arXiv subjects

S. Tsygankov

Publications and source records attributed to S. Tsygankov.

At least 19 recordsLinked to original sources

arXiv2024

Multiwavelength coverage of the 2024 periastron passage of PSR B1259-63 / LS 2883

PSR B1259-63 is a gamma-ray binary system with a 48 ms radio pulsar orbiting around an O9.5Ve star, LS 2883, in a highly eccentric ~3.4 yr long orbit. Close to the periastron the system is detected from radio up to the TeV energies due to the interaction of LS 2883 and pulsar's outflows. The observations of last 4 periastra passages taken in 2010-2021 demonstrate periastron to periastron variability at all wavelength, probably linked to the state of the decretion disk. In this paper we present the results of our optical, radio and X-ray observational campaigns on PSR B1259-63 performed in 2024 accompanied with the analysis of the publicly available GeV FERMI/LAT data. We show that this periastron passage was characterised by the early flaring of X-rays before the periastron passage and GeV emission after the periastron passage, which can be explained by a larger size of the decretion disk as supported by the optical observations. The structure of the GeV flare is also in agreement with the disruption of the large dense disk. The possible X-ray/radio correlation was observed only during the post-periastron rise of X-ray and radio emission.

astro-ph.HE↗

arXiv2024

Searching for redshifted 2.2 MeV neutron-capture lines from accreting neutron stars: Theoretical X-ray luminosity requirements and INTEGRAL/SPI observations

Accreting neutron stars (NSs) are expected to emit a redshifted 2.2 MeV line due to the capture of neutrons produced through the spallation processes of $^4$He and heavier ions in their atmospheres. Detecting this emission would offer an independent method for constraining the equation of state of NSs and provide valuable insights into nuclear reactions occurring in extreme gravitational and magnetic environments. Typically, a higher mass accretion rate is expected to result in a higher 2.2 MeV line intensity. However, when the mass accretion rate approaches the critical threshold, the accretion flow is decelerated by the radiative force, leading to a less efficient production of free neutrons and a corresponding drop in the flux of the spectral line. This makes the brightest X-ray pulsars unsuitable candidates for gamma-ray line detection. In this work, we present a theoretical framework for predicting the optimal X-ray luminosity required to detect a redshifted 2.2 MeV line in a strongly magnetized NS. As the INTEGRAL mission nears its conclusion, we have undertaken a thorough investigation of the SPI data of this line in a representative sample of accreting NSs. No redshifted 2.2 MeV line was detected. For each spectrum, we have determined the 3-sigma upper limits of the line intensity, assuming different values of the line width. Our findings suggest that advancing our understanding of the emission mechanism of the 2.2 MeV line, as well as the accretion flow responsible for it, will require a substantial increase in sensitivity from future MeV missions. For example, for a bright X-ray binary such as Sco X-1, we would need at least a 3-sigma line point source sensitivity of ~1E-6 ph/cm^2/s, that is, about two orders of magnitude better than that currently achieved. [Abridged]

astro-ph.HE↗

arXiv2024

X-ray polarization measurement of the gold standard of radio-quiet active galactic nuclei : NGC 1068

We used the Imaging X-ray Polarimetry Explorer (IXPE) satellite to measure, for the first time, the 2-8 keV polarization of NGC 1068. We pointed IXPE for a net exposure time of 1.15 Ms on the target, in addition to two ~ 10 ks each Chandra snapshots in order to account for the potential impact of several ultraluminous X-ray source (ULXs) within IXPE's field-of-view. We measured a 2 - 8 keV polarization degree of 12.4% +/- 3.6% and an electric vector polarization angle of 101° +/- 8° at 68% confidence level. If we exclude the spectral region containing the bright Fe K lines and other soft X-ray lines where depolarization occurs, the polarization fraction rises up to 21.3% +/- 6.7% in the 3.5 - 6.0 keV band, with a similar polarization angle. The observed polarization angle is found to be perpendicular to the parsec scale radio jet. Using a combined Chandra and IXPE analysis plus multi-wavelength constraints, we estimated that the circumnuclear "torus" may sustain a half-opening angle of 50° - 55° (from the vertical axis of the system). Thanks to IXPE, we have measured the X-ray polarization of NGC 1068 and found comparable results, both in terms of polarization angle orientation with respect to the radio-jet and torus half-opening angle, to the X-ray polarimetric measurement achieved for the other archetypal Compton-thick AGN : the Circinus galaxy. Probing the geometric arrangement of parsec-scale matter in extragalactic object is now feasible thanks to X-ray polarimetry.

