Лаборатория фундаментальной и прикладной рентгеновской астрофизики
2024
2023
1
[article]
12/2023
Monthly Notices of the Royal Astronomical Society
The first X-ray polarimetric observation of the black hole binary LMC X-1
We report on an X-ray polarimetric observation of the high-mass X-ray binary LMC X-1 in the high/soft state, obtained by the Imaging X-ray Polarimetry Explorer (IXPE) in 2022 October. The measured polarization is below the minimum detectable polarization of 1.1 per cent (at the 99 per cent confidence level). Simultaneously, the source was observed with the Neutron Star Interior Composition Explorer (NICER), Nuclear Spectroscopic Telescope Array (NuSTAR), and Spectrum-Rontgen-Gamma (SRG)/Astronomical Roentgen Telescope - X-ray Concentrator (ART-XC) instruments, which enabled spectral decomposition into a dominant thermal component and a Comptonized one. The low 2-8 keV polarization of the source did not allow for strong constraints on the black hole spin and inclination of the accretion disc. However, if the orbital inclination of about 36° is assumed, then the upper limit is consistent with predictions for pure thermal emission from geometrically thin and optically thick discs. Assuming the polarization degree of the Comptonization component to be 0, 4, or 10 per cent, and oriented perpendicular to the polarization of the disc emission (in turn assumed to be perpendicular to the large-scale ionization cone orientation detected in the optical band), an upper limit to the polarization of the disc emission of 1.0, 0.9, or 0.9 per cent, respectively, is found (at the 99 per cent confidence level).
[article]
10/2023
Astronomy and Astrophysics
X-ray polarimetry of the accreting pulsar GX 301−2
The phase- and energy-resolved polarization measurements of accreting X-ray pulsars (XRPs) allow us to test different theoretical models of their emission, and they also provide an avenue to determine the emission region geometry. We present the results of the observations of the XRP GX 301−2 performed with the Imaging X-ray Polarimetry Explorer (IXPE). A persistent XRP, GX 301−2 has one of the longest spin periods known: ∼680 s. A massive hyper-giant companion star Wray 977 supplies mass to the neutron star via powerful stellar winds. We did not detect significant polarization in the phase-averaged data when using spectro-polarimetric analysis, with the upper limit on the polarization degree (PD) of 2.3% (99% confidence level). Using the phase-resolved spectro-polarimetric analysis, we obtained a significant detection of polarization (above 99% confidence level) in two out of nine phase bins and a marginal detection in three bins, with a PD ranging between ∼3% and ∼10% and a polarization angle varying in a very wide range from ∼0° to ∼160°. Using the rotating vector model, we obtained constraints on the pulsar geometry using both phase-binned and unbinned analyses, finding excellent agreement. Finally, we discuss possible reasons for a low observed polarization in GX 301−2.
[article]
10/2023
Monthly Notices of the Royal Astronomical Society
RX J0440.9+4431: another supercritical X-ray pulsar
In the beginning of 2023, the Be transient X-ray pulsar RX J0440.9+4431 underwent a first-ever giant outburst observed from the source peaking in the beginning of February and reaching peak luminosity of ≈4.3 × 1037 erg s-1. Here, we present the results of a detailed spectral and temporal study of the source based on NuSTAR, Swift, INTEGRAL, and NICER observations performed during this period and covering wide range of energies and luminosities. We find that both the pulse profile shape and spectral hardness change abruptly around ≈2.8 × 1037 erg s-1, which we associate with a transition to supercritical accretion regime and erection of the accretion column. The observed pulsed fraction decreases gradually with energy up to 20 keV (with a local minimum around fluorescence iron line), which is unusual for an X-ray pulsar, and then rises rapidly at higher energies with the pulsations significantly detected up to ≈120 keV. The broad-band energy spectra of RX J0440.9+4431 at different luminosity states can be approximated with a two-hump model with peaks at energies of about 10-20 and 50-70 keV previously suggested for other pulsars without additional features. In particular, an absorption feature around 30 keV previously reported and interpreted as a cyclotron line in the literature appears to be absent when using this model, so the question regarding the magnetic field strength of the neutron star remains open. Instead, we attempted to estimate field using several indirect methods and conclude that all of them point to a relatively strong field of around B ~ 1013 G.
[article]
09/2023
Monthly Notices of the Royal Astronomical Society
X-ray polarimetry of X-ray pulsar X Persei: another orthogonal rotator?
X Persei is a persistent low-luminosity X-ray pulsar of period of ≈ 835 s in a Be binary system. The field strength at the neutron star surface is not known precisely, but indirect signs indicate a magnetic field above 1013 G, which makes the object one of the most magnetized known X-ray pulsars. Here we present the results of observations X Persei performed with the Imaging X-ray Polarimetry Explorer (IXPE). The X-ray polarization signal was found to be strongly dependent on the spin phase of the pulsar. The energy-averaged polarization degree in 3-8 keV band varied from several to ~20 per cent over the pulse with a phase dependence resembling the pulse profile. The polarization angle shows significant variation and makes two complete revolutions during the pulse period, resulting in nearly nil pulse-phase averaged polarization. Applying the rotating vector model to the IXPE data we obtain the estimates for the rotation axis inclination and its position angle on the sky, as well as for the magnetic obliquity. The derived inclination is close to the orbital inclination, reported earlier for X Persei. The polarimetric data imply a large angle between the rotation and magnetic dipole axes, which is similar to the result reported recently for the X-ray pulsar GRO J1008-57. After eliminating the effect of polarization angle rotation over the pulsar phase using the best-fitting rotating vector model, the strong dependence of the polarization degree with energy was discovered, with its value increasing from 0 at ~2 keV to 30per cent at 8 keV.
[article]
08/2023
Astronomy and Astrophysics
Simultaneous and panchromatic observations of the fast radio burst FRB 20180916B
Aims. Fast radio bursts are bright radio transients whose origins are not yet understood. The search for a multi-wavelength counterpart of those events can set a tight constraint on the emission mechanism and the progenitor source. Methods. We conducted a multi-wavelength observational campaign on FRB 20180916B between October 2020 and August 2021 over eight activity cycles of the source. Observations were carried out in the radio band by the SRT both at 336 and 1547 MHz and the uGMRT at 400 MHz. Simultaneous observations were conducted by the optical telescopes Asiago (Galileo and Copernico), CMO SAI MSU, CAHA 2.2 m, RTT-150 and TNG, and X/γ-ray detectorss on board the AGILE, Insight-HXMT, INTEGRAL, and Swift satellites. Results. We present the detection of 14 new radio bursts detected with the SRT at 336 MHz and seven new bursts with the uGMRT from this source. We provide the deepest prompt upper limits in the optical band for FRB 20180916B to date. In fact, the TNG/SiFAP2 observation simultaneous to a burst detection by uGMRT gives an upper limit Eoptical/Eradio < 1.3 × 102. Another burst detected by the SRT at 336 MHz was also co-observed by Insight-HXMT. The non-detection in the X-rays yields an upper limit (1-30 keV band) of EX-ray/Eradio in the range of (0.9-1.3) × 107, depending on the model that is considered for the X-ray emission.
[article]
05/2023
Astronomy Letters
Spectral and Temporal Analysis of the Supergiant Fast X-ray Transient IGR J16195-4945 with SRG/ART-XC
We present the results of the analysis of the SRG/ART-XC observation of the Supergiant Fast X-ray Transient IGR J16195-4545 performed on March 3, 2021. Six bright flares are present in the light curve, with no significant change in hardness occuring during these flares. The spectrum is described with an absorbed power law model with a high energy exponential cutoff showing heavy absorption, with $N_{\textrm{H}}=(12± 2)× 10^{22} cm^{-2}$ and $Γ=0.56± 0.15$, $E_{\textrm{cut}}=13± 2$ keV. Adopting the Bayesian block decomposition of the light curve, we measured the properties of the observed flares (duration, rise time, waiting time, released energy and pre-flare luminosity), which are consistent with the quasi-spherical subsonic accretion model. The stellar wind velocity of the supergiant is estimated to be $vw≈ 500$ km s${}^{-1}$. Additionally, the system was found to have an unusual near-IR variability.
