arXiv subjects

K. Atapin

Publications and source records attributed to K. Atapin.

14 recordsLinked to original sources

arXiv2024

I-band photometry of the quiescent black hole X-ray nova GROJ0422+32=V518 Per: modelling of the orbital light curve and estimation of the black hole mass

We present new photometric observations of the X-ray nova GROJ0422+32 (V518 Per) carried out in the I$_c$ band over 14 nights in 2020 -- 2023. We had to revise the orbital period of the system, the new value $P_{\rm orb}=5^{\rm h}04^{\rm m}35.50^{\rm s} \pm 0.04^{\rm s}$ is about one minute shorter than the period by Webb et al. (2000) but close to the result reported earlier by Filippenko et al. (1995). The obtained folded light curve has a regular shape with a clear ellipticity effect and signatures of a slight heating effect. The simulations of this light curve in terms of the model of an interacting binary system allowed us to estimate the orbital inclination $i = 33^\circ - 49^\circ$ of the system and derive masses of the black hole $M_x=(6.5\pm2.9)M_\odot$ and companion star $M_v=(0.47\pm0.21)M_\odot$. This range of the black hole masses overlaps with the known gap of (2--5)\,M$_\odot$ in the distribution of compact objects masses but mostly it lies above the upper boundary of this gap. To obtain more precise estimates one needs to know the degree of heating of the donor star, so synchronous X-ray/optical observations of this system are desirable.

astro-ph.HE↗

arXiv2022

Search for LBVs in the Local Volume galaxies: study of four stars in NGC 4449

We continue to search for LBV stars in galaxies outside the Local Group. In this work, we have investigated four luminous stars in NGC 4449. Multiple spectral observations carried out for J122810.94+440540.6, J122811.70+440550.9, and J122809.72+440514.8 revealed the emission features in their spectra that are characteristic of LBVs. Photometry showed noticeable brightness changes of J122809.72+440514.8 ($\Delta I=0.69\pm0.13^m$) and J122817.83+440630.8 ($\Delta R=2.15\pm0.13^m$), while the variability of J122810.94+440540.6 and J122811.70+440550.9 does not exceed $0.3^m$ regardless of the filter. We have obtained estimates of the interstellar reddening, photosphere temperatures, and bolometric luminosities $\log(\text{L}_\text{Bol}/\text{L}_{\odot}) \approx 5.24-6.42$. Using the CMFGEN code, we have modelled the spectrum of the cold state of J122809.72+440514.8 ($T_{\text{eff}}=9300\,$K) and have obtained possible value of the mass loss rate $\dot{M} = 5.2\times10^{-3}\,M_{\odot}\,yr^{-1}$. Based on the observational properties, J122809.72+440514.8 and J122817.83+440630.8 were classified as LBVs, while the other two stars were classified as LBV candidates or B[e]-supergiants candidates

astro-ph.SR↗

arXiv2022

Search for LBVs in the Local Volume galaxies: study of two stars in NGC 1156

We continue the search for luminous blue variables (LBVs) in Local Volume galaxies in order to study their fundamental parameters. In this paper, we report the discovery of two new LBVs in the dwarf irregular galaxy NGC 1156. Both stars exhibit spectral variability simultaneously with strong brightness variations: $\Delta \text{R}_c = 0.84 \pm 0.23^m$ for J025941.21+251412.2 and $\Delta \text{R}_c = 2.59 \pm 0.10^m$ for J025941.54+251421.8. The bolometric luminosities of the stars are in the range of L$_\text{Bol} \approx (0.8 - 1.6) \times 10^6$ L$_\odot$. These values are corrected for reddening A$_\text{V} \approx 0.9$ and are given for the distance to the galaxy D=7.0$\pm$0.4 Mpc, which we have measured by the TRGB method. Both stars are above the Humphreys-Davidson limit in the region of relatively low temperatures, T$_\text{eff} \lesssim 10$ kK on the temperature-luminosity diagram. J025941.54+251421.8 had a temperature below the hydrogen ionisation threshold at maximum brightness, exhibiting behaviour very similar to that of the known LBV R71 during its 2012 outburst. We have estimated the masses of the detected LBVs and studied the properties of their stellar environment. We discuss our results within the framework of both a single star and a binary system evolution scenario for LBVs.

astro-ph.GA↗

arXiv2022

Simultaneous X-ray/UV observations of ultraluminous X-ray source Holmberg II X-1 with Indian space mission AstroSat

