bright clumps. The declining trend of the cloud emission, if linearly fitted, is consistent with half-life decay time of similar to 8 yr. Such strong variations have been observed in several other molecular clouds in the Galactic Centre, including the giant molecular cloud Sgr B2, and point towards a similar propagation of illuminating fronts, presumably induced by the past flaring activity of Sgr A(star). We also detect a significant drop of the equivalent width of the fluorescent Fe Ka line, which could mean either that the new clumps have a different position along the line of sight or that the contribution of cosmic ray has become more dominant.

  • 11.
    Krivonos, Roman
    et al.
    Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia..
    Sazonov, Sergey
    Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia.;Natl Res Univ Higher Sch Econ, Myasnitskaya Ul 20, Moscow 101000, Russia..
    Tsygankov, Sergey S.
    Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia.;Univ Turku, Tuorla Observ, Dept Phys & Astron, FI-20014 Turku, Finland..
    Poutanen, Juri
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia.;Univ Turku, Tuorla Observ, Dept Phys & Astron, FI-20014 Turku, Finland.;Stockholm Univ, Roslagstullsbacken 23, SE-10691 Stockholm, Sweden..
    NuSTAR observations of the ultraluminous X-ray source M33 X-8: a black hole in a very high state?2018In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 480, no 2, p. 2357-2364Article in journal (Refereed)
    Abstract [en]

    The closest known ultraluminous X-ray source (ULX), M33 X-8, has been recently observed with NuSTAR during its Extragalactic Legacy program, which includes a hard X-ray survey of the M33 galaxy. We present results of two long observations of M33 taken in 2017 March and July, with M33 X-8 in the field of view. The source demonstrates a nearly constant flux during the observations, and its 3-20 keV spectrum can be well described by two distinct components: a standard accretion disc with a temperature of similar to 1 keV at the inner radius and a power law with a photon index Gamma approximate to 3, which is significantly detected up to 20 keV. There is also an indication of a high-energy cut-off in the spectrum, corresponding to a temperature of the Comptonizing medium of greater than or similar to 10 keV. The broad-band spectral properties of M33 X-8 resemble black hole X-ray binaries in their very high states, suggesting that M33 X-8 is a black hole accreting at a nearly Eddington rate, in contrast to super-Eddington accretion believed to take place in more luminous ULXs.

  • 12.
    Kuuttila, J.
    et al.
    Univ Turku, Dept Phys & Astron, Tuorla Observ, Vaisalantie 20, Piikkio 21500, Finland..
    Kajava, J. J. E.
    Univ Turku, Dept Phys & Astron, Tuorla Observ, Vaisalantie 20, Piikkio 21500, Finland.;Univ Turku, Finnish Ctr Astron ESO FINCA, Vaisalantie 20, Piikkio 21500, Finland.;ESA, European Space Astron Ctr, Sci Operat Dept, Madrid 28691, Spain..
    Nättila, Joonas
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Univ Turku, Dept Phys & Astron, Tuorla Observ, Vaisalantie 20, Piikkio 21500, Finland; Stockholm Univ, Roslagstullsbacken 23, S-10691 Stockholm, Sweden..
    Motta, S. E.
    Univ Oxford, Dept Phys, Astrophys, Denys Wilkinson Bldg,Keble Rd, Oxford OX1 3RH, England..
    Sanchez-Fernandez, C.
    ESA, European Space Astron Ctr, Sci Operat Dept, Madrid 28691, Spain..
    Kuulkers, E.
    ESA, European Space Astron Ctr, Sci Operat Dept, Madrid 28691, Spain.;ESA, Estec, Keplerlaan 1, NL-2201 AZ Noordwijk, Netherlands..
    Cumming, A.
    McGill Univ, Dept Phys, 3550 Univ St, Montreal, PQ H3A 2T8, Canada.;McGill Univ, McGill Space Inst, 3550 Univ St, Montreal, PQ H3A 2T8, Canada..
    Poutanen, Juri
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Univ Turku, Dept Phys & Astron, Tuorla Observ, Vaisalantie 20, Piikkio 21500, Finland.;Stockholm Univ, Roslagstullsbacken 23, S-10691 Stockholm, Sweden.;Univ Calif Santa Barbara, Kavli Inst Theoret Phys, Santa Barbara, CA 93106 USA..
    Flux decay during thermonuclear X-ray bursts analysed with the dynamic power-law index method2017In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 604, article id A77Article in journal (Refereed)
    Abstract [en]

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

  • 13.
    Li, Z.
    et al.
    Xiangtan Univ, Dept Phys, Xiangtan 411105, Peoples R China.;ISSI, Hallerstr 6, CH-3012 Bern, Switzerland.;Univ Bern, Albert Einstein Ctr Fundamental Phys, Inst Theoret Phys, Lab High Energy Phys, Bern, Switzerland..
    De Falco, V.
    ISSI, Hallerstr 6, CH-3012 Bern, Switzerland.;Univ Basel, Dept Phys, Klingelbergstr 82, CH-4056 Basel, Switzerland..
    Falanga, M.
    ISSI, Hallerstr 6, CH-3012 Bern, Switzerland.;Int Space Sci Inst Beijing, 1 Nanertiao, Beijing 100190, Peoples R China..
    Bozzo, E.
    Univ Geneva, Dept Astron, Chemin Ecogia 16, CH-1290 Versoix, Switzerland..
    Kuiper, L.
    SRON, Netherlands Inst Space Res, Sorbonnelaan 2, NL-3584 CA Utrecht, Netherlands..
    Poutanen, Juri
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Univ Turku, Dept Phys & Astron, Tuorla Observ, Turku 20014, Finland.;Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia.
    Cumming, A.
    McGill Univ, Dept Phys, 3600 Rue Univ, Montreal, PQ H3A 2T8, Canada.;McGill Univ, McGill Space Inst, 3600 Rue Univ, Montreal, PQ H3A 2T8, Canada..
    Galloway, D. K.
    Monash Univ, Sch Phys & Astron, Clayton, Vic 3800, Australia.;Monash Univ, Monash Ctr Astrophys, Clayton, Vic 3800, Australia..
    Zhang, S.
    Inst High Energy Phys, Lab Particle Astrophys, Beijing 100049, Peoples R China..
    Mixed H/He bursts in SAX J1748.9-2021 during the spectral change of its 2015 outburst2018In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 620, article id A114Article in journal (Refereed)
    Abstract [en]

    SAX J1748.9-2021 is a transiently accreting X-ray millisecond pulsar. It is also known as an X-ray burster source discovered by Beppo-SAX. We analyzed the persistent emission and type-I X-ray burst properties during its 2015 outburst. The source changed from hard to soft state within half day. We modeled the broadband spectra of the persistent emission in the (1-250) keV energy band for both spectral states using the quasi-simultaneous INTEGRAL and Swift data. The broadband spectra are well fitted by an absorbed thermal Componization model, COMPPS, in a slab geometry. The best-fits for the two states indicate significantly different plasma temperature of 18 and 5 keV and the Thomson optical depths of three and four, respectively. In total, 56 type-I X-ray bursts were observed during the 2015 outburst, of which 26 detected by INTEGRAL in the hard state, 25 by XMM-Newton in the soft state, and five by Swift in both states. As the object transited from the hard to the soft state, the recurrence time for X-ray bursts decreased from approximate to 2 to approximate to 1 h. The relation between the recurrence time, Delta t(rec )and the local mass accretion rate per unit area onto the compact object, in, is fitted by a power-law model, and yielded as best fit at Delta t(rec )similar to <(m) over dot >(-1)(.0)(2 +/-)(0.)(03) using all X-ray bursts. In both cases, the observed recurrence times are consistent with the mixed hydrogen and helium bursts. We also discuss the effects of type-I X-ray bursts prior to the hard to soft transition.