[article]
05/2023
The Astrophysical Journal
Properties of the Extremely Energetic GRB 221009A from Konus-WIND and SRG/ART-XC Observations
We report on Konus-WIND (KW) and Mikhail Pavlinsky Astronomical Roentgen Telescope - X-ray Concentrator (ART-XC) observations and analysis of a nearby GRB 221009A, the brightest γ-ray burst (GRB) detected by KW for >28 yr of observations. The prompt, pulsed phase of the burst emission lasts for ~600 s and is followed by a steady power-law decay lasting for more than 25 ks. From the analysis of the KW and ART-XC light curves and the KW spectral data, we derive time-averaged spectral peak energy of the burst E p ≈ 2.6 MeV, E p at the brightest emission peak ≈ 3.0 MeV, the total 20 keV-10 MeV energy fluence of ≈0.22 erg cm-2, and the peak energy flux in the same band of ≈0.031 erg cm-2 s-1. The enormous observed fluence and peak flux imply, at redshift z = 0.151, huge values of isotropic energy release E iso ≈ 1.2 × 1055 erg (or ≳6.5 solar rest mass) and isotropic peak luminosity L iso ≈ 3.4 × 1054 erg s-1 (64 ms scale), making GRB 221009A the most energetic and one of the most luminous bursts observed since the beginning of the GRB cosmological era in 1997. The isotropic energetics of the burst fit nicely both "Amati" and "Yonetoku" hardness-intensity correlations for >300 KW long GRBs, implying that GRB 221009A is most likely a very hard, super-energetic version of a "normal" long GRB.
2022
1
[article]
10/2022
Monthly Notices of the Royal Astronomical Society
ULX pulsar Swift J0243.6+6124 observations with NuSTAR: dominance of reflected emission in the super-Eddington state
We report the discovery of the bright reflected emission component in the super-Eddington state of the ultraluminous X-ray pulsar Swift J0243.6+6124, based on the NuSTAR observations of the source during its 2017 outburst. The flux of the reflected emission is weakly variable over the pulsar phase while the direct emission shows significantly larger pulsation amplitude. We propose that in this system the neutron star finds itself in the centre of the well formed by the inner edge of the geometrically thick super-Eddington accretion disc truncated by the magnetic field of the pulsar. The aspect ratio of the well is H/R ~ 1. The inner edge of the truncated disc is continuously illuminated by the emission of the accretion column giving rise to the weakly variable reflected emission. As the neutron star rotates, its emission sweeps through the line of sight, giving rise to the pulsating direct emission. From Doppler broadening of the iron line, we measure the truncation radius of the accretion disc ~50 Rg. The inferred dipole component of the magnetic field is consistent with previous estimates favouring a not very strong field. The uniqueness of this system is determined by its moderately super-Eddington accretion rate and the moderate magnetic field so that the inner edge of the truncated geometrically thick accretion disc is seen from the neutron star at a large solid angle.
[article]
09/2022
Monthly Notices of the Royal Astronomical Society
Constraints on the magnetic field structure in accreting compact objects from aperiodic variability
We investigate the aperiodic variability for a relatively large sample of accreting neutron stars and intermediate polars, focusing on the properties of the characteristic break commonly observed in power spectra of accreting objects. In particular, we investigate the relation of the break frequency and the magnetic field strength, both of which are connected to the size of the magnetosphere. We find that for the majority of objects in our sample the measured break frequency values indeed agree with estimated inner radii of the accretion disc, which allows to use observed break frequencies to independently assess the magnetic field strength and structure in accreting compact objects. As a special case, we focus on Hercules X-1 which is a persistent, medium-luminosity X-ray pulsar accreting from its low-mass companion. In the literature, it has been suggested that the complex pulse profiles, the spin-up behaviour and the luminosity-correlation of the cyclotron energy seen in Her X-1 can be explained with a complex magnetic field structure of the neutron star. Here, we connect the measured break frequency to the magnetospheric radius and show that the magnetic field strength derived assuming a dipole configuration is nearly an order of magnitude smaller than the magnetic field strength corresponding to the cyclotron energy. Accordingly, this discrepancy can be explained with the magnetic field having strong multipole components. The multipolar structure would also increase the accreting area on the neutron star surface, explaining why the critical luminosity for accretion column formation is puzzlingly high in this source.
[article]
05/2022
Astronomy and Astrophysics
Phase-resolved X-ray spectroscopy of PSR B0656+14 with SRG/eROSITA and XMM-Newton
We present a detailed spectroscopic and timing analysis of X-ray observations of the bright pulsar PSR B0656+14. The observations were obtained simultaneously with eROSITA and XMM-Newton during the calibration and performance verification phase of the Spektrum-Roentgen-Gamma mission (SRG). The analysis of the 100 ks deep observation of eROSITA is supported by archival observations of the source, including XMM-Newton, NuSTAR, and NICER. Using XMM-Newton and NICER, we first established an X-ray ephemeris for the time interval 2015 to 2020, which connects all X-ray observations in this period without cycle count alias and phase shifts. The mean eROSITA spectrum clearly reveals an absorption feature originating from the star at 570 eV with a Gaussian σ of about 70 eV that was tentatively identified in a previous long XMM-Newton observation. A second previously discussed absorption feature occurs at 260-265 eV and is described here as an absorption edge. It could be of atmospheric or of instrumental origin. These absorption features are superposed on various emission components that are phenomenologically described here as the sum of hot (120 eV) and cold (65 eV) blackbody components, both of photospheric origin, and a power law with photon index Γ = 2 from the magnetosphere. We created energy-dependent light curves and phase-resolved spectra with a high signal-to-noise ratio. The phase-resolved spectroscopy reveals that the Gaussian absorption line at 570 eV is clearly present throughout ~60% of the spin cycle, but it is otherwise undetected. Likewise, its parameters were found to be dependent on phase. The visibility of the line strength coincides in phase with the maximum flux of the hot blackbody. If the line originates from the stellar surface, it nevertheless likely originates from a different location than the hot polar cap. We also present three families of model atmospheres: a magnetized atmosphere, a condensed surface, and a mixed model. They were applied to the mean observed spectrum, whose continuum fit the observed data well. The atmosphere model, however, predicts distances that are too short. For the mixed model, the Gaussian absorption may be interpreted as proton cyclotron absorption in a field as high as 1014 G, which is significantly higher than the field derived from the moderate observed spin-down. Based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and NASA.
[article]
05/2022
Astronomy and Astrophysics
SRG/ART-XC, Swift, NICER, and NuSTAR study of different states of the transient X-ray pulsar MAXI J0903-531
The results of the broadband spectral and timing study of the recently discovered transient X-ray pulsar MAXI J0903-531 in a wide range of luminosities that differ by a factor of ~30 are reported. The observed X-ray spectrum in both states can be described as a classical pulsar-like spectrum consisting of a power law with a high-energy cutoff. We argue that the absence of the spectrum transformation to the two-hump structure that is expected at low fluxes indicates that the magnetic field of the neutron star is relatively weak below (2-3) × 1012 G. This estimate is consistent with other indirect constraints and non-detection of any absorption features that might be interpreted as a cyclotron absorption line. The timing analysis of the NuSTAR data revealed only slight variations of a single-peaked pulse profile of the source as a function of the energy band and mass accretion rate. In both intensity states, the pulsed fraction increases from 40% to roughly 80% with the energy. Finally, we were also able to obtain the orbital solution for the binary system using data from the Fermi/GBM, NICER, and NuSTAR instruments.