astro-ph.HE↗

arXiv2021

Discovery of the 5 keV cyclotron line followed by three harmonics in SwiftJ1626.6-5156

We report on observations of the Be/X-ray binary system SwiftJ1626.6-5156 performed with NuSTAR during a short outburst in March 2021, following its detection of by the MAXI monitor and Spektrum-Roentgen-Gamma (SRG) observatory. Our analysis of the broadband X-ray spectrum of the source confirms the presence of two absorption-like features at energies ~9 and ~17 keV previously reported in literature and interpreted as the fundamental cyclotron resonance scattering feature (CRSF) and its first harmonic (based on RXTE data). The better sensitivity and energy resolution of NuSTAR, combined with the low energy coverage of NICER, allowed us to detect two additional absorption-like features at ~4.9 keV and ~13 keV. We conclude, therefore, that in total four cyclotron lines are observed in the spectrum of SwiftJ1626.6-5156: the fundamental CRSF at ~4.9 keV and three higher spaced harmonics. This discovery makes SwiftJ1626.6-5156 the second accreting pulsar, after 4U0115+63, whose spectrum is characterized by more than three lines of a cyclotronic origin, and implies the source has the weakest confirmed magnetic field among all X-ray pulsars B~4E11 G. This discovery makes SwiftJ1626.6-5156 one of prime targets for the upcoming X-ray polarimetry missions covering soft X-ray band such as IXPE and eXTP.

astro-ph.HE↗

arXiv2021

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 at GeV energies around the periastron, during which the energy release substantially exceeds the spin down luminosity under the assumption of the isotropic energy release. 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 of Chernyakova et al (2020) in the case of a sparser and clumpier disk of the Be star.

astro-ph.HE↗

arXiv2021

Time Domain Astronomy with the THESEUS Satellite

THESEUS is a medium size space mission of the European Space Agency, currently under evaluation for a possible launch in 2032. Its main objectives are to investigate the early Universe through the observation of gamma-ray bursts and to study the gravitational waves electromagnetic counterparts and neutrino events. On the other hand, its instruments, which include a wide field of view X-ray (0.3-5 keV) telescope based on lobster-eye focusing optics and a gamma-ray spectrometer with imaging capabilities in the 2-150 keV range, are also ideal for carrying out unprecedented studies in time domain astrophysics. In addition, the presence onboard of a 70 cm near infrared telescope will allow simultaneous multi-wavelegth studies. Here we present the THESEUS capabilities for studying the time variability of different classes of sources in parallel to, and without affecting, the gamma-ray bursts hunt.

astro-ph.IM↗

arXiv2021

SRG/ART-XC and NuSTAR observations of the X-ray pulsar GRO J1008-57 in the lowest luminosity state

We report results of the first broadband observation of the transient X-ray pulsar GRO J1008-57 performed in the quiescent state. Observations were conducted quasi-simultaneously with the Mikhail Pavlinsky ART-XC telescope on board SRG and NuSTAR right before the beginning of a Type I outburst. GRO J1008-57 was detected in the state with the lowest observed luminosity around several $\times 10^{34}$ erg s$^{-1}$ and consequently accreting from the cold disk. Timing analysis allowed to significantly detect pulsations during this state for the first time. The observed pulsed fraction of about 20\% is, however, almost three times lower than in brighter states when the accretion proceeds through the standard disk. We traced the evolution of the broadband spectrum of the source on a scale of three orders of magnitude in luminosity and found that at the lowest luminosities the spectrum transforms into the double-hump structure similarly to other X-ray pulsars accreting at low luminosities (X Persei, GX 304-1, A0535+262) reinforcing conclusion that this spectral shape is typical for these objects.

