| 1. |
- Tsygankov, Sergey S., et al.
(författare)
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X-Ray Pulsar XTE J1858+034 : Discovery of the Cyclotron Line and the Revised Optical Identification
- 2021
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Ingår i: Astrophysical Journal. - : IOP PUBLISHING LTD. - 0004-637X .- 1538-4357. ; 909:2
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Tidskriftsartikel (refereegranskat)abstract
- We present the results of a detailed investigation of the poorly studied X-ray pulsar (XRP) XTE J1858+034 based on the data obtained with the NuSTAR observatory during the outburst of the source in 2019. The spectral analysis resulted in the discovery of a cyclotron absorption feature in the source spectrum at similar to 48 keV in both the pulse phase-averaged and resolved spectra. Accurate X-ray localization of the source using the NuSTAR and Chandra observatories allowed us to accurately determine the position of the X-ray source and identify the optical companion of the pulsar. The analysis of the counterpart properties suggested that the system is likely a symbiotic binary hosting an XRP and a late-type companion star of the K-M classes rather than a Be X-ray binary as previously suggested.
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| 2. |
- Doroshenko, Victor, et al.
(författare)
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Luminosity dependence of the cyclotron line and evidence for the accretion regime transition in V 0332+53
- 2017
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press. - 0035-8711 .- 1365-2966. ; 466:2, s. 2143-2150
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 3. |
- Tsygankov, Sergey S., et al.
(författare)
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On the magnetic field of the first Galactic ultraluminous X-ray pulsar Swift J0243.6+6124
- 2018
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press. - 0035-8711 .- 1365-2966 .- 1745-3925 .- 1745-3933. ; 479:1, s. L134-L138
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 4. |
- Tsygankov, Sergey S., et al.
(författare)
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The unusual behavior of the young X-ray pulsar SXP 1062 during the 2019 outburst
- 2020
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 637
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Tidskriftsartikel (refereegranskat)abstract
- We present the results of the first dedicated observation of the young X-ray pulsar SXP 1062 in the broad X-ray energy band obtained during its 2019 outburst with the NuSTAR and XMM-Newton observatories. The analysis of the pulse-phase averaged and phase-resolved spectra in the energy band from 0.5 to 70 keV did not reveal any evidence for the presence of a cyclotron line. The spin period of the pulsar was found to have decreased to 979.48 +/- 0.06 s implying a similar to 10% reduction compared to the last measured period during the monitoring campaign conducted about five years ago, and is puzzling considering that the system apparently has not shown major outbursts ever since. The switch of the pulsar to the spin-up regime supports the common assumption that torques acting on the accreting neutron star are nearly balanced and thus SXP 1062 likely also spins with a period close to the equilibrium value for this system. The current monitoring of the source also revealed a sharp drop in its soft X-ray flux right after the outburst, which is in drastic contrast to the behavior during the previous outburst when the pulsar remained observable for years with only a minor flux decrease after the end of the outburst. This unexpected off state of the source lasted for at most 20 days after which SXP 1062 returned to the level observed during previous campaigns. We discuss this and other findings in context of the modern models of accretion onto strongly magnetized neutron stars.
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| 5. |
- Mushtukov, Alexander A., et al.
(författare)
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Positive correlation between the cyclotron line energy and luminosity in sub-critical X-ray pulsars : Doppler effect in the accretion channel
- 2015
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press. - 0035-8711 .- 1365-2966. ; 454:3, s. 2714-2721
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 6. |
- Monkkonen, Juhani, et al.
(författare)
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Evidence for the radiation-pressure dominated accretion disk in bursting pulsar GRO J1744-28 using timing analysis
- 2019
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Ingår i: Astronomy and Astrophysics. - : EDP SCIENCES S A. - 0004-6361 .- 1432-0746. ; 626
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 7. |
- Mushtukov, Alexander A., et al.
(författare)
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Compton scattering S matrix and cross section in strong magnetic field
- 2016
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Ingår i: Physical Review D. - : American Psychological Association (APA). - 2470-0010 .- 2470-0029. ; 93:10
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 8. |
- Mushtukov, Alexander A., et al.
(författare)
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Pulsating ULXs : large pulsed fraction excludes strong beaming
- 2021
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 501:2, s. 2424-2429
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Tidskriftsartikel (refereegranskat)abstract
- The recent discovery of pulsating ultraluminous X-ray sources (ULXs) shows that the apparent luminosity of accreting neutron stars can exceed the Eddington luminosity by a factor of 100s. The relation between the actual and apparent luminosity is a key ingredient in theoretical models of ULXs, but it is still under debate. A typical feature of the discovered pulsating ULXs is a large pulsed fraction (PF). Using Monte Carlo simulations, we consider a simple geometry of accretion flow and test the possibility of simultaneous presence of a large luminosity amplification due the geometrical beaming and a high PF. We argue that these factors largely exclude each other and only a negligible fraction of strongly beamed ULX pulsars can show PF above 10 per cent. Discrepancy between this conclusion and current observations indicates that pulsating ULXs are not strongly beamed and their apparent luminosity is close to the actual one.
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| 9. |
- Mushtukov, Alexander A., et al.
(författare)
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Ultraluminous X-ray sources as neutrino pulsars
- 2018
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Ingår i: Monthly notices of the Royal Astronomical Society. - : OXFORD UNIV PRESS. - 0035-8711 .- 1365-2966. ; 476:3, s. 2867-2873
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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