  • 14.
    Li, Zhaosheng
    et al.
    Xiangtan Univ, Dept Phys, Xiangtan 411105, Peoples R China.;Int Space Sci Inst, Hallerstr 6, CH-3012 Bern, Switzerland..
    Suleimanov, Valery F.
    Univ Tubingen, Kepler Ctr Astro & Particle Phys, Inst Astron & Astrophys, Sand 1, D-72076 Tubingen, Germany.;Kazan Volga Reg Fed Univ, Kremlevskaya Str 18, Kazan 420008, Russia.;Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia..
    Poutanen, Juri
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia.;Univ Turku, Dept Phys & Astron, Tuorla Observ, FI-20014 Turku, Finland.;Stockholm Univ, Roslagstullsbacken 23, SE-10691 Stockholm, Sweden..
    Salmi, Tuomo
    Univ Turku, Dept Phys & Astron, Tuorla Observ, FI-20014 Turku, Finland..
    Falanga, Maurizio
    Int Space Sci Inst, Hallerstr 6, CH-3012 Bern, Switzerland.;Int Space Sci Inst Beijing, 1 Nanertiao, Beijing 100190, Peoples R China..
    Nättilä, Joonas
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Xu, Renxin
    Peking Univ, Sch Phys, Beijing 100871, Peoples R China.;Peking Univ, State Key Lab Nucl Phys & Technol, Beijing 100871, Peoples R China..
    Evidence for the Photoionization Absorption Edge in a Photospheric Radius Expansion X-Ray Burst from GRS 1747-312 in Terzan 62018In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 866, no 1, article id 53Article in journal (Refereed)
    Abstract [en]

    Thermonuclear X-ray bursts on the surface of neutron stars (NSs) can enrich the photosphere with metals, which may imprint photoionization edges on the burst spectra. We report here the discovery of absorption edges in the spectra of the type I X-ray burst from the NS low-mass X-ray binary GRS 1747-312 in Terzan 6 during observations by the Rossi X-ray Timing Explorer. We find that the edge energy evolves from 9.45 +/- 0.51 to similar to 6 keV and then back to 9.44 +/- 0.40 keV during the photospheric radius expansion phase and remains at 8.06 +/- 0.66 keV in the cooling tail. The photoionization absorption edges of hydrogen-like Ni, Fe, or an Fe/Ni mixture and the bound-bound transitions of metals may be responsible for the observed spectral features. The ratio of the measured absorption edge energy in the cooling tail to the laboratory value of the hydrogen-like Ni(Fe) edge energy allows us to estimate the gravitational redshift factor 1 + z = 1.34 +/- 0.11(1 + z = 1.15 +/- 0.09). The evolution of the spectral parameters during the cooling tail are well described by metal-rich atmosphere models. The combined constraints on the NS mass and radius from the direct cooling method and the tidal deformability strongly suggest very high atmospheric abundance of the iron group elements and limit the distance to the source to 11 +/- 1 kpc.

  • 15. Lutovinov, A. A.
    et al.
    Tsygankov, S. S.
    Krivonos, R. A.
    Molkov, S. V.
    Poutanen, Juri
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    PROPELLER EFFECT in the TRANSIENT X-RAY PULSAR SMC X-22017In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 834, no 2, article id 209Article in journal (Refereed)
    Abstract [en]

    We report the results of the monitoring campaign of the transient X-ray pulsar SMC X-2 performed with the Swift/XRT telescope over the period of 2015 September-2016 January during the Type II outburst. During this event, the bolometric luminosity of the source ranged from ≃1039 down to several ×1034 erg s-1. Moreover, we discovered its dramatic drop by a factor of more than 100 below the limiting value of erg s-1, which can be interpreted as a transition to the propeller regime. These measurements make SMC X-2 the sixth pulsating X-ray source where such a transition is observed and allow us to estimate the magnetic field of the neutron star in the system B ≃ 3 × 1012 G, which is in agreement with independent results of the spectral analysis. 

  • 16.
    Mereminskiy, Ilya A.
    et al.
    Russian Acad Sci, Space Res Inst, Profsoyuznaya 84-32, Moscow 117997, Russia..
    Semena, Andrey N.
    Russian Acad Sci, Space Res Inst, Profsoyuznaya 84-32, Moscow 117997, Russia..
    Bykov, Sergey D.
    Russian Acad Sci, Space Res Inst, Profsoyuznaya 84-32, Moscow 117997, Russia.;Bauman Moscow State Tech Univ, Baumanskaya 2, Moscow 105005, Russia..
    Filippova, Ekaterina V.
    Russian Acad Sci, Space Res Inst, Profsoyuznaya 84-32, Moscow 117997, Russia..
    Lutovinov, Alexander A.
    Russian Acad Sci, Space Res Inst, Profsoyuznaya 84-32, Moscow 117997, Russia.;Higher Sch Econ, Myasnitskaya 20, Moscow 101000, Russia..
    Poutanen, Juri
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Studying temporal variability of GRS 1739-278 during the 2014 outburst2019In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 482, no 1, p. 1392-1405Article in journal (Refereed)
    Abstract [en]

    We report a discovery of low-frequency quasi-periodic oscillation at 0.3-0.7 Hz in the power spectra of the accreting black hole GRS 1739-278 in the hard-intermediate state during its 2014 outburst based on the NuSTAR and Swift/XRT data. The QPO frequency strongly evolved with the source flux during the NuSTAR observation. The source spectrum became softer with rising QPO frequency and simultaneous increasing of the power-law index and decreasing of the cut-off energy. In the power spectrum, a prominent harmonic is clearly seen together with the main QPO peak. The fluxes in the soft and the hard X-ray bands are coherent, however, the coherence drops for the energy bands separated by larger gaps. The phase lags are generally positive (hard) in the 0.1-3 Hz frequency range, and negative below 0.1 Hz. The accretion disc inner radius estimated with the relativistic reflection spectral model appears to be R-in < 7.3R(g). In the framework of the relativistic precession model, in order to satisfy the constraints from the observed QPO frequency and the accretion disc truncation radius, a massive black hole with M-BH a parts per thousand 100 M-aS (TM) is required.