[article]
05/2022
Astronomy and Astrophysics
Losing a minute every two years: SRG X-ray view of the rapidly accelerating X-ray pulsar SXP 1323
The source SXP 1323 is a peculiar high-mass X-ray binary located in the Small Magellanic Cloud. It is renowned for its rapid spin-up. We investigate for the first time broadband X-ray properties of SXP 1323 as observed by the Mikhail Pavlinsky ART-XC and eROSITA telescopes on board the Spectrum-Roentgen-Gamma observatory. Using ART-XC and eROSITA data, we produced a first broadband 1−20 keV X-ray spectrum and estimated the pulsed fraction box 8 keV. With the addition of archival XMM-Newton observations, we traced the evolution of the spin period of SXP 1323 over the last five years and found that after 2016, the source switched to a linear spin-up with a rate of −29.9 s yr−1. The broadband X-ray spectrum is typical for accreting X-ray pulsars. It has a steep power-law index (Γ = −0.15) and an exponential cutoff energy of 5.1 keV. No significant difference between spectra obtained in states with and without pulsations were found.
[article]
04/2022
Astronomy Letters
All-Sky Monitor for High-Accuracy Measurement of the Cosmic X-ray Background Onboard the International Space Station
The MVN (Monitor Vsego Neba, all-sky monitor) experiment onboard the International Space Station for high-accuracy measurement of the cosmic X-ray background by the aperture modulation method being planned in 2022-2025 is considered. The technique that allows the measurement error within the MVN experiment to be minimized is described. Simulations of the main results of the experiment have been performed. They have shown that a record accuracy of measuring the cosmic X-ray background can be achieved. The main MVN systems are considered, and the results of the flight and ground-based experiments to measure the parameters of these systems are presented.
[article]
02/2022
Monthly Notices of the Royal Astronomical Society
Physical modelling of viscous disc evolution around magnetized neutron star. Aql X-1 2013 outburst decay
We present a model of a viscously evolving accretion disc around a magnetized neutron star. The model features the varying outer radius of the hot ionized part of the disc due to cooling and the varying inner radius of the disc due to interaction with the magnetosphere. It also includes hindering of accretion on the neutron star because of the centrifugal barrier and irradiation of the outer disc and companion star by X-rays from the neutron star and disc. When setting inner boundary conditions, we take into account that processes at the inner disc occur on a time-scale much less than the viscous time-scale of the whole disc. We consider three types of outflow from the disc inner edge: zero outflow, one based on MHD calculations, and a very efficient propeller mechanism. The light curves of an X-ray transient after the outburst peak can be calculated by a corresponding, publicly available code. We compare observed light curves of the 2013 burst of Aql X-1 in X-ray and optical bands with modelled ones. We find that the fast drop of the 0.3-10 keV flux can be solely explained by a radial shrinking of the hot disc. At the same time, models with the neutron star magnetic field >108 G have better fits because the accretion efficiency behaviour emphasizes the 'knee' on the light curve. We also find that a plato emission can be produced by a `disc-reservoir' with stalled accretion.
[article]
02/2022
Monthly Notices of the Royal Astronomical Society
On the nature of the X-ray pulsar XTE J1859+083 and its broad-band properties
This work is devoted to the study of the broad-band 0.8-79 keV spectral and timing properties of the poorly studied X-ray pulsar XTE J1859+083 during its 2015 outburst based on the data from the NuSTAR and Swift observatories. We show that the source pulse profile has complex shape that depends on the energy band. Pulse fraction of XTE J1859+083 has constant value around 35 per cent in the broad energy band, this behaviour is atypical for X-ray pulsars. At the same time its energy spectrum is typical of this class of objects and has a power-law shape with an exponential cutoff at high energies. No cyclotron absorption line was discovered in the source spectrum. On the basis of indirect method and the absence of a cyclotron line, an estimation was made for the magnetic field strength as less than 5 × 1011 G or belonging to the interval from 5 × 1012 to $2.0^{+0.9}_{-1.2}\times 10^{13}$ G. Data from the NOT and SALT telescopes as well as optical and IR sky surveys allowed us also to study the nature of its optical companion. We have proposed and studied new possible candidates for the optical companion of XTE J1859+083 and the most likely candidate was identified. The results of the optical and IR photometry and spectroscopy of these possible companions showed that the system is a Be X-ray binary, showing Brγ, He I, and strong H α spectral lines.
[article]
01/2022
Astronomy and Astrophysics
Broad-band analysis of X-ray pulsar 2S 1845-024
We present the results of a detailed investigation of the poorly studied X-ray pulsar 2S 1845−024 based on data obtained at the NuSTAR observatory during the type I outburst in 2017. Neither pulse phase-averaged nor phase-resolved spectra of the source show evidence for a cyclotron absorption feature. We also used data obtained from other X-ray observatories (Swift, XMM-Newton and Chandra) to study the spectral properties as a function of orbital phase. The analysis reveals a high hydrogen column density for the source reaching ∼1024 cm−2 around periastron. Using high-quality Chandra data we were able to obtain an accurate localization of 2S 1845−024 at RA = 18h48m16.s8 and Dec = −2°25'25.″1 (J2000), which allowed us to use infrared (IR) data to roughly classify the optical counterpart of the source as an OB supergiant at a distance of ≳15 kpc.
2021
1
[article]
09/2021
Monthly Notices of the Royal Astronomical Society
Pulsating iron spectral features in the emission of X-ray pulsar V 0332+53
We present results of phase- and time-resolved study of iron spectral features in the emission of the Be/X-ray transient pulsar V 0332+53 during its type II outburst in 2004 using archival RXTE/PCA data. Coherent pulsations of both fluorescent iron line at ≈6.4 keV and neutral iron K-edge at ≈7.1 keV have been detected throughout the entire outburst. The pulsating iron K-edge is reported for the first time for this object. Near the peak of the outburst, the 3-12 keV pulse profile shows two deep, Fmax/Fmin ~ 2, and narrow dips of nearly identical shape, separated by exactly Δϕ = 0.5 in phase. The dip spectra are nearly identical to each other and very similar in shape to the spectra outside the dips. The iron K-edge peaks at the phase intervals corresponding to the dips, although its optical depth τK ~ 0.05 is by far insufficient to explain the dips. The iron line shows pulsations with a complex pulse profile without any obvious correlation with the total flux or optical depth of the K-edge. Accounting for the component associated with reprocessing of the pulsar emission by the surface of the donor star and circumstellar material, we find a very high pulsation amplitude of the iron line flux, Fmax/Fmin ~ 10. We demonstrate that these properties of V 0332+53 cannot be explained by contemporary emission models for accreting X-ray pulsars and speculate about the origin of the observed iron spectral features.
[article]
09/2021
Monthly Notices of the Royal Astronomical Society
Two-blackbody portraits of radiation from magnetized neutron stars
We study a simple model describing thermal radiation spectra from magnetized neutron stars. The model assumes that a star is nearly spherical and isothermal inside and possesses dipole magnetic fields (B ≲ 1014 G) near the surface, which make the surface temperature distribution non-uniform. We assume further that any surface element emits a blackbody (BB) spectrum with a local effective temperature. We show that such thermal spectra (including phase-resolved) are accurately approximated by simple equivalent two-BB (2BB) models. We introduce and study phase-space maps of 2BB parameters and show that these maps can be useful for interpreting neutron star observations, in which 2BB spectral fits have been done.