We present the results of 8 epochs of simultaneous UV and X-ray observations of the highly variable ultraluminous X-ray source (ULX) Holmberg II X-1 with AstroSat -- Indian multiwavelength space satellite. During the entire observation period from late 2016 to early 2020, Holmberg II X-1 showed a moderate X-ray luminosity of $8\times10^{39}$ erg/s and a hard power-law spectrum with $\Gamma \lesssim 1.9$. Due to low variability of the object in X-rays (by a factor 1.5) and insignificant variability in the UV range (upper limit $\approx25$%) we could not find reliable correlation between flux changes in these ranges. Inside each particular observation, the X-ray variability amplitude is higher, it reaches a factor of 2-3 respect to the mean level at the time scales of $\sim10$ ks or even shorter. We discussed our results in terms of three models of a heated donor star, a heated disk and a heated wind, and estimated the lower limit to the variability which would allow to reject at least part of them.

astro-ph.HE↗

arXiv2021

New data for the LBV in NGC 4736

We present new spectral and photometric data of confirmed LBV star from the NGC4736 galaxy. The star NGC4736_1 (Mbol = -11.5 mag) showed noticeable spectral variability from 2015 to 2018, which was accompanied by a significant change in the brightness. We also have estimated possible initial mass of the object NGC4736_1 as ~130 Msun.

astro-ph.SR↗

arXiv2021

Optical Counterparts of ULXs in Two Dwarf Galaxies: NGC 4861 and NGC 4449

We present the results of a search for optical candidates of Ultraluminous X-ray sources (ULXs) in two dwarf galaxies: NGC 4861 and NGC 4449 using Hubble Space Telescope {\it HST} archival data. With a precise astrometry, we confirm that NGC 4861 X1 associated with an HII complex as reported by \cite{2014MNRAS.441.1841T} and we conclude that NGC 4861 X2 resides in a young star group with 400$\pm$80 M$\odot$. We also find that NGC 4449 X7 is associated with three optical candidates within an error radius of 0$\farcs$2 at the 90$\%$ confidence level. Absolute magnitudes (M$_{v}$) of these candidates are determined as $-$5.0 and $-$4.1. The age and mass values for the three candidates by using stellar evolutionary tracks are estimated as 40-50 Myr and $\sim$8 M$\odot$, respectively. The locations of optical candidates suggest possible association with a nearby group of stars. In addition, we analyzed the previously unused archival data of {\it XMM-Newton}, {\it Chandra} and {\it Swift} where the sources detected. Although the X-ray spectral data do not allow us to discriminate between physical models, long-term data at hand are consistent with the sources being in luminous hard states.

astro-ph.HE↗

arXiv2020

Observations of ultraluminous X-ray sources with the 6-m telescope of the SAO RAS

The nature of ultraluminous X-ray sources (ULXs) is not yet fully understood but it can be done in the most efficient way by analyzing its optical emission and environment. We present preliminary results of photometric and spectral studies of some newly identified ULX optical counterparts. Based on the X-ray catalog of ULX candidates by Liu 2011 we have unequivocally identified in the optical range more than 100 sources, for 49 of them we carried out a long slit spectroscopy. According to these results, the majority of the most brightest candidates (m$_V < 22^m$) turned out to be background objects of different types, but the observed parameters of other sources of the sample allow us to classify them as real ULX. Here we present results for ten selected sources.

astro-ph.HE↗

arXiv2020

New luminous blue variable candidates in the NGC 247 galaxy

We search for LBV stars in galaxies outside the Local Group. Here we present a study of two bright $H\alpha$ sources in the NGC 247 galaxy. Object j004703.27-204708.4 ($M_V=-9.08 \pm 0.15^m$) shows the spectral lines typical for well-studied LBV stars: broad and bright emission lines of hydrogen and helium He I with P Cyg profiles, emission lines of iron Fe II, silicon Si II, nitrogen N II and carbon C II, forbidden iron [Fe II] and nitrogen [N II] lines. The variability of the object is $\Delta B = 0.74\pm0.09^m$ and $\Delta V = 0.88\pm0.09^m$, which makes it reliable LBV candidate. The star j004702.18-204739.93 ($M_V=-9.66 \pm 0.23^m$) shows many emission lines of iron Fe II, forbidden iron lines [Fe II], bright hydrogen lines with broad wings, and also forbidden lines of oxygen [O I] and calcium [Ca II] formed in the circumstellar matter. The study of the light curve of this star also did not reveal significant variations in brightness ($\Delta V = 0.29\pm0.09^m$). We obtained estimates of interstellar absorption, the photosphere temperature, as well as bolometric magnitudes $M_\text{bol}=-10.5^{+0.5}_{-0.4}$ and $M_\text{bol}=-10.8^{+0.5}_{-0.6}$, which corresponds to bolometric luminosities $\log(L_\text{bol}/L_{\odot})=6.11^{+0.20}_{-0.16}$ and $6.24^{+0.20}_{-0.25}$ for j004703.27-204708.4 and j004702.18-204739.93 respectively. Thus, the object j004703.27-204708.4 remains a reliable LBV candidate, while the object j004702.18-204739.93 can be classified as B[e]-supergiant.