  • 17.
    Monkkonen, Juhani
    et al.
    Univ Turku, Dept Phys & Astron, Turku 20014, Finland..
    Tsygankov, Sergey S.
    Univ Turku, Dept Phys & Astron, Turku 20014, Finland.;Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia..
    Mushtukov, Alexander A.
    Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia.;Leiden Univ, Leiden Observ, NL-2300 RA Leiden, Netherlands.;Univ Amsterdam, Anton Pannekoek Inst, Sci Pk 904, NL-1098 XH Amsterdam, Netherlands..
    Doroshenko, Victor
    Univ Tubingen, Inst Astron & Astrophys, Sand 1, D-72076 Tubingen, Germany..
    Suleimanov, Valery F.
    Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia.;Univ Tubingen, Inst Astron & Astrophys, Sand 1, D-72076 Tubingen, Germany.;Kazan Volga Reg Fed Univ, Kremlevskaya Str 18, Kazan 420008, Russia..
    Poutanen, Juri
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Univ Turku, Dept Phys & Astron, Turku 20014, Finland.;Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia..
    Evidence for the radiation-pressure dominated accretion disk in bursting pulsar GRO J1744-28 using timing analysis2019In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 626, article id A106Article in journal (Refereed)
    Abstract [en]

    The X-ray pulsar GRO J1744-28 is a unique source that shows both pulsations and type-II X-ray bursts, allowing studies of the interaction of the accretion disk with the magnetosphere at huge mass-accretion rates exceeding 10(19) g s(-1) during its super-Eddington outbursts. The magnetic field strength in the source, B approximate to 5 x 10(11) G, is known from the cyclotron absorption feature discovered in the energy spectrum around 4.5 keV. Here, we have explored the flux variability of the source in context of interaction of its magnetosphere with the radiation-pressure dominated accretion disk. Specifically, we present the results of the analysis of noise power density spectra (PDS) using the observations of the source in 1996-1997 by the Rossi X-ray Timing Explorer (RXTE). Accreting compact objects commonly exhibit a broken power-law PDS shape with a break corresponding to the Keplerian orbital frequency of matter at the innermost disk radius. The observed frequency of the break can thus be used to estimate the size of the magnetosphere. We find, however, that the observed PDS of GRO J1744-28 differs dramatically from the canonical shape. The observed break frequency appears to be significantly higher than expected based on the magnetic field estimated from the cyclotron line energy. We argue that these observational facts can be attributed to the existence of the radiation-pressure dominated region in the accretion disk at luminosities above similar to 2 x10(37) erg s(-1). We discuss a qualitative model for the PDS formation in such disks, and show that its predictions are consistent with our observational findings. The presence of the radiation-pressure dominated region can also explain the observed weak luminosity dependence of the inner radius, and we argue that the small inner radius can be explained by a quadrupole component dominating the magnetic field of the neutron star.

  • 18. Mushtukov, Alexander A.
    et al.
    Nagirner, Dmitrij I.
    Poutanen, Juri
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Compton scattering S matrix and cross section in strong magnetic field2016In: PHYSICAL REVIEW D, ISSN 2470-0010, Vol. 93, no 10, article id 105003Article in journal (Refereed)
    Abstract [en]

    Compton scattering of polarized radiation in a strong magnetic field is considered. The recipe for calculation of the scattering matrix elements, the differential and total cross sections based on quantum electrodynamic second-order perturbation theory is presented for the case of arbitrary initial and final Landau level, electron momentum along the field and photon momentum. Photon polarization and electron spin state are taken into account. The correct dependence of natural Landau level width on the electron spin state is taken into account in a general case of arbitrary initial photon momentum for the first time. A number of steps in the calculations were simplified analytically making the presented recipe easy to use. The redistribution functions over the photon energy, momentum and polarization states are presented and discussed. The paper generalizes already known results and offers a basis for the accurate calculation of radiation transfer in a strong B field, for example, in strongly magnetized neutron stars.

  • 19. Mushtukov, Alexander A.
    et al.
    Tsygankov, Sergey S.
    Serber, Alexander V.
    Suleimanov, Valery F.
    Poutanen, Juri
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. University of Turku, Finland.
    Positive correlation between the cyclotron line energy and luminosity in sub-critical X-ray pulsars: Doppler effect in the accretion channel2015In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 454, no 3, p. 2714-2721Article in journal (Refereed)
    Abstract [en]

    Cyclotron resonance scattering features observed in the spectra of some X-ray pulsars show significant changes of the line centroid energy with the pulsar luminosity. Whereas for bright sources above the so-called critical luminosity, these variations are established to be connected with the appearance of the high-accretion column above the neutron star surface, at low, sub-critical luminosities the nature of the variations (but with the opposite sign) has not been discussed widely. We argue here that the cyclotron line is formed when the radiation from a hotspot propagates through the plasma falling with a mildly relativistic velocity on to the neutron star surface. The position of the cyclotron resonance is determined by the Doppler effect. The change of the cyclotron line position in the spectrum with luminosity is caused by variations of the velocity profile in the line-forming region affected by the radiation pressure force. The presented model has several characteristic features: (i) the line centroid energy is positively correlated with the luminosity; (ii) the line width is positively correlated with the luminosity as well; (iii) the position and the width of the cyclotron absorption line are variable over the pulse phase; (iv) the line has a more complicated shape than widely used Lorentzian or Gaussian profiles; (v) the phase-resolved cyclotron line centroid energy and the width are negatively and positively correlated with the pulse intensity, respectively. The predictions of the proposed theory are compared with the variations of the cyclotron line parameters in the Xray pulsar GX 304-1 over a wide range of sub-critical luminosities as seen by the INTEGRAL observatory.

  • 20.
    Mushtukov, Alexander A.
    et al.
    Univ Amsterdam, Anton Pannekoek Inst, Sci Pk 904, NL-1098 XH Amsterdam, Netherlands.;Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia.;Russian Acad Sci, Pulkovo Observ, St Petersburg 196140, Russia..
    Tsygankov, Sergey S.
    Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia.;Univ Turku, Dept Phys & Astron, Tuorla Observ, Vaisalantie 20, FI-21500 Piikkio, Finland..
    Suleimanov, Valery F.
    Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia.;Univ Tubingen, Kepler Ctr Astro & Particle Phys, Inst Astron & Astrophys, Sand 1, D-72076 Tubingen, Germany.;Kazan Volga Reg Fed Univ, Kremlevskaja Str 18, Kazan 420008, Russia..
    Poutanen, Juri
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Univ Turku, Dept Phys & Astron, Tuorla Observ, Vaisalantie 20, FI-21500 Piikkio, Finland.;Stockholm Univ, Roslagstullsbacken 23, SE-10691 Stockholm, Sweden.;Univ Calif Santa Barbara, Kavii Inst Theoret Phys, Santa Barbara, CA 93106 USA..
    Ultraluminous X-ray sources as neutrino pulsars2018In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 476, no 3, p. 2867-2873Article in journal (Refereed)
    Abstract [en]