[article]
08/2021
Astronomy and Astrophysics
Spectral evolution of X-ray pulsar 4U 1901+03 during the 2019 outburst based on Insight-HXMT and NuSTAR observations
We report on a detailed spectral analysis of emission from X-ray pulsar 4U 1901+03 using data obtained by the Insight-HXMT and NuSTAR observatories during the 2019 outburst of the source. Thanks to the extensive coverage of the outburst by Insight-HXMT, we were able to investigate the spectral evolution of the source as a function of flux, and compare these results to the previous reports, focusing on the properties of a putative absorption feature at around 10 keV. In particular, we demonstrate that the broadband X-ray continuum of 4U 1901+03 can be well described with a two-component continuum model without an absorption line at 10 keV, which casts doubt on its interpretation as a cyclotron line. The high quality of the data also allowed us to perform both phase-averaged and phase-resolved spectral analyses as a function of luminosity. Finally, we performed a detailed investigation of another absorption feature in the spectrum of the source around 30 keV recently reported in the NuSTAR data. We show that this feature appears to be significantly detected both in phase-averaged and phase-resolved spectra irrespective of the continuum model.
[article]
07/2021
Physics Reports
Heat blanketing envelopes of neutron stars
Near the surface of any neutron star there is a thin heat blanketing envelope that produces substantial thermal insulation of warm neutron star interiors and that relates the internal temperature of the star to its effective surface temperature. Physical processes in the blanketing envelopes are reasonably clear but the chemical composition is not. The latter circumstance complicates inferring physical parameters of matter in the stellar interiors from observations of the thermal surface radiation of the stars and urges one to elaborate the models of blanketing envelopes. We outline physical properties of these envelopes, particularly, the equation of state, thermal conduction, ion diffusion and others. Various models of heat blankets are reviewed, such as composed of separate layers of different elements, or containing diffusive binary ion mixtures in or out of diffusion equilibrium. The effects of strong magnetic fields in the envelopes are outlined as well as the effects of high temperatures which induce strong neutrino emission in the envelopes themselves. Finally, we discuss how the properties of the heat blankets affect thermal evolution of neutron stars and the ability to infer important information on internal structure of neutron stars from observations.
[article]
07/2021
Universe
Multi-Wavelength Properties of the 2021 Periastron Passage of PSR B1259-63
PSR B1259-63 is a gamma-ray binary system hosting a radio pulsar orbiting around a O9.5Ve star, LS 2883, with a period of ∼3.4 years. The interaction of the pulsar wind with the LS 2883 outflow leads to unpulsed broadband emission in the radio, X-ray, GeV, and TeV domains. One of the most unusual features of the system is an outburst of GeV energies around the periastron, during which the energy release substantially exceeds the spin down luminosity under the assumption of the isotropic emission. In this paper, we present the first results of a recent multi-wavelength campaign (radio, optical, and X-ray bands) accompanied by the analysis of publicly available GeV Fermi/LAT data. The campaign covered a period of more than 100 days around the 2021 periastron and revealed substantial differences from previously observed passages. We report a major delay of the GeV flare, weaker X-ray flux during the peaks, which are typically attributed to the times when the pulsar crosses the disk, and the appearance of a third X-ray peak never observed before. We argue that these features are consistent with the emission cone model proposed by us previously, in the case of a sparser and clumpier disk of the Be star.
[article]
05/2021
Astronomy and Astrophysics
Broadband X-ray spectra and timing of the accreting millisecond pulsar Swift J1756.9-2508 during its 2018 and 2019 outbursts
The accreting millisecond X-ray pulsar Swift J1756.9-2508 launched into an outburst in April 2018 and June 2019 - 8.7 years after the previous period of activity. We investigated the temporal, timing, and spectral properties of these two outbursts using data from NICER, XMM-Newton, NuSTAR, INTEGRAL, Swift, and Insight-HXMT. The two outbursts exhibited similar broadband spectra and X-ray pulse profiles. For the first time, we report the detection of the pulsed emission up to ∼100 keV that was observed by Insight-HXMT during the 2018 outburst. We also found the pulsation up to ∼60 keV that was observed by NICER and NuSTAR during the 2019 outburst. We performed a coherent timing analysis combining the data from the two outbursts. The binary system is well described by a constant orbital period over a time span of ∼12 years. The time-averaged broadband spectra are well fitted by the absorbed thermal Comptonization model COMPPS in a slab geometry with an electron temperature, kTe = 40-50 keV, Thomson optical depth τ ∼ 1.3, blackbody seed photon temperature kTbb, seed ∼ 0.7-0.8 keV, and hydrogen column density of NH ∼ 4.2 × 1022 cm−2. We searched the available data for type-I (thermonuclear) X-ray bursts, but found none, which is unsurprising given the estimated low peak accretion rate (≈0.05 of the Eddington rate) and generally low expected burst rates for hydrogen-poor fuel. Based on the history of four outbursts to date, we estimate the long-term average accretion rate at roughly 5 × 10−12 M⊙ yr−1 for an assumed distance of 8 kpc. The expected mass transfer rate driven by gravitational radiation in the binary implies the source may be no closer than 4 kpc. Swift J1756.9-2508 is the third low mass X-ray binary exhibiting "double" outbursts, which are separated by much shorter intervals than what we typically see and are likely to result from interruption of the accretion flow from the disk onto the neutron star. Such behavior may have important implications for the disk instability model.
[article]
04/2021
Experimental Astronomy
MVN experiment - All sky monitor for measuring cosmic X-ray background of the universe onboard the ISS
The article describes the MVN experiment (MVN - Monitor Vsego Neba in Russian transliteration) - All Sky Monitor onboard the ISS, which will start in 2021. The main scientific task of the experiment is to measure the cosmic X-ray background in energy range of 6 - 70 keV with unprecedented high absolute and spectral accuracy (about 1%). To reach this goal, Space Research Institute of the Russian Academy of Sciences has developed scientific payload called MVN. The main instrument of MVN is the SPIN-X1-MVN X-ray monitor, which is equipped with four CdTe based semiconductor detectors. The X-ray monitor will be mounted on the external surface of the ISS with zenith orientation. The structure of the monitor including the key systems (detectors, gears, thermal control system) is considered. Measuring of the cosmic X-ray background is carried out by the aperture modulation method described in detail in this article. Also we describe an optimal algorithm of the obtained in experiment data filtering, which will provide us with highest quality of the CXB measurement.
[article]
04/2021
Astronomy Letters
Phase-Resolved Spectroscopy of the Magnetar SGR J1745-2900 Based on Data from the NuSTAR Observatory
The magnetar SGR J1745-2900 located in the vicinity of the supermassive black hole Sgr A${}^{\star}$ was detected during its X-ray outburst with the Swift/XRT telescope in April 2013. For several months after its detection the source was observed with the NuSTAR observatory, which allowed pulsations with a period ${∼}3.76$ s to be recorded. Using these observations, we have studied in detail the dependence of the pulse profile and the pulsed fraction on the energy and intensity of the magnetar. The pulsed fraction in the 3-5 and 5-10 keV energy bands is shown to be 40-50$%$, slightly increasing with decreasing flux. We have performed phase-resolved spectroscopy for the source in the energy range from 3 to ${∼}40$ keV and show that the temperature of the emitting regions remains fairly stable during the pulse, while their apparent size changes significantly with phase.
[article]
02/2021
Monthly Notices of the Royal Astronomical Society
Pulsating ULXs: large pulsed fraction excludes strong beaming
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.