astro-ph.HE↗

arXiv2020

AT 2017gbl: a dust obscured TDE candidate in a luminous infrared galaxy

We present the discovery with Keck of the extremely infrared (IR) luminous transient AT 2017gbl, coincident with the Northern nucleus of the luminous infrared galaxy (LIRG) IRAS 23436+5257. Our extensive multi-wavelength follow-up spans ~900 days, including photometry and spectroscopy in the optical and IR, and (very long baseline interferometry) radio and X-ray observations. Radiative transfer modelling of the host galaxy spectral energy distribution and long-term pre-outburst variability in the mid-IR indicate the presence of a hitherto undetected dust obscured active galactic nucleus (AGN). The optical and near-IR spectra show broad 2000 km/s hydrogen, He I and O I emission features that decrease in flux over time. Radio imaging shows a fast evolving compact source of synchrotron emission spatially coincident with AT 2017gbl. We infer a lower limit for the radiated energy of 7.3 x 10^50 erg from the IR photometry. An extremely energetic supernova would satisfy this budget, but is ruled out by the radio counterpart evolution. Instead, we propose AT 2017gbl is related to an accretion event by the central supermassive black hole, where the spectral signatures originate in the AGN broad line region and the IR photometry is consistent with re-radiation by polar dust. Given the fast evolution of AT 2017gbl, we deem a tidal disruption event (TDE) of a star a more plausible scenario than a dramatic change in the AGN accretion rate. This makes AT 2017gbl the third TDE candidate to be hosted by a LIRG, in contrast to the so far considered TDE population discovered at optical wavelengths and hosted preferably by post-starburst galaxies.

astro-ph.HE↗

arXiv2020

Switches between accretion structures during flares in 4U 1901+03

We report on our analysis of the 2019 outburst of the X-ray accreting pulsar 4U 1901+03 observed with Insight-HXMT and NICER. Both spectra and pulse profiles evolve significantly in the decaying phase of the outburst. Dozens of flares are observed throughout the outburst. They are more frequent and brighter at the outburst peak. We find that the flares, which have a duration from tens to hundreds of seconds, are generally brighter than the persistent emission by a factor of $\sim$ 1.5. The pulse profile shape during the flares can be significantly different than that of the persistent emission. In particular, a phase shift is clearly observed in many cases. We interpret these findings as direct evidence of changes of the pulsed beam pattern, due to transitions between the sub- and super-critical accretion regimes on a short time scale. We also observe that at comparable luminosities the flares' pulse profiles are rather similar to those of the persistent emission. This indicates that the accretion on the polar cap of the neutron star is mainly determined by the luminosity, i.e., the mass accretion rate.

astro-ph.HE↗

arXiv2019

First characterization of Swift J1845.7-0037 with NuSTAR

The hard X-ray transient source Swift J1845.7-0037 was discovered in 2012 by Swift/BAT. However, at that time no dedicated observations of the source were performed. On Oct 2019 the source became active again, and X-ray pulsations with a period of ~199s were detected with Swift/XRT. This triggered follow-up observations with NuSTAR. Here we report on the timing and spectral analysis of the source properties using NuSTAR and Swift/XRT. The main goal was to confirm pulsations and search for possible cyclotron lines in the broadband spectrum of the source to probe its magnetic field. Despite highly significant pulsations with period of 207.379(2) were detected, no evidence for a cyclotron line was found in the spectrum of the source. We therefore discuss the strength of the magnetic field based on the source flux and the detection of the transition to the "cold-disc" accretion regime during the 2012 outburst. Our conclusion is that, most likely, the source is a highly magnetized neutron star with B 1e13G at a large distance of d~10 kpc. The latter one consistent with the non-detection of a cyclotron line in the NuSTAR energy band.

