arXiv subjects

Kirill Atapin

Publications and source records attributed to Kirill Atapin.

6 recordsLinked to original sources

arXiv2023

NGC 5474 X-1: a neutron star ULX in an old stellar cluster?

We present the optical and X-ray study of the ultraluminous X-ray source (ULX) NGC 5474 X-1. The X-ray spectrum taken during the bright state of the source ($L_X \sim 2\times10^{40}$ egs/s) shows signatures of a broad absorption line at $\simeq 8$ keV which may be a cyclotron resonant scattering feature. This implies that this system may host a neutron star with a magnetic field $\sim 10^{12}$ G. The first observation of this area with the Hubble Space Telescope (HST) carried out 14 months later revealed that the source was bright in the optical range as well. The subsequent observations have shown that the source faded in both ranges (more than $2.8^m$ in the U band and by a factor of 50-100 in X-rays) and has never become bright again. Deeper HST observations made it possible to impose constraints on the donor star spectral class and mass ($<\,7$ M$_\odot$), as well as to identify a stellar cluster of about 1 Gyr, the centre of which is located at a projected distance of $\simeq 2$ pc from NGC 5474 X-1. The ULX can be a member of this old cluster, however, the presence of stars with ages of $\sim10$ Myr within 300 pc around the ULX does not completely exclude the possibility that it is just an accidental projection.

astro-ph.HE↗

arXiv2020

Optical Counterpart to the Ultraluminous X-Ray Source in the UGC6456 Galaxy

We report the identification of the optical counterpart to the transient ultraluminuos X-ray source in the blue dwarf galaxy UGC6456 (VII Zw 403). The source is highly variable in both the X-ray (more 100 times, 0.3-10 keV) and optical (more 3 times, V band) ranges. The peak X-ray luminosity of UGC6456 ULX exceeds $10^{40}$ erg/s; the absolute magnitude when the source is optically bright is M$_V = -8.24\pm0.11$, which makes this source one of the brightest ULXs in the optical range. We found a correlation between the optical and X-ray fluxes (with a coefficient of $0.9\pm 0.3$), which may indicate that the optical emission is produced by re-processing of the X-rays in outer parts of the optically-thick wind coming from the supercritical accretion disk. Optical spectra of UGC6456 ULX show broad and variable hydrogen and helium emission lines, which also confirms the presence of the strong wind.

astro-ph.HE↗

arXiv2018

Ultraluminous X-ray sources

Ultraluminous X-ray sources (ULXs) represent a class of binary systems that are more luminous than any black hole in our Galaxy. The nature of these objects remained unclear for a long time. The most popular models for the ULXs involve either intermediate mass black holes (IMBHs) or stellar-mass black holes accreting at super-Eddington rates. In the last few years our understating of these objects was significantly improved, which made the model of super-Eddington accretion much preferable. Both the X-ray and optical spectra provide evidence for the strong outflows coming from supercritical accretion disk. Another surprising result was discovery of pulsations in four ULXs, which claims that these systems must host neutron stars. Besides the presence of pulsations, there is no sharp difference between ultraluminous pulsars and normal ULXs. This fact implies that significant number of known ULXs might eventually be neutron stars.

astro-ph.HE↗

arXiv2016

X-ray Variability of SS 433: Evidence for Supercritical Accretion

We study the X-ray variability of SS433 based on data from the ASCA observatory and the MAXI and RXTE/ASM monitoring missions. Based on the ASCA data, we have constructed the power spectrum of SS433 in the frequency range from $10^{-6}$ to 0.1 Hz, which confirms the presence of a flat portion (flat-topped noise) in the spectrum at frequencies $3\times 10^{-5}$ - $10^{-3}$ Hz. The periodic variability (precession, nutation, eclipses) begins to dominate significantly over the stochastic variability at lower frequencies, which does not allow the stochastic variability to be studied reliably. The best agreement with the observations is reached by the model with the flat portion extending to $9.5\times10^{-6}$ Hz and a power-law spectrum with index of 2.6 below that frequency. The jet nutation with a period of about three days suggests that the time for the passage of material through the disk is less than this value. Therefore, at frequencies below $4\times10^{-6}$ Hz, the power spectrum probably does not reflect the disk structure. It may depend on other factors, for example, a variable mass accretion rate supplied by the donor. The flat portion may arise from a rapid decrease in the viscous time in the supercritical or radiative disk zones. It could be related to the variability of the X-ray jets that are formed in the supercritical region.

astro-ph.HE↗

arXiv2016

Ultraluminous X-ray sources as super-Eddington accretion disks

The origin of Ultraluminous X-ray sources (ULXs) in external galaxies whose X-ray luminosities exceed those of the brightest black holes in our Galaxy by hundreds and thousands of times is mysterious. The most popular models for the ULXs involve either intermediate mass black holes (IMBHs) or stellar-mass black holes accreting at super-Eddington rates. Here we review the ULX properties, their X-ray spectra indicate a presence of hot winds in their accretion disks supposing the supercritical accretion. However, the strongest evidences come from optical spectroscopy. The spectra of the ULX counterparts are very similar to that of SS 433, the only known supercritical accretor in our Galaxy.

astro-ph.HE↗

arXiv2014

X-ray variability of SS433: effects of the supercritical accretion disc

We study a stochastic variability of SS433 in the $10^{-4} - 5\times 10^{-2}$ Hz frequency range based on RXTE data, and on simultaneous observations with RXTE and optical telescopes. We find that the cross-correlation functions and power spectra depend drastically on the precession phase of the supercritical accretion disc. When the wind funnel of the disc is maximally open to the observer, a flat part emerges in the power spectrum; a break is observed at the frequency $1.7\times10^{-3}$ Hz, with a power-law index $β\approx 1.67$ at higher frequencies. The soft emission forming mostly in the jets, lags behind the hard and optical emission. When the observer does not see the funnel and jets (the `edge-on' disc), the power spectrum is described by a single power-law with $β\approx 1.34$ and no correlations between X-ray ranges are detected. We investigated two mechanisms to explain the observed variability at the open disc phase, 1) reflection of radiation at the funnel wall (X-rays and optical) and 2) the gas cooling in the jets (X-rays only). The X-ray variability is determined by the contribution of both mechanisms, however the contribution of the jets is much higher. We found that the funnel size is $(2-2.5)\times10^{12}$ cm, and the opening angle is $\vartheta_f\sim 50^\circ$. X-ray jets may consist of three fractions with different densities: $8\times10^{13}$, $3\times10^{13}$ and $5\times10^{11}$ cm$^{-3}$, with most of the jet's mass falling within the latter fraction. We suppose that revealed flat part in the power spectrum may be related to an abrupt change in the disc structure and viscous time-scale at the spherization radius, because the accretion disc becomes thick at this radius, $h/r \sim 1$. The extent of the flat spectrum depends on the variation of viscosity at the spherization radius.

astro-ph.HE↗