2020
1
[article]
12/2020
New Astronomy Reviews
The INTEGRAL view of the pulsating hard X-ray sky: from accreting and transitional millisecond pulsars to rotation-powered pulsars and magnetars
In the last 25 years a new generation of X-ray satellites imparted a significant leap forward in our knowledge of X-ray pulsars. The discovery of accreting and transitional millisecond pulsars proved that disk accretion can spin up a neutron star to a very high rotation speed. The detection of MeV-GeV pulsed emission from a few hundreds of rotation-powered pulsars probed particle acceleration in the outer magnetosphere, or even beyond. Also, a population of two dozens of magnetars has emerged. INTEGRAL played a central role to achieve these results by providing instruments with high temporal resolution up to the hard X-ray/soft, γ-ray band and a large field of view imager with good angular resolution to spot hard X-ray transients. In this article we review the main contributions by INTEGRAL to our understanding of the pulsating hard X-ray sky, such as the discovery and characterization of several accreting and transitional millisecond pulsars, the generation of the first catalog of hard X-ray/soft γ-ray rotation-powered pulsars, the detection of polarization in the hard X-ray emission from the Crab pulsar, and the discovery of persistent hard X-ray emission from several magnetars.
[article]
09/2020
Astronomy and Astrophysics
Magnetospheric return-current-heated atmospheres of rotation-powered millisecond pulsars
We computed accurate atmosphere models of rotation-powered millisecond pulsars in which the polar caps of a neutron star (NS) are externally heated by magnetospheric return currents. The external ram pressure, energy losses, and stopping depth of the penetrating charged particles were computed self-consistently with the atmosphere model, instead of assuming a simplified deep-heated atmosphere in radiative equilibrium. We used exact Compton scattering formalism to model the properties of the emergent X-ray radiation. The deep-heating approximation was found to be valid only if most of the heat originates from ultra-relativistic bombarding particles with Lorentz factors of γ ≳ 100. In the opposite regime, the atmosphere attains a distinct two-layer structure with an overheated optically thin skin on top of an optically thick cool plasma. The overheated skin strongly modifies the emergent radiation: It produces a Compton-upscattered high-energy tail in the spectrum and alters the radiation beaming pattern from limb darkening to limb brightening for emitted hard X-rays. This kind of drastic change in the emission properties can have a significant impact on the inferred NS pulse profile parameters as performed, for example, by Neutron star Interior Composition ExploreR. Finally, the connection between the energy distribution of the return current particles and the atmosphere emission properties offers a new tool to probe the exact physics of pulsar magnetospheres.
[article]
09/2020
Astronomy and Astrophysics
High-energy characteristics of the accretion-powered millisecond pulsar IGR J17591-2342 during its 2018 outburst. XMM-Newton, NICER, NuSTAR, and INTEGRAL view of the 0.3-300 keV X-ray band
IGR J17591-2342 is an accreting millisecond X-ray pulsar, discovered with INTEGRAL, which went into outburst around July 21, 2018. To better understand the physics acting in these systems during the outburst episode, we performed detailed temporal-, timing-, and spectral analyses across the 0.3-300 keV band using data from NICER, XMM-Newton, NuSTAR, and INTEGRAL. The hard X-ray 20-60 keV outburst profile covering ∼85 days is composed of four flares. Over the course of the maximum of the last flare, we discovered a type-I thermonuclear burst in INTEGRAL JEM-X data, posing constraints on the source distance. We derived a distance of 7.6 ± 0.7 kpc, adopting Eddington-limited photospheric radius expansion and assuming anisotropic emission. In the timing analysis, using all NICER 1-10 keV monitoring data, we observed a rather complex set of behaviours starting with a spin-up period (MJD 58345-58364), followed by a frequency drop (MJD 58364-58370), an episode of constant frequency (MJD 58370-58383), concluded by irregular behaviour till the end of the outburst. The 1-50 keV phase distributions of the pulsed emission, detected up to ∼120 keV using INTEGRAL ISGRI data, was decomposed in three Fourier harmonics showing that the pulsed fraction of the fundamental increases from ∼10% to ∼17% going from ∼1.5 to ∼4 keV, while the harder photons arrive earlier than the soft photons for energies ≲10 keV. The total emission spectrum of IGR J17591-2342 across the 0.3-150 keV band could adequately be fitted in terms of an absorbed COMPPS model yielding as best fit parameters a column density of NH = (2.09 ± 0.05) × 1022 cm-2, a blackbody seed photon temperature kTbb, seed of 0.64 ± 0.02 keV, electron temperature kTe = 38.8 ± 1.2 keV and Thomson optical depth τT = 1.59 ± 0.04. The fit normalisation results in an emission area radius of 11.3 ± 0.5 km adopting a distance of 7.6 kpc. Finally, the results are discussed within the framework of accretion physics- and X-ray thermonuclear burst theory.
[article]
08/2020
Astronomy and Astrophysics
Accurate analytic formula for light bending in Schwarzschild metric
We propose new analytic formulae describing light bending in the Schwarzschild metric. For an emission radii above the photon orbit at the 1.5 Schwarzschild radius, the formulae have an accuracy of better than 0.2% for the bending angle and 3% for the lensing factor for any trajectories that turn around a compact object by less than about 160°. In principle, they can be applied to any emission point above the horizon of the black hole. The proposed approximation can be useful for problems involving emission from neutron stars and accretion discs around compact objects when fast accurate calculations of light bending are required. It can also be used to test the codes that compute light bending using exact expressions via elliptical integrals.
[article]
07/2020
Astronomy and Astrophysics
Observational appearance of rapidly rotating neutron stars. X-ray bursts, cooling tail method, and radius determination
Neutron stars (NSs) in low-mass X-ray binaries rotate at frequencies high enough to significantly deviate from sphericity (ν* ∼ 200-600 Hz). First, we investigate the effects of rapid rotation on the observational appearance of a NS. We propose analytical formulae relating gravitational mass and equatorial radius of the rapidly rotating NS to the mass M and radius R of a non-rotating NS of the same baryonic mass using accurate fully relativistic computations. We assume that the NS surface emission is described by the Planck function with two different emission patterns: the isotropic intensity and that corresponding to the electron-scattering dominated atmosphere. For these two cases we compute spectra from an oblate rotating NS observed at different inclination angles using the modified oblate Schwarzschild approximation, where light bending is computed in Schwarzschild metric, but frame dragging and quadrupole moment of a NS are approximately accounted for in the photon redshift calculations. In particular, we determine the solid angle at which a rotating NS is seen by a distant observer, the observed colour temperature and the blackbody normalization. Then, we investigate how rapid rotation affects the results of NS radius determination using the cooling tail method applied to the X-ray burst spectral evolution. We approximate the local spectra from the NS surface by a diluted blackbody with the luminosity-dependent dilution factor using previously computed NS atmosphere models. We then generalize the cooling tail method to the case of a rapidly rotating NS to obtain the most probable values of M and R of the corresponding non-rotating NS with the same baryonic mass. We show that the NS radius could be overestimated by 3-3.5 km for face-on stars of R ≈ 11 km rotating at ν*= 700 Hz if the version of the cooling tail method for a non-rotating NS is used. We apply the method to an X-ray burst observed from the NS rotating at ν* ≈ 532 Hz in SAX J1810.8-2609. The resulting radius of the non-rotating NS (assuming M = 1.5 M⊙) becomes 11.8 ± 0.5 km if it is viewed at inclination i = 60° and R = 11.2 ± 0.5 km for a face-on view, which are smaller by 0.6 and 1.2 km than the radius obtained using standard cooling tail method ignoring rotation. The corresponding equatorial radii of these rapidly rotating NSs are 12.3 ± 0.6 km (for i = 60°) and 11.6 ± 0.6 km (for i = 0°).