    The classical limit on the accretion luminosity of a neutron star is given by the Eddington luminosity. The advanced models of accretion on to magnetized neutron stars account for the appearance of magnetically confined accretion columns and allow the accretion luminosity to be higher than the Eddington value by a factor of tens. However, the recent discovery of pulsations from ultraluminous X-ray source (ULX) in NGC 5907 demonstrates that the accretion luminosity can exceed the Eddington value up to by a factor of 500. We propose a model explaining observational properties of ULX-1 in NGC 5907 without any ad hoc assumptions. We show that the accretion column at extreme luminosity becomes advective. Enormous energy release within a small geometrical volume and advection result in very high temperatures at the bottom of accretion column, which demand to account for the energy losses due to neutrino emission which can be even more effective than the radiation energy losses. We show that the total luminosity at the mass accretion rates above 1021 g s(-1) is dominated by the neutrino emission similarly to the case of core-collapse supernovae. We argue that the accretion rate measurements based on detected photon luminosity in case of bright ULXs powered by neutron stars can be largely underestimated due to intense neutrino emission. The recently discovered pulsating ULX-1 in galaxy NGC 5907 with photon luminosity of similar to 10(41) erg s(-1) is expected to be even brighter in neutrinos and is thus the first known Neutrino Pulsar.

  • 21.
    Nabizadeh, Armin
    et al.
    Univ Turku, Dept Phys & Astron, Turku 20014, Finland..
    Tsygankov, Sergey S.
    Univ Turku, Dept Phys & Astron, Turku 20014, Finland.;Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia..
    Karasev, Dmitrij I.
    Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia..
    Monkkonen, Juhani
    Univ Turku, Dept Phys & Astron, Turku 20014, Finland..
    Lutovinov, Alexander A.
    Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia..
    Nagirner, Dmitrij I.
    St Petersburg State Univ, Sobolev Astron Inst, St Petersburg 198504, Russia..
    Poutanen, Juri
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Properties of the transient X-ray pulsar Swift J1816.7-1613 and its optical companion2019In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 622, article id A198Article in journal (Refereed)
    Abstract [en]

    We present results of investigation of the poorly studied X-ray pulsar Swift J1816.7-1613 during its transition from the type I outburst to the quiescent state. Our studies are based on the data obtained from X-ray observatories Swift, NuSTAR, and Chandra alongside with the latest IR data from UKIDSS/GPS and Spitzer/GLIMPSE surveys. The aim of the work is to determine the parameters of the system, namely the strength of the neutron star magnetic field and the distance to the source, which are required for the interpretation of the source behaviour in the framework of physically motivated models. No cyclotron absorption line was detected in the broad-band energy spectrum. However, the timing analysis hints at the typical for the X-ray pulsars magnetic field from a few x10(11) to a few x10(12) G. We also estimated the type of the IR-companion as a B0-2e star located at a distance of 7-13 kpc.

  • 22. Nättilä, J.
    et al.
    Miller, M. C.
    Steiner, A. W.
    Kajava, J. J. E.
    Suleimanov, V. F.
    Poutanen, Juri
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. University of Turku, Finland.
    Neutron star mass and radius measurements from atmospheric model fits to X-ray burst cooling tail spectra2017In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 608, article id A31Article in journal (Refereed)
    Abstract [en]

    Observations of thermonuclear X-ray bursts from accreting neutron stars (NSs) in low-mass X-ray binary systems can be used to constrain NS masses and radii. Most previous work of this type has set these constraints using Planck function fits as a proxy: the models and the data are both fit with diluted blackbody functions to yield normalizations and temperatures that are then compared with each other. For the first time, we here fit atmosphere models of X-ray bursting NSs directly to the observed spectra. We present a hierarchical Bayesian fitting framework that uses current X-ray bursting NS atmosphere models with realistic opacities and relativistic exact Compton scattering kernels as a model for the surface emission. We test our approach against synthetic data and find that for data that are well described by our model, we can obtain robust radius, mass, distance, and composition measurements. We then apply our technique to Rossi X-ray Timing Explorer observations of five hard-state X-ray bursts from 4U 1702-429. Our joint fit to all five bursts shows that the theoretical atmosphere models describe the data well, but there are still some unmodeled features in the spectrum corresponding to a relative error of 1-5% of the energy flux. After marginalizing over this intrinsic scatter, we find that at 68% credibility, the circumferential radius of the NS in 4U 1702-429 is R = 12.4 +/- 0.4 km, the gravitational mass is M = 1.9 +/- 0.3 M-circle dot, the distance is 5.1 < D/kpc < 6.2, and the hydrogen mass fraction is X < 0.09.

  • 23. Nättilä, J.
    et al.
    Steiner, A. W.
    Kajava, J. J. E.
    Suleimanov, V. F.
    Poutanen, Juri
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Stockholm University, Sweden.
    Equation of state constraints for the cold dense matter inside neutron stars using the cooling tail method2016In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 591, article id A25Article in journal (Refereed)
    Abstract [en]

    The cooling phase of thermonuclear (type-I) X-ray bursts can be used to constrain neutron star (NS) compactness by comparing the observed cooling tracks of bursts to accurate theoretical atmosphere model calculations. By applying the so-called cooling tail method, where the information from the whole cooling track is used, we constrain the mass, radius, and distance for three different NSs in low-mass X-ray binaries 4U 1702-429, 4U 1724-307, and SAX J1810.8-260. Care is taken to use only the hard state bursts where it is thought that the NS surface alone is emitting. We then use a Markov chain Monte Carlo algorithm within a Bayesian framework to obtain a parameterized equation of state (EoS) of cold dense matter from our initial mass and radius constraints. This allows us to set limits on various nuclear parameters and to constrain an empirical pressure-density relationship for the dense matter. Our predicted EoS results in NS a radius between 10.5-12.8 km (95% confidence limits) for a mass of 1.4 M, depending slightly on the assumed composition. Because of systematic errors and uncertainty in the composition, these results should be interpreted as lower limits for the radius.

  • 24.
    Poutanen, Juri
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. University of Turku, Finland.
    Rosseland and Flux Mean Opacities for Compton Scattering2017In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 835, no 2, article id 119Article in journal (Refereed)
    Abstract [en]

    Rosseland mean opacity plays an important role in theories of stellar evolution and X-ray burst models. In the high-temperature regime, when most of the gas is completely ionized, the opacity is dominated by Compton scattering. Our aim here is to critically evaluate previous works on this subject and to compute the exact Rosseland mean opacity for Compton scattering over a broad range of temperature and electron degeneracy parameter. We use relativistic kinetic equations for Compton scattering and compute the photon mean free path as a function of photon energy by solving the corresponding integral equation in the diffusion limit. As a byproduct we also demonstrate the way to compute photon redistribution functions in the case of degenerate electrons. We then compute the Rosseland mean opacity as a function of temperature and electron degeneracy and present useful approximate expressions. We compare our results to previous calculations and find a significant difference in the low-temperature regime and strong degeneracy. We then proceed to compute the flux mean opacity in both free-streaming and diffusion approximations, and show that the latter is nearly identical to the Rosseland mean opacity. We also provide a simple way to account for the true absorption in evaluating the Rosseland and flux mean opacities.