astro-ph.HE↗

arXiv2019

Hot disk of the Swift J0243.6+6124 revealed by Insight-HXMT

We report on analysis of observations of the bright transient X-ray pulsar \src obtained during its 2017-2018 giant outburst with Insight-HXMT, \emph{NuSTAR}, and \textit{Swift} observatories. We focus on the discovery of a sharp state transition of the timing and spectral properties of the source at super-Eddington accretion rates, which we associate with the transition of the accretion disk to a radiation pressure dominated (RPD) state, the first ever directly observed for magnetized neutron star. This transition occurs at slightly higher luminosity compared to already reported transition of the source from sub- to super-critical accretion regime associate with onset of an accretion column. We argue that this scenario can only be realized for comparatively weakly magnetized neutron star, not dissimilar to other ultra-luminous X-ray pulsars (ULPs), which accrete at similar rates. Further evidence for this conclusion is provided by the non-detection of the transition to the propeller state in quiescence which strongly implies compact magnetosphere and thus rules out magnetar-like fields.

astro-ph.HE↗

arXiv2019

Discovery of a pulse-phase-transient cyclotron line in the X-ray pulsar GROJ2058+42

We report the discovery of absorption features in the X-ray spectrum of the transient X-ray pulsar GROJ2058+42. The features are detected around $\sim10$, $\sim20$ and $\sim30$ keV in both NuSTAR observations carried out during the source type II outburst in spring 2019. The most intriguing property is that the deficit of photons around these energies is registered only in the narrow phase interval covering around 10% of the pulsar spin period. We interpret these absorption lines as a cyclotron resonant scattering line (fundamental) and two higher harmonics. The measured energy allow us to estimate the magnetic field strength of the neutron star as $\sim10^{12}$ G.

astro-ph.HE↗

arXiv2019

GROJ1750-27: a neutron star far behind the Galactic Center switching into the propeller regime

We report on analysis of properties of the X-ray binary pulsar GROJ1750-27 based on X-ray (Chandra, Swift, and Fermi/GBM), and near-infrared (VVV and UKIDSS surveys) observations. An accurate position of the source is determined for the first time and used to identify its infrared counterpart. Based on the VVV data we investigate the spectral energy distribution (SED) of the companion, taking into account a non-standard absorption law in the source direction. A comparison of this SED with those of known Be/X-ray binaries and early type stars has allowed us to estimate a lower distance limit to the source at $>12$ kpc. An analysis of the observed spin-up torque during a giant outburst in 2015 provides an independent distance estimate of $14-22$ kpc, and also allows to estimate the magnetic field on the surface of the neutron star at $B\simeq(3.5-4.5)\times10^{12}$ G. The latter value is in agreement with the possible transition to the propeller regime, a strong hint for which was revealed by Swift/XRT and Chandra. We conclude that GROJ1750-27 is located far behind the Galactic Center, which makes it one of the furthest Galactic X-ray binaries known.

astro-ph.HE↗

arXiv2018

A dust-enshrouded tidal disruption event with a resolved radio jet in a galaxy merger

Tidal disruption events (TDEs) are transient flares produced when a star is ripped apart by the gravitational field of a supermassive black hole (SMBH). We have observed a transient source in the western nucleus of the merging galaxy pair Arp 299 that radiated >1.5x10^52 erg in the infrared and radio, but was not luminous at optical or X-ray wavelengths. We interpret this as a TDE with much of its emission re-radiated at infrared wavelengths by dust. Efficient reprocessing by dense gas and dust may explain the difference between theoretical predictions and observed luminosities of TDEs. The radio observations resolve an expanding and decelerating jet, probing the jet formation and evolution around a SMBH.

astro-ph.GA↗

arXiv2018

An upper limit on nickel overabundance in the supercritical accretion disk wind of SS 433 from X-ray spectroscopy