[article]
06/2020
Astronomy and Astrophysics
Kilohertz quasi-periodic oscillations from neutron star spreading layers
When the accretion disc around a weakly magnetised neutron star (NS) meets the stellar surface, it should brake down to match the rotation of the NS, forming a boundary layer. As the mechanisms potentially responsible for this braking are apparently inefficient, it is reasonable to consider this layer as a spreading layer (SL) with negligible radial extent and structure. We perform hydrodynamical 2D spectral simulations of an SL, considering the disc as a source of matter and angular momentum. Interaction of new, rapidly rotating matter with the pre-existing, relatively slow material co-rotating with the star leads to instabilities capable of transferring angular momentum and creating variability on dynamical timescales. For small accretion rates, we find that the SL is unstable for heating instability that disrupts the initial latitudinal symmetry and produces large deviations between the two hemispheres. This instability also results in breaking of the axial symmetry as coherent flow structures are formed and escape from the SL intermittently. At enhanced accretion rates, the SL is prone to shearing instability and acts as a source of oblique waves that propagate towards the poles, leading to patterns that again break the axial symmetry. We compute artificial light curves of an SL viewed at different inclination angles. Most of the simulated light curves show oscillations at frequencies close to 1 kHz. We interpret these oscillations as inertial modes excited by shear instabilities near the boundary of the SL. Their frequencies, dependence on flux, and amplitude variations can explain the high-frequency pair quasi-periodic oscillations observed in many low-mass X-ray binaries.
[article]
05/2020
Monthly Notices of the Royal Astronomical Society
Discovery of a retrogradely rotating neutron star in the X-ray pulsar GX 301-2
We report on the analysis of the spin evolution of a slow X-ray pulsar GX 301-2 along the orbit using long-term monitoring by Fermi/Gamma-ray Burst Monitor. Based on the observationally confirmed accretion scenario and an analytical model for the accretion of angular momentum, we demonstrate that in this system, the neutron star spins retrogradely, that is, in a direction opposite to the orbital motion. This first-of-a-kind discovery of such a system proves the principal possibility of retrograde rotation in accreting systems with suitable accretion torque, and might have profound consequences for our understanding of the spin evolution of X-ray pulsars, estimates of their initial spin periods, and the ultimate result of their evolution.
[article]
05/2020
Astronomy and Astrophysics
The unusual behavior of the young X-ray pulsar SXP 1062 during the 2019 outburst
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 ∼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.
[article]
03/2020
Monthly Notices of the Royal Astronomical Society
Spectral and temporal properties of Compton scattering by mildly relativistic thermal electrons
We have obtained new solutions and methods for the process of thermal Comptonization. We modify the solution to the kinetic equation of Sunyaev and Titarchuk to allow its application up to mildly relativistic electron temperatures and optical depths ≳ {1}. The solution can be used for spectral fitting of X-ray spectra from astrophysical sources. We also have developed an accurate Monte Carlo method for calculating spectra and timing properties of thermal Comptonization sources. The accuracy of our kinetic equation solution is verified by comparison with the Monte Carlo results. We also compare our results with those of other publicly available methods. Furthermore, based on our Monte Carlo code, we present distributions of the photon emission times and the evolution of the average photon energy for both up and down scattering.
[article]
02/2020
Astronomy and Astrophysics
Burst-induced coronal cooling in GS 1826-24. The clock wagging its tail
Type I X-ray bursts in GS 1826-24, and in several other systems, may induce cooling of the hot inner accretion flow that surrounds the bursting neutron star. Given that GS 1826-24 remained persistently in the hard state over the period 2003-2008 and presented regular bursting properties, we stacked the spectra of the X-ray bursts detected by INTEGRAL (JEM-X and ISGRI) and XMM-Newton (RGS) during that period to study the effect of the burst photons on the properties of the Comptonizing medium. The extended energy range provided by these instruments allows the simultaneous observation of the burst and persistent emission spectra. We detect an overall change in the shape of the persistent emission spectrum in response to the burst photon shower. For the first time, we observe simultaneously a drop in the hard X-ray emission, together with a soft X-ray excess with respect to the burst blackbody emission. The hard X-ray drop can be explained by burst-induced coronal cooling, while the bulk of the soft X-ray excess can be described by fitting the burst emission with an atmosphere model, instead of a simple blackbody model. Traditionally, the persistent emission was assumed to be invariant during X-ray bursts, and more recently to change only in normalization but not in spectral shape; the observed change in the persistent emission level during X-ray bursts may thus trigger the revision of existing neutron star mass-radius constraints, as the derived values rely on the assumption that the persistent emission does not change during X-ray bursts. The traditional burst fitting technique leads to up to a 10% overestimation of the bolometric burst flux in GS 1826-24, which significantly hampers the comparisons of the KEPLER and MESA model against this "textbook burster".
2019
1
[article]
12/2019
Experimental Astronomy
On-ground calibration of the ART-XC/SRG mirror system and detector unit at IKI. Part III
We have performed tests of the SRG project's ART-XC telescope's flight spare units of the mirror system (MS) and the URD focal plane X-ray detector. During the tests, the MS was illuminated by a quasi-parallel X-ray beam from a microfocus tube. By changing the angle between the MS optical axis and the incoming beam we obtained data on vignetting of the effective area of the MS-URD pair at the energies of Cu K-lines. The measurements are in good agreement with our model. Using the model vignetting of the effective area, we determined the ART-XC grasp as a function of energy for singly and doubly reflected events. The grasp reaches ∼31 cm2deg2 for singly reflected events at 7.2 keV, ∼44 cm2deg2 for doubly reflected events at 8.6 keV and ∼74 cm2deg2 for the sum of singly and doubly reflected events at 7.6 keV. The grasp values at 8.1 keV are ∼29.9 cm2deg2 for singly reflected events, ∼43.8 cm2deg2 for doubly reflected events and ∼73.7 cm2deg2 for all events. The ART-XC field of view is ∼2 deg2. In its central ∼0.3 deg2, sources are imaged and the instrument works in a "telescope" mode. The outer ∼1.7 deg2 do not have an imaging capability and the instrument works as a "concentrator" there. We also discuss the possibility of using ART-XC for monitoring bright transient X-ray sources when the observatory will be operating in the survey mode.
[article]
12/2019
Monthly Notices of the Royal Astronomical Society
Observational constraints on the magnetic field of the bright transient Be/X-ray pulsar SXP 4.78
We report results of the spectral and timing analysis of the Be/X-ray pulsar SXP 4.78 using the data obtained during its recent outburst with NuSTAR, Swift, Chandra, and NICER observatories. Using an overall evolution of the system luminosity, spectral analysis, and variability power spectrum we obtain constraints on the neutron star magnetic field strength. We found a rapid evolution of the variability power spectrum during the rise of the outburst, and absence of the significant changes during the flux decay. Several low frequency quasi-periodic oscillation features are found to emerge on the different stages of the outburst, but no clear clues on their origin were found in the energy spectrum and overall flux behaviour. We use several indirect methods to estimate the magnetic field strength on the neutron star surface and found that most of them suggest magnetic field B ≲ 2 × 1012 G. The strictest upper limit comes from the absence of the cyclotron absorption features in the energy spectra and suggests relatively weak magnetic field B < 6 × 1011 G.)