  • 25. Salmi, T.
    et al.
    Nättilä, Joonas
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Poutanen, Juri
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Tuorla Observatory, Department of Physics and Astronomy, 20014 University of Turku, Finland e-mail: thjsal@utu.fi; Space Research Institute of the Russian Academy of Sciences, Profsoyuznaya str. 84/32, 117997 Moscow, Russia .
    Bayesian parameter constraints for neutron star masses and radii using X-ray timing observations of accretion-powered millisecond pulsars2018In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 618, article id 33348Article in journal (Refereed)
    Abstract [en]

    We present a Bayesian method to constrain the masses and radii of neutron stars (NSs) using the information encoded in the X-ray pulse profiles of accreting millisecond pulsars. We model the shape of the pulses using "oblate Schwarzschild" approximation, which takes into account the deformed shape of the star together with the special and general relativistic corrections to the photon trajectories and angles. The spectrum of the radiation is obtained from an empirical model of Comptonization in a hot slab in which a fraction of seed black-body photons is scattered into a power-law component. By using an affine-invariant Markov chain Monte Carlo ensemble sampling method, we obtain posterior probability distributions for the different model parameters, especially for the mass and the radius. To test the robustness of our method, we first analysed self-generated synthetic data with known model parameters. Similar analysis was then applied for the observations of SAX J1808.4-3658 by the Rossi X-ray Timing Explorer (RXTE). The results show that our method can reproduce the model parameters of the synthetic data, and that accurate constraints for the radius can be obtained using the RXTE pulse profile observations if the mass is a priori known. For a mass in the range 1.5-1.8 M, the radius of the NS in SAX J1808.4-3658 is constrained between 9 and 13 km. If the mass is accurately known, the radius can be determined with an accuracy of 5% (68% credibility). For example, for the mass of 1.7 M the equatorial radius is Req = 11.9+0.5 -0.4 km. Finally, we show that further improvements can be obtained when the X-ray polarization data from the Imaging X-ray Polarimeter Explorer will become available.

  • 26.
    Salmi, Tuomo
    et al.
    Univ Turku, Dept Phys & Astron, Tuorla Observ, Turku 20014, Finland..
    Suleimanov, Valery F.
    Univ Tubingen, Kepler Ctr Astro & Particle Phys, Inst Astron & Astrophys, Sand 1, D-72076 Tubingen, Germany.;Kazan Volga Reg Fed Univ, Astron Dept, Kremlyovskaya Str 18, Kazan 420008, Russia.;Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia..
    Poutanen, Juri
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Effects of Compton scattering on the neutron star radius constraints in rotation-powered millisecond pulsars2019In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 627, article id A39Article in journal (Refereed)
    Abstract [en]

    The aim of this work is to study the possible effects and biases on the radius constraints for rotation-powered millisecond pulsars when using Thomson approximation to describe electron scattering in the atmosphere models, instead of using exact formulation for Compton scattering. We compare the differences between the two models in the energy spectrum and angular distribution of the emitted radiation. We also analyse a self-generated, synthetic, phase-resolved energy spectrum, based on Compton atmosphere and the most X-ray luminous, rotation-powered millisecond pulsars observed by the Neutron star Interior Composition ExploreR (NICER). We derive constraints for the neutron star parameters using both the Compton and Thomson models. The results show that the method works by reproducing the correct parameters with the Compton model. However, biases are found in both the size and the temperature of the emitting hotspot, when using the Thomson model. The constraints on the radius are still not significantly changed, and therefore the Thomson model seems to be adequate if we are interested only in the radius measurements using NICER.

  • 27. Suleimanov, V. F.
    et al.
    Kajava, J. J. E.
    Molkov, S. V.
    Nättilä, J.
    Lutovinov, A. A.
    Werner, K.
    Poutanen, Juri
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Basic parameters of the helium-accreting X-ray bursting neutron star in 4U 1820-302017In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 472, no 4, p. 3905-3913, article id stx2234Article in journal (Refereed)
    Abstract [en]

    The ultracompact low-mass X-ray binary 4U 1820-30 situated in the globular cluster NGC 6624 has an orbital period of only ≈11.4 min, which likely implies a white dwarf companion. The observed X-ray bursts demonstrate a photospheric radius expansion phase and therefore are believed to reach the Eddington luminosity, allowing us to estimate the mass and the radius of the neutron star (NS) in this binary. Here, we re-analyse all Rossi X-ray Timing Explorer observations of the system and confirm that almost all the bursts took place during the hard persistent state of the system. This allows us to use the recently developed direct cooling tail method to estimate the NS mass and radius. However, because of the very short, about a second, duration of the cooling tail phases that can be described by the theoretical atmosphere models, the obtained constraints on the NS radius are not very strict. Assuming a pure helium NS atmosphere, we found that the NS radius is in the range 10-12 km, if the NS mass is below 1.7 MO˙, and in a wider range of 8-12 km for a higher 1.7-2.0 Mo˙ NS mass. The method also constrains the distance to the system to be 6.5 ± 0.5 kpc, which is consistent with the distance to the cluster. For the solar composition atmosphere, the NS parameters are in strong contradiction with the generally accepted range of possible NS masses and radii.

  • 28. Suleimanov, V. F.
    et al.
    Klochkov, D.
    Poutanen, Juri
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. University of Turku, Finland.
    Werner, K.
    Probing the possibility of hotspots on the central neutron star in HESSJ1731-3472017In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 600, article id A43Article in journal (Refereed)
    Abstract [en]

    The X-ray spectra of the neutron stars located in the centers of supernova remnants Cas A and HESS J1731-347 are well fit with carbon atmosphere models. These fits yield plausible neutron star sizes for the known or estimated distances to these supernova remnants. The evidence in favor of the presence of a pure carbon envelope at the neutron star surface is rather indirect and is based on the assumption that the emission is generated uniformly by the entire stellar surface. Although this assumption is supported by the absence of pulsations, the observational upper limit on the pulsed fraction is not very stringent. In an attempt to quantify this evidence, we investigate the possibility that the observed spectrum of the neutron star in HESS J1731-347 is a combination of the spectra produced in a hydrogen atmosphere of the hotspots and of the cooler remaining part of the neutron star surface. The lack of pulsations in this case has to be explained either by a sufficiently small angle between the neutron star spin axis and the line of sight, or by a sufficiently small angular distance between the hotspots and the neutron star rotation poles. As the observed flux from a non-uniformly emitting neutron star depends on the angular distribution of the radiation emerging from the atmosphere, we have computed two new grids of pure carbon and pure hydrogen atmosphere model spectra accounting for Compton scattering. Using new hydrogen models, we have evaluated the probability of a geometry that leads to a pulsed fraction below the observed upper limit to be about 8.2%. Such a geometry thus seems to be rather improbable but cannot be excluded at this stage.