We take advantage of a long (with a total exposure time of 120 ks) X-ray observation of the unique Galactic microquasar SS 433, carried out with the XMM-Newton space observatory, to search for a fluorescent line of neutral (or weakly ionized) nickel at the energy 7.5 keV. We consider two models of the formation of fluorescent lines in the spectrum of SS 433: 1) due to reflection of hard X-ray radiation from a putative central source on the optically thick walls of the accretion disk "funnel"; and 2) due to scattering of the radiation coming from the hottest parts of the jets in the optically thin wind of the system. It is shown, that for these cases, the photon flux of Ni I K$_α$ fluorescent line is expected to be 0.45 of the flux of Fe I K$_α$ fluorescent line at 6.4 keV, for the relative nickel overabundance $Z_{Ni}/Z = 10$, as observed in the jets of SS 433. For the continuum model without the absorption edge of neutral iron, we set a 90 per cent upper limit on the flux of the narrow Ni I K$_α$ line at the level of $0.9 \times 10^{-5}$ ph s$^{-1}$ cm$^{-2}$. For the continuum model with the absorption edge, the corresponding upper limit is $2.5 \times 10^{-5}$ ph s$^{-1}$ cm$^{-2}$. At the same time, for the Fe I K$_α$ line, we measure the flux of $9.9_{8.4}^{11.2} \times 10^{-5}$ ph s$^{-1}$ cm$^{-2}$. Taken at the face value, the results imply that the relative overabundance of nickel in the wind of the accretion disc should be at least 1.5 times less than the corresponding excess of nickel observed in the jets of SS 433.

astro-ph.HE↗

arXiv2017

Study of orbital and superorbital variability of LSI +61 303 with X-ray data

LSI +61 303 is one of the few X-ray binaries with a Be star companion from which radio, X-rays and high-energy gamma-ray (GeV and TeV) emission have been observed. The nature of the high energy activity of the system is not yet fully understood, but it is widely believed that it is generated due to the interaction of the relativistic electrons leaving the compact object with the photons and non-relativistic wind of the Be star. The superorbital variability of the system has been observed in the radio, optical and X-ray domains and could be due to the cyclic change of the Be star disk size. In this paper we systematically review all publicly available data from Suzaku, XMM-Newton, Chandra and Swift observatories in order to measure the absorption profile of the circumstellar Be disk as a function of orbital and superorbital phases. We also discuss short-term variability of the system, found during the analysis and its implications for the understanding of the physical processes in this system.

astro-ph.HE↗

arXiv2016

NuSTAR observations of the supergiant X-ray pulsar IGR J18027-2016: accretion from the stellar wind and possible cyclotron absorption line

We report on the first focused hard X-ray view of the absorbed supergiant system IGRJ18027-2016 performed with the NuSTAR observatory. The pulsations are clearly detected with a period of $P_{spin}=139.866(1)$ s and a pulse fraction of about 50-60% at energies from 3 to 80 keV. The source demonstrates an approximately constant X-ray luminosity on a time scale of more than dozen years with an average spin-down rate of $dP/dt\simeq6\times10^{-10}$ s/s. This behaviour of the pulsar can be explained in terms of the wind accretion model in the settling regime. The detailed spectral analysis at energies above 10 keV was performed for the first time and revealed a possible cyclotron absorption feature at energy ~23 keV. This energy corresponds to the magnetic field $B\simeq3\times10^{12}$ G at the surface of the neutron star, which is typical for X-ray pulsars.

astro-ph.HE↗

arXiv2016

Propeller effect in the transient X-ray pulsar SMC X-2

We report the results of the monitoring campaign of the transient X-ray pulsar SMC X-2 performed with the Swift/XRT telescope over the period of 2015 September - 2016 January during the Type II outburst. During this event, the bolometric luminosity of the source ranged from $\simeq10^{39}$ down to several$\times10^{34}$ erg/s. Moreover, we discovered its dramatic drop by a factor of more than 100 below the limiting value of $L_{\rm lim}\simeq4\times10^{36}$ erg/s, which can be interpreted as a transition to the propeller regime. These measurements make SMC X-2 the sixth pulsating X-ray source where such a transition is observed and allow us to estimate the magnetic field of the neutron star in the system $B\simeq3\times10^{12}$ G, which is in agreement with independent results of the spectral analysis.

astro-ph.HE↗