[article]
10/2019
The Astrophysical Journal
Pulsar Wind-heated Accretion Disk and the Origin of Modes in Transitional Millisecond Pulsar PSR J1023+0038
Transitional millisecond pulsars provide a unique set of observational data for understanding accretion at low rates onto magnetized neutron stars. In particular, PSR J1023+0038 exhibits a remarkable bimodality of the X-ray luminosity (low and high modes), pulsations extending from the X-ray to the optical band, GeV emission, and occasional X-ray flares. We discuss a scenario for the pulsar interaction with the accretion disk capable of explaining the observed behavior. We suggest that during the high mode the disk is truncated outside the light cylinder, allowing the pulsar wind to develop near the equatorial plane and strike the disk. The dissipative wind-disk collision energizes the disk particles and generates synchrotron emission, which peaks in the X-ray band and extends down to the optical band. The emission is modulated by the pulsar wind rotation, resulting in a pulse profile with two peaks 180° apart. This picture explains the high mode luminosity, spectrum, and pulse profile (X-ray and optical) of PSR J1023+0038. It may also explain the X-ray flares as events of sudden increase in the effective disk cross section intercepting the wind. In contrast to previously proposed models, we suggest that the disk penetrates the light cylinder only during the low X-ray mode. This penetration suppresses the dissipation caused by the pulsar wind-disk collision, and the system enters the propeller regime. The small duty cycle of the propeller explains the low spin-down rate of the pulsar.
[article]
09/2019
The Astrophysical Journal
Pulsating in Unison at Optical and X-Ray Energies: Simultaneous High Time Resolution Observations of the Transitional Millisecond Pulsar PSR J1023+0038
PSR J1023+0038 is the first millisecond pulsar discovered to pulsate in the visible band; such a detection took place when the pulsar was surrounded by an accretion disk and also showed X-ray pulsations. We report on the first high time resolution observational campaign of this transitional pulsar in the disk state, using simultaneous observations in the optical (Telescopio Nazionale Galileo, Nordic Optical Telescope, Telescopi Joan Oró), X-ray (XMM-Newton, NuSTAR, NICER), infrared (Gran Telescopio Canarias), and UV (Swift) bands. Optical and X-ray pulsations were detected simultaneously in the X-ray high-intensity mode in which the source spends ∼70% of the time, and both disappeared in the low mode, indicating a common underlying physical mechanism. In addition, optical and X-ray pulses were emitted within a few kilometers and had similar pulse shapes and distributions of the pulsed flux density compatible with a power-law relation F ν ∝ ν -0.7 connecting the optical and the 0.3-45 keV X-ray band. Optical pulses were also detected during flares with a pulsed flux reduced by one-third with respect to the high mode; the lack of a simultaneous detection of X-ray pulses is compatible with the lower photon statistics. We show that magnetically channeled accretion of plasma onto the surface of the neutron star cannot account for the optical pulsed luminosity (∼1031 erg s-1). On the other hand, magnetospheric rotation-powered pulsar emission would require an extremely efficient conversion of spin-down power into pulsed optical and X-ray emission. We then propose that optical and X-ray pulses are instead produced by synchrotron emission from the intrabinary shock that forms where a striped pulsar wind meets the accretion disk, within a few light cylinder radii away, ∼100 km, from the pulsar.
[article]
09/2019
Monthly Notices of the Royal Astronomical Society
Soft excess in the quiescent Be/X-ray pulsar RX J0812.4-3114
We report a 72 ks XMM-Newton observation of the Be/X-ray pulsar (BeXRP) RX J0812.4-3114 in quiescence (L_X ≈ 1.6× 10^{33} erg s^{-1}). Intriguingly, we find a two-component spectrum, with a hard power-law (Γ ≈ 1.5) and a soft blackbody-like excess below ≈1 keV. The blackbody component is consistent in kT with a prior quiescent Chandra observation reported by Tsygankov et al. and has an inferred blackbody radius of ≈10 km, consistent with emission from the entire neutron star (NS) surface. There is also mild evidence for an absorption line at ≈ 1 and/or ≈ 1.4 keV. The hard component shows pulsations at P ≈ 31.908 s (pulsed fraction 0.84 ± 0.10), agreeing with the pulse period seen previously in outbursts, but no pulsations were found in the soft excess (pulsed fraction ≲ 31 per cent). We conclude that the pulsed hard component suggests low-level accretion on to the NS poles, while the soft excess seems to originate from the entire NS surface. We speculate that, in quiescence, the source switches between a soft thermal-dominated state (when the propeller effect is at work) and a relatively hard state with low-level accretion, and use the propeller cut-off to estimate the magnetic field of the system to be ≲ 8.4× 10^{11} G. We compare the quiescent thermal LX predicted by the standard deep crustal heating model to our observations and find that RX J0812.4-3114 has a high thermal LX, at or above the prediction for minimum cooling mechanisms. This suggests that RX J0812.4-3114 either contains a relatively low-mass NS with minimum cooling, or that the system may be young enough that the NS has not fully cooled from the supernova explosion.
[article]
07/2019
Astronomy and Astrophysics
Effects of Compton scattering on the neutron star radius constraints in rotation-powered millisecond pulsars
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.
[article]
07/2019
Monthly Notices of the Royal Astronomical Society
Broad-band aperiodic variability in X-ray pulsars: accretion rate fluctuations propagating under the influence of viscous diffusion
We investigate aperiodic X-ray flux variability in accreting highly magnetized neutron stars - X-ray pulsars (XRPs). The X-ray variability is largely determined by mass accretion rate fluctuations at the NS surface, which replicate accretion rate fluctuations at the inner radius of the accretion disc. The variability at the inner radius is due to fluctuations arising all over the disc and propagating inwards under the influence of viscous diffusion. The inner radius varies with mean mass accretion rate and can be estimated from the known magnetic field strength and accretion luminosity of XRPs. Observations of transient XRPs covering several orders of magnitude in luminosity give a unique opportunity to study effects arising due to the changes of the inner disc radius. We investigate the process of viscous diffusion in XRP accretion discs and construct new analytical solutions of the diffusion equation applicable for thin accretion discs truncated both from inside and outside. Our solutions are the most general ones derived in the approximation of Newtonian mechanics. We argue that the break observed at high frequencies in the power density spectra of XRPs corresponds to the minimal time-scale of the dynamo process, which is responsible for the initial fluctuations. Comparing data from the bright X-ray transient A 0535+26 with our model, we conclude that the time-scale of initial variability in the accretion disc is a few times longer than the local Keplerian time-scale.
[article]
06/2019
Astronomy and Astrophysics
Super-Eddington accretion discs with advection and outflows around magnetized neutron stars
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 Alfvén 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 NGC 5907 X-1, the brightest ultra-luminous pulsar currently known, and NGC 300 ULX1, which is apparently a Be/X-ray binary experiencing a very bright super-Eddington outburst.
[article]
06/2019
Astronomy and Astrophysics
Evidence for the radiation-pressure dominated accretion disk in bursting pulsar GRO J1744-28 using timing analysis
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 1019 g s-1 during its super-Eddington outbursts. The magnetic field strength in the source, B ≈ 5 × 1011 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 ∼2 × 1037 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.
[article]
04/2019
Experimental Astronomy
On-ground calibration of the ART-XC/SRG mirror system and detector unit at IKI. Part II
We have performed calibration tests of a spare CdTe detector unit of the ART-XC/SRG telescope at the Space Research Institute's 60-m-long X-ray test facility. During the tests, we illuminated the detector with a quasi-parallel X-ray beam generated by interchangeable X-ray tubes with different targets. To monitor the X-ray flux, we used two commercial detectors Amptek XR-100T-CdTe and XR-100SDD. We present some features of the ART-XC detectors operation, limitations of reducing the lower energy thresholds for photon detection and results of verification of the detector's flat field characteristics at different energies. We estimate the ART-XC X-ray detector efficiency based on data obtained by the Amptek detectors and the ratio of their model efficiency. Using the measured detector efficiency, we have updated the ART-XC telescope's on-axis effective area for the sum of doubly and singly reflected events. The ART-XC effective area reaches a maximum of 410 ± 16 cm2 at 9.6 keV.