  • 29. Suleimanov, V. F.
    et al.
    Poutanen, Juri
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. University of Turku, Finland.
    Klochkov, D.
    Werner, K.
    Measuring the basic parameters of neutron stars using model atmospheres2016In: European Physical Journal A, ISSN 1434-6001, E-ISSN 1434-601X, Vol. 52, no 2, article id 20Article in journal (Refereed)
    Abstract [en]

    Model spectra of neutron star atmospheres are nowadays widely used to fit the observed thermal X-ray spectra of neutron stars. This fitting is the key element in the method of the neutron star radius determination. Here, we present the basic assumptions used for the neutron star atmosphere modeling as well as the main qualitative features of the stellar atmospheres leading to the deviations of the emergent model spectrum from blackbody. We describe the properties of two of our model atmosphere grids: i) pure carbon atmospheres for relatively cool neutron stars (1-4MK) and ii) hot atmospheres with Compton scattering taken into account. The results obtained by applying these grids to model the X-ray spectra of the central compact object in supernova remnant HESS 1731-347, and two X-ray bursting neutron stars in low-mass X-ray binaries, 4U 1724-307 and 4U 1608-52, are presented. Possible systematic uncertainties associated with the obtained neutron star radii are discussed.

  • 30.
    Suleimanov, V. F.
    et al.
    Univ Tubingen, Kepler Ctr Astro & Particle Phys, Inst Astron & Astrophys, Sand 1, D-72076 Tubingen, Germany.;Kazan Volga Reg Fed Univ, Astron Dept, Kremlyovskaya Str 18, Kazan 420008, Russia.;Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia..
    Poutanen, Juri
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia.;Univ Turku, Dept Phys & Astron, Tuorla Observ, Turku 20014, Finland.;Stockholm Univ, Roslagstullsbacken 23, S-10691 Stockholm, Sweden..
    Werner, K.
    Univ Tubingen, Kepler Ctr Astro & Particle Phys, Inst Astron & Astrophys, Sand 1, D-72076 Tubingen, Germany..
    Accretion heated atmospheres of X-ray bursting neutron stars2018In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 619, article id A114Article in journal (Refereed)
    Abstract [en]

    Some thermonuclear (type I) X-ray bursts at the neutron star surfaces in low-mass X-ray binaries take place during hard persistent states of the systems. Spectral evolution of these bursts is well described by the atmosphere model of a passively cooling neutron star when the burst luminosity is high enough. The observed spectral evolution deviates from the model predictions when the burst luminosity drops below a critical value of 20-70% of the maximum luminosity. The amplitude of the deviations and the critical luminosity correlate with the persistent luminosity, which leads us to suggest that these deviations are induced by the additional heating of the accreted particles. We present a method for computation of the neutron star atmosphere models heated by accreted particles assuming that their energy is released via Coulomb interactions with electrons. We computed the temperature structures and the emergent spectra of the atmospheres of various chemical compositions and investigate the dependence of the results on the velocity of accreted particles, their temperature and the penetration angle. We show that the heated atmosphere develops two different regions. The upper one is the hot (20-100 keV) corona-like surface layer cooled by Compton scattering, and the deeper, almost isothermal optically thick region with a temperature of a few keV. The emergent spectra correspondingly have two components: a blackbody with the temperature close to that of the isothermal region and a hard Comptonized component (a power law with an exponential decay). Their relative contribution depends on the ratio of the energy dissipation rate of the accreted particles to the intrinsic flux from the neutron star surface. These spectra deviate strongly from those of undisturbed, passively cooling neutron star atmospheres, with the main differences being the presence of a high-energy tail and a strong excess in the low-energy part of the spectrum. They also lack the iron absorption edge, which is visible in the spectra of undisturbed low-luminosity atmospheres with solar chemical composition. Using the computed spectra, we obtained the dependences of the dilution and color-correction factors as functions of relative luminosities for pure helium and solar abundance atmospheres. We show that the helium model atmosphere heated by accretion corresponding to 5% of the Eddington luminosity describes well the late stages of the X-ray bursts in 4U 1820-30.

  • 31. Suleimanov, Valery F.
    et al.
    Poutanen, Juri
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Univ Turku, Finland.
    Nattila, Joonas
    Kajava, Jari J. E.
    Revnivtsev, Mikhail G.
    Werner, Klaus
    The direct cooling tail method for X-ray burst analysis to constrain neutron star masses and radii2017In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 466, no 1, p. 906-913Article in journal (Refereed)
    Abstract [en]

    Determining neutron star (NS) radii and masses can help to understand the properties of matter at supra-nuclear densities. Thermal emission during thermonuclear X-ray bursts from NSs in low-mass X-ray binaries provides a unique opportunity to study NS parameters, because of the high fluxes, large luminosity variations and the related changes in the spectral properties. The standard cooling tail method uses hot NS atmosphere models to convert the observed spectral evolution during cooling stages of X-ray bursts to the Eddington flux F-Edd and the stellar angular size Omega. These are then translated to the constraints on the NS massMand radius R. Here we present the improved, direct cooling tail method that generalizes the standard approach. First, we adjust the cooling tail method to account for the bolometric correction to the flux. Then, we fit the observed dependence of the blackbody normalization on flux with a theoretical model directly on theM-R plane by interpolating theoretical dependences to a given gravity, hence ensuring only weakly informative priors for M and R instead of FEdd and Omega. The direct cooling method is demonstrated using a photospheric radius expansion burst from SAX J1810.8-2609, which has happened when the system was in the hard state. Comparing to the standard cooling tail method, the confidence regions are shifted by 1 sigma towards larger radii, giving R = 11.5-13.0 km at M = 1.3-1.8M(circle dot) for this NS.

  • 32. Tsygankov, S. S.
    et al.
    Doroshenko, V.
    Lutovinov, A. A.
    Mushtukov, A. A.
    Poutanen, Juri
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    SMC X-3: The closest ultraluminous X-ray source powered by a neutron star with non-dipole magnetic field2017In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 605, article id A39Article in journal (Refereed)
    Abstract [en]

    Aims. The magnetic field of accreting neutron stars determines their overall behavior including the maximum possible luminosity. Some models require an above-average magnetic field strength (≥ 1013 G) in order to explain super-Eddington mass accretion rate in the recently discovered class of pulsating ultraluminous X-ray sources (ULX). The peak luminosity of SMC X-3 during its major outburst in 2016-2017 reached ~2.5 × 1039 erg s-1 comparable to that in ULXs thus making this source the nearest ULX-pulsar. Determination of the magnetic field of SMC X-3 is the main goal of this paper. Methods. SMC X-3 belongs to the class of transient X-ray pulsars with Be optical companions, and exhibited a giant outburst in July 2016-March 2017. The source has been observed over the entire outburst with the Swift/XRT and Fermi/GBM telescopes, as well as the NuSTAR observatory. Collected data allowed us to estimate the magnetic field strength of the neutron star in SMC X-3 using several independent methods. Results. Spin evolution of the source during and between the outbursts, and the luminosity of the transition to the so-called propeller regime in the range of (0.3-7) × 1035 erg s-1 imply a relatively weak dipole field of (1-5) × 1012 G. On the other hand, there is also evidence for a much stronger field in the immediate vicinity of the neutron star surface. In particular, transition from super- to sub-critical accretion regime associated with the cease of the accretion column and very high peak luminosity favor a field that is an order of magnitude stronger. This discrepancy makes SMC X-3 a good candidate for possessing significant non-dipolar components of the field, and an intermediate source between classical X-ray pulsars and accreting magnetars which may constitute an appreciable fraction of ULX population.