[article]
02/2019
The Astrophysical Journal
Thermal Emission and Magnetic Beaming in the Radio and X-Ray Mode-switching PSR B0943+10
PSR B0943+10 is a mode-switching radio pulsar characterized by two emission modes with different radio and X-ray properties. Previous studies, based on simple combinations of blackbody and power-law models, showed that its X-ray flux can be decomposed in a pulsed thermal plus an unpulsed nonthermal components. However, if PSR B0943+10 is a nearly aligned rotator seen pole-on, as suggested by the radio data, it is difficult to reproduce the high observed pulsed fraction unless magnetic beaming is included. In this work, we reanalyze all of the available X-ray observations of PSR B0943+10 with simultaneous radio coverage, modeling its thermal emission with polar caps covered by a magnetized hydrogen atmosphere or with a condensed iron surface. The condensed surface model provides good fits to the spectra of both pulsar modes, but, similarly to the blackbody, it cannot reproduce the observed pulse profiles, unless an additional power law with an ad hoc modulation is added. Instead, the pulse profiles and phase-resolved spectra are well described using the hydrogen atmosphere model to describe the polar cap emission plus an unpulsed power law. For the X-ray brighter state (Q-mode) we obtain a best fit with a temperature {kT}∼ 0.09 {keV}, an emitting radius R∼ 260 m, a magnetic field consistent with the value of the dipole field of 4 × 1012 G inferred from the timing parameters, and a small angle between the magnetic and spin axis, ξ = 5.° The corresponding parameters for the X-ray fainter state (B-mode) are {kT}∼ 0.08 {keV} and R ∼ 170 m.
[article]
02/2019
Monthly Notices of the Royal Astronomical Society
Dramatic spectral transition of X-ray pulsar GX 304-1 in low luminous state
We report on the discovery of a dramatic change in the energy spectrum of the X-ray pulsar GX 304-1 appearing at low luminosity. Particularly, we found that the cut-off power-law spectrum typical for accreting pulsars, including GX 304-1 at higher luminosities of LX ∼ 1036-1037 erg s-1, transformed at lower luminosity of LX ∼ 1034 erg s-1 to a two-component spectrum peaking around 5 and 40 keV. We suggest that the observed transition corresponds to a change of the dominant mechanism responsible for the deceleration of the accretion flow. We argue that the accretion flow energy at low accretion rates is released in the atmosphere of the neutron star, and the low-energy component in the source spectrum corresponds to the thermal emission of the optically thick, heated atmospheric layers. The most plausible explanations for the high-energy component are either the cyclotron emission reprocessed by the magnetic Compton scattering or the thermal radiation of deep atmospheric layers partly Comptonized in the overheated upper layers. Alternative scenarios are also discussed.
[article]
01/2019
Astronomy and Astrophysics
Study of the X-ray pulsar IGR J19294+1816 with NuSTAR: Detection of cyclotron line and transition to accretion from the cold disk
In the work we present the results of two deep broadband observations of the poorly studied X-ray pulsar IGR J19294+1816 obtained with the NuSTAR observatory. The source was observed during Type I outburst and in the quiescent state. In the bright state a cyclotron absorption line in the energy spectrum was discovered at Ecyc = 42.8 ± 0.7 keV. Spectral and timing analysis prove the ongoing accretion also during the quiescent state of the source. Based on the long-term flux evolution, particularly on the transition of the source to the bright quiescent state with luminosity around 1035 erg s-1, we conclude that IGR J19294+1816 switched to the accretion from the "cold" accretion disk between Type I outbursts. We also report the updated orbital period of the system.
[article]
01/2019
Monthly Notices of the Royal Astronomical Society
Discovery of a cyclotron absorption line in the transient X-ray pulsar XTE J1829-098
We report results of a spectral and timing analysis of the X-ray pulsar XTE J1829-098 using data obtained with the NuSTAR observatory during an outburst in 2018 August. A strong absorption feature was detected at the energy of Ecyc ≃ 15 keV in the source spectrum. This feature was interpreted as a cyclotron resonance scattering line corresponding to the magnetic field strength of the neutron star surface B ≃ 1.7 × 1012 G. The pulse phase-resolved spectroscopy shows that the cyclotron line is significantly detected at all phases of the pulse and its energy and other parameters are variable over the pulse period. The timing analysis of the source emission revealed strong pulsations with a period of P = 7.84480(2) s. The pulsed fraction is changed with the energy, including its local increase in the vicinity of the cyclotron line. Using the archival data of the RXTE observatory the presence of the cyclotron line in the spectrum of XTE J1829-098 was independently confirmed and tentative hint for the possible anticorrelation of the line energy with the source flux was revealed.
2018
1
[article]
12/2018
Monthly Notices of the Royal Astronomical Society
Interplay of spectral components in timing properties of accreting compact objects
X-ray variability of accreting black hole binaries is believed to be produced by the fluctuations propagating towards the compact object. Observations suggest the light curves in different energy bands are connected, but the fluctuations at harder energies are delayed with respect to the softer ones. The standard interpretation involves dependence of the X-ray spectral hardness on the radial distance, with harder spectra emitted closer to the black hole. Recently, a number of challenges to this scenario have been found, both at qualitative and quantitative level. The model does not predict the large magnitude of the delay between different energies, as well as the dependence of variability amplitude on the spectral range and frequency. We study timing properties of accreting black hole X-ray binaries taking into account peculiarities of spectral formation in these sources. We show that the simultaneous presence of two components leads to a complex shape of power spectra and phase lags between the X-ray bands: the amplitude of the phase lag becomes independent of the delay between the components, and the power spectra show an artificial decrease or, on the contrary, enhancement of power at low Fourier frequencies. This provides an essential ingredient for the reinstatement of the propagating fluctuations model. The work draws attention to the importance of considering conditions of spectral formation when studying timing properties.
[article]
12/2018
Astronomy and Astrophysics
Mixed H/He bursts in SAX J1748.9-2021 during the spectral change of its 2015 outburst
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 ≈2 to ≈1 h. The relation between the recurrence time, Δtrec, and the local mass accretion rate per unit area onto the compact object, ṁ, is fitted by a power-law model, and yielded as best fit at Δtrec ∼ ⟨ṁ⟩-1.02±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.
[article]
11/2018
Astronomy and Astrophysics
Accretion heated atmospheres of X-ray bursting neutron stars
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.
[article]
10/2018
The Astrophysical Journal
Discovery of X-Rays from the Old and Faint Pulsar J1154-6250
We report on the first X-ray observation of the 0.28 s isolated radio pulsar PSR J1154-6250 obtained with the XMM-Newton observatory in 2018 February. A point-like source is firmly detected at a position consistent with that of PSR J1154-6250. The two closest stars are outside the 3σ confidence limits of the source position and thus unlikely to be responsible for the observed X-ray emission. The energy spectrum of the source can be fitted equally well either with an absorbed power law with a steep photon index Γ ≈ 3.3 or with an absorbed blackbody with temperature kT = 0.21 ± 0.04 keV and emitting radius R BB ≈ 80 m (assuming a distance of 1.36 kpc). The X-ray luminosity of 4.4 × 1030 erg s-1 derived with the power-law fit corresponds to an efficiency of {η }X={L}Xunabs}/\dot{E}=4.5× {10}-3, similar to those of other old pulsars. The X-ray properties of PSR J1154-6250 are consistent with an old age and suggest that the spatial coincidence of this pulsar with the OB association Cru OB1 is due to a chance alignment.
[article]
10/2018
Astronomy and Astrophysics
Bayesian parameter constraints for neutron star masses and radii using X-ray timing observations of accretion-powered millisecond pulsars
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.
[article]
10/2018
The Astrophysical Journal
Evidence for the Photoionization Absorption Edge in a Photospheric Radius Expansion X-Ray Burst from GRS 1747-312 in Terzan 6
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 ∼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.