  • 33. Tsygankov, S. S.
    et al.
    Lutovinov, A. A.
    Doroshenko, V.
    Mushtukov, A. A.
    Suleimanov, V.
    Poutanen, Juri
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA.
    Propeller effect in two brightest transient X-ray pulsars: 4U 0115+63 and v 0332+532016In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 593, article id A16Article in journal (Refereed)
    Abstract [en]

    Aims. We present the results of the monitoring programmes performed with the Swift/XRT telescope and aimed specifically to detect an abrupt decrease of the observed flux associated with a transition to the propeller regime in two well-known X-ray pulsars 4U 0115+63 and V 0332+53. Methods. Both sources form binary systems with Be optical companions and undergo so-called giant outbursts every 3-4 years. The current observational campaigns were performed with the Swift/XRT telescope in the soft X-ray band (0.5-10 keV) during the declining phases of the outbursts exhibited by both sources in 2015. Results. The transitions to the propeller regime were detected at the threshold luminosities of (1.4 ± 0.4) × 1036 erg s-1 and (2.0 ± 0.4) × 1036 erg s-1 for 4U 0115+63 and V 0332+53, respectively. Spectra of the sources are shown to be significantly softer during the low state. In both sources, the accretion at rates close to the aforementioned threshold values briefly resumes during the periastron passage following the transition into the propeller regime. The strength of the dipole component of the magnetic field required to inhibit the accretion agrees well with estimates based on the position of the cyclotron lines in their spectra, thus excluding presence of a strong multipole component of the magnetic field in the vicinity of the neutron star.

  • 34. Tsygankov, S. S.
    et al.
    Mushtukov, A. A.
    Suleimanov, V. F.
    Doroshenko, V.
    Abolmasov, P. K.
    Lutovinov, A. A.
    Poutanen, Juri
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. University of Turku, Finland.
    Stable accretion from a cold disc in highly magnetized neutron stars2017In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 608, article id A17Article in journal (Refereed)
    Abstract [en]

    Aims. The aim of this paper is to investigate the transition of a strongly magnetized neutron star into the accretion regime with very low accretion rate.& para;& para;Methods. For this purpose, we monitored the Be-transient X-ray pulsar GRO J1008-57 throughout a full orbital cycle. The current observational campaign was performed with the Swift/XRT telescope in the soft X-ray band (0.5-10 keV) between two subsequent Type I outbursts in January and September 2016.& para;& para;Results. The expected transition to the propeller regime was not observed. However, transitions between different regimes of accretion were detected. In particular, after an outburst, the source entered a stable accretion state characterised by an accretion rate of similar to 10(14)-10(15) g s(-1). We associate this state with accretion from a cold (low-ionised) disc of temperature below similar to 6500 K. We argue that a transition to this accretion regime should be observed in all X-ray pulsars that have a certain combination of the rotation frequency and magnetic field strength. The proposed model of accretion from a cold disc is able to explain several puzzling observational properties of X-ray pulsars.

  • 35.
    Tsygankov, Sergey S.
    et al.
    Univ Turku, Dept Phys & Astron, Tuorla Observ, FI-20014 Turku, Finland.;Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia..
    Doroshenko, Victor
    Univ Tubingen, Inst Astron & Astrophys, Sand 1, D-72076 Tubingen, Germany..
    Mushtukov, Alexander A.
    Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia.;Univ Amsterdam, Anton Pannekoek Inst Astron, Sci Pk 904, NL-1098 XH Amsterdam, Netherlands..
    Lutovinov, Alexander A.
    Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia..
    Poutanen, Juri
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Univ Turku, Dept Phys & Astron, Tuorla Observ, FI-20014 Turku, Finland.;Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia.
    On the magnetic field of the first Galactic ultraluminous X-ray pulsar Swift J0243.6+61242018In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 479, no 1, p. L134-L138Article in journal (Refereed)
    Abstract [en]

    We report on the monitoring of the final stage of the outburst from the first Galactic ultraluminous X-ray pulsar Swift J0243.6+6124, which reached similar to 40 Eddington luminosities. The main aim of the monitoring program with the Swift/XRT telescope was to measure the magnetic field of the neutron star using the luminosity of transition to the 'propeller' state. The visibility constraints, unfortunately, did not permit us to observe the source down to the fluxes low enough to detect such a transition. The tight upper limit on the propeller luminosity L-prop < 6.8x10(35) erg s(-1) implies the dipole component of the magnetic field B< 10(13) G. On the other hand, the observed evolution of the pulse profile and of the pulsed fraction with flux points to a change of the emission region geometry at the critical luminosity L-crit similar to 3 x 10(38) erg s(-1) both in the rising and declining parts of the outburst. We associate the observed change with the onset of the accretion column, which allows us to get an independent estimate of the magnetic field strength close to the neutron stars surface of B > 10(13) G. Given the existing uncertainty in the effective magnetosphere size, we conclude that both estimates are marginally compatible with each other.

  • 36. Tsygankov, Sergey S.
    et al.
    Lutovinov, Alexander A.
    Krivonos, Roman A.
    Molkov, Sergey V.
    Jenke, Peter J.
    Finger, Mark H.
    Poutanen, Juri
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. University of Turku, Finland.
    NuSTAR discovery of a cyclotron absorption line in the transient X-ray pulsar 2S 1553-5422016In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 457, no 1, p. 258-266Article in journal (Refereed)
    Abstract [en]

    We report the results of a spectral and timing analysis of the poorly studied transient X-ray pulsar 2S 1553-542 using data collected with the NuSTAR and Chandra observatories and the Fermi/GBM instrument during an outburst in 2015. The properties of the source at high energies (>30 keV) are studied for the first time and the sky position has been essentially improved. The source broad-band spectrum has a quite complicated shape and can be reasonably described by a composite model with two continuum components - a blackbody emission with the temperature about 1 keV at low energies and a power law with an exponential cut-off at high energies. Additionally, an absorption feature at similar to 23.5 keV is discovered both in phase-averaged and phase-resolved spectra and interpreted as the cyclotron resonance scattering feature corresponding to the magnetic field strength of the neutron star B similar to 3 x 10(12) G. Based on the Fermi/ GBM data, the orbital parameters of the system were substantially improved, which allowed us to determine the spin period of the neutron star P = 9.27880(3) s and a local spin-up. (P) over dot similar or equal to -7.5 x 10(-10) s s(-1) due to the mass accretion during the NuSTAR observations. Assuming accretion from the disc and using standard torque models, we estimated the distance to the system as d = 20 +/- 4 kpc.

  • 37. Tsygankov, Sergey S.
    et al.
    Wijnands, Rudy
    Lutovinov, Alexander A.
    Degenaar, Nathalie
    Poutanen, Juri
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. University of Turku, Finland.
    The X-ray properties of Be/X-ray pulsars in quiescence2017In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 470, no 1, p. 126-141Article in journal (Refereed)
    Abstract [en]

    Observations of accreting neutron stars (NSs) with strong magnetic fields can be used not only for studying the accretion flow interaction with the NS magnetospheres, but also for understanding the physical processes inside NSs and for estimating their fundamental parameters. Of particular interest are (i) the interaction of a rotating NS (magnetosphere) with the infalling matter at different accretion rates, and (ii) the theory of deep crustal heating and the influence of a strong magnetic field on this process. Here, we present results of the first systematic investigation of 16 X-ray pulsars with Be optical companions during their quiescent states, based on data from the Chandra, XMM-Newton and Swift observatories. The whole sample of sources can be roughly divided into two distinct groups: (i) relatively bright objects with a luminosity around similar to 10(34) erg s(-1) and (hard) power-law spectra, and (ii) fainter ones showing thermal spectra. X-ray pulsations were detected from five objects in group (i) with quite a large pulse fraction of 50-70 per cent. The obtained results are discussed within the framework of the models describing the interaction of the infalling matter with the NS magnetic field and those describing heating and cooling in accreting NSs.

  • 38.
    Veledina, Alexandra
    et al.
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Univ Turku, Dept Phys & Astron, Turku 20014, Finland.;Stockholm Univ, Roslagstullsbacken 23, S-10691 Stockholm, Sweden.;Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia..
    Berdyugin, Andrei, V
    Univ Turku, Dept Phys & Astron, Turku 20014, Finland..
    Kosenkov, Ilia A.
    Univ Turku, Dept Phys & Astron, Turku 20014, Finland.;St Petersburg State Univ, Dept Astrophys, Univ Skiy Pr 28, St Petersburg 198504, Russia..
    Kajava, Jari J. E.
    Univ Turku, Finnish Ctr Astron ESO FINCA, Turku 20014, Finland..
    Tsygankov, Sergey S.
    Univ Turku, Dept Phys & Astron, Turku 20014, Finland.;Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia..
    Piirola, Vilppu
    Univ Turku, Dept Phys & Astron, Turku 20014, Finland..
    Berdyugin, Svetlana, V
    Kiepenheuer Inst Sonnenphys, Schoneckstr 6, D-79104 Freiburg, Germany.;Univ Hawaii, Inst Astron, 2680 Woodlawn Dr, Honolulu, HI 96822 USA..
    Sakanoi, Takeshi
    Tohoku Univ, Grad Sch Sci, Aoba Ku, Sendai, Miyagi 9808578, Japan..
    Kagitani, Masato
    Tohoku Univ, Grad Sch Sci, Aoba Ku, Sendai, Miyagi 9808578, Japan..
    Kravtsov, Vadim
    St Petersburg State Univ, Dept Astrophys, Univ Skiy Pr 28, St Petersburg 198504, Russia..
    Poutanen, Juri
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. Univ Turku, Dept Phys & Astron, Turku 20014, Finland.;Stockholm Univ, Roslagstullsbacken 23, S-10691 Stockholm, Sweden.;Russian Acad Sci, Space Res Inst, Profsoyuznaya Str 84-32, Moscow 117997, Russia..
    Evolving optical polarisation of the black hole X-ray binary MAXI J1820+0702019In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 623, article id A75Article in journal (Refereed)
    Abstract [en]

    Aims. The optical emission of black hole transients increases by several magnitudes during the X-ray outbursts. Whether the extra light arises from the X-ray heated outer disc, from the inner hot accretion flow, or from the jet is currently debated. Optical polarisation measurements are able to distinguish the relative contributions of these components. Methods. We present the results of BVR polarisation measurements of the black hole X-ray binary MAXI J1820+070 during the period of March-April 2018. Results. We detect small, similar to 0.7%, but statistically significant polarisation, part of which is of interstellar origin. Depending on the interstellar polarisation estimate, the intrinsic polarisation degree of the source is between similar to 0.3% and 0.7%, and the polarisation position angle is between similar to 10 degrees-30 degrees. We show that the polarisation increases after MJD 58222 (2018 April 14). The change is of the order of 0.1% and is most pronounced in the R band. The change of the source Stokes parameters occurs simultaneously with the drop of the observed V-band flux and a slow softening of the X-ray spectrum. The Stokes vectors of intrinsic polarisation before and after the drop are parallel, at least in the V and R filters. Conclusions. We suggest that the increased polarisation is due to the decreasing contribution of the non-polarized component, which we associate with the the hot flow or jet emission. The low polarisation can result from the tangled geometry of the magnetic field or from the Faraday rotation in the dense, ionised, and magnetised medium close to the black hole. The polarized optical emission is likely produced by the irradiated disc or by scattering of its radiation in the optically thin outflow.

  • 39.
    Veledina, Alexandra
    et al.
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. University of Turku, Finland.
    Gandhi, Poshak
    Hynes, Robert
    Kajava, Jari J. E.
    Tsygankov, Sergey S.
    Revnivtsev, Michail G.
    Durant, Martin
    Poutanen, Juri
    KTH, Centres, Nordic Institute for Theoretical Physics NORDITA. University of Turku, Finland.
    Expanding hot flow in the black hole binary SWIFT J1753.5-0127: evidence from optical timing2017In: Monthly notices of the Royal Astronomical Society, ISSN 0035-8711, E-ISSN 1365-2966, Vol. 470, no 1, p. 48-59Article in journal (Refereed)
    Abstract [en]

    We describe the evolution of optical and X-ray temporal characteristics during the outburst decline of the black hole X-ray binary SWIFT J1753.5-0127. The optical/ X-ray crosscorrelation function demonstrates a single positive correlation at the outburst peak, then it has multiple dips and peaks during the decline stage, which are then replaced by the precognition dip plus peak structure in the outburst tail. Power spectral densities and phase lags show a complex evolution, revealing the presence of intrinsically connected optical and X-ray quasi-periodic oscillations. For the first time, we quantitatively explain the evolution of these timing properties during the entire outburst within one model, the essence of which is the expansion of the hot accretion flow towards the tail of the outburst. The pivoting of the spectrum produced by synchrotron Comptonization in the hot flow is responsible for the appearance of the anticorrelation with the X-rays and for the optical quasi-periodic oscillations. Our model reproduces well the cross-correlation and phase lag spectrum during the decline stage, which could not be understood with any model proposed before.

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