astro-ph.SR↗

arXiv2020

Modeling of extended atmospheres with temperatures below 40000K

We calculate models of extended stellar atmospheres with a temperature in the range of 12000-40000K and a mass loss rate of $10^{-6}-10^{-4} M_{\odot}$ yr$^{-1}$. A large number of objects with emission spectra, such as luminous blue variables (LBV), FeII - emission line stars, Of/late-WN stars, and even ultraluminous X-ray sources (ULXs) often have effective temperatures in this range. The paper presents the results of model grids calculating in the form of equivalent width diagrams for the selected lines of hydrogen, He, Si, and Fe, as well as the results of studies of some emission objects using the calculated models.

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arXiv2019

Ultraluminous X-ray sources with flat-topped noise and QPO

We analyzed the X-ray power density spectra of five ultraluminous X-ray sources (ULXs) NGC5408 X-1, NGC6946 X-1, M82 X-1, NGC1313 X-1 and IC342 X-1 that are the only ULXs which display both flat-topped noise (FTN) and quasi-periodic oscillations (QPO). We studied the QPO frequencies, fractional root-mean-square (rms) variability, X-ray luminosity and spectral hardness. We found that the level of FTN is anti-correlated with the QPO frequency. As the frequency of the QPO and brightness of the sources increase, their fractional variability decreases. We propose a simple interpretation using the spherizarion radius, viscosity time and $α$-parameter as basic properties of these systems. The main physical driver of the observed variability is the mass accretion rate which varies >3 between different observations of the same source. As the accretion rate decreases the spherization radius reduces and the FTN plus the QPO move toward higher frequencies resulting in a decrease of the fractional rms variability. We also propose that in all ULXs when the accretion rate is low enough (but still super-Eddington) the QPO and FTN disappear. Assuming that the maximum X-ray luminosity depends only on the black hole (BH) mass and not on the accretion rate (not considering the effects of either the inclination of the super-Eddington disc nor geometrical beaming of radiation) we estimate that all the ULXs have about similar BH masses, with the exception of M82 X-1, which might be 10 times more massive.

astro-ph.HE↗

arXiv2019

New Luminous Blue Variable Candidates in NGC 4736

We have found three new LBV candidates in the star-forming galaxy NGC 4736. They show typical well-known LBV spectra, broad and strong hydrogen lines, He I lines, many Fe II lines, and forbidden [Fe II] and [Fe III]. Using archival Hubble Space Telescope and ground-based telescope data, we have estimated the bolometric magnitudes of these objects from -8.4 to -11.5, temperatures, and reddening. Source NGC 4736_1 (Mv = -10.2 +/- 0.1 mag) demonstrated variability between 2005 and 2018 as Delta V = 1.1 mag and Delta B = 0.82 mag, the object belongs to LBV stars. NGC 4736_2 (Mv < -8.6 mag) shows P Cyg profiles and its spectrum has changed from 2015 to 2018. The brightness variability of NGC 4736_2 is Delta V = 0.5 mag and Delta B = 0.4 mag. In NGC 4736_3 (Mv = -8.2 +/- 0.2 mag), we found strong nebular lines, broad wings of hydrogen; the brightness variation is only 0.2 mag. Therefore, the last two objects may reside to LBV candidates.

astro-ph.SR↗

arXiv2016

Optical counterparts of two ultraluminous X-ray sources NGC4559 X-10 and NGC4395 ULX-1

We study the optical counterparts of ultraluminous X-ray sources NGC4559 X-10 and NGC4395 ULX-1. Their absolute magnitudes, after taking the reddening into account, are $M_V \approx -5.3$ and $M_V \approx -6.2$, respectively. The spectral energy distribution of the NGC4559 X-10 counterpart is well fitted by a spectrum of an F-type star, whereas NGC4395 ULX-1 has a blue power-law spectrum. Optical spectroscopy of NGC4395 ULX-1 has shown a broad and variable HeII~$λ$4686 emission, which puts this object in line with all the other spectrally-studied ULXs. Using the Swift archival X-ray data for NGC4395 ULX-1, we have found a period of $62.8\pm 2.3$ days. The X-ray phase curve of the source is very similar to the precession curve of SS433. The optical variation of the counterpart (between two accurate measurements) amounts to 0.10 mag. Analyzing the absolute magnitudes of 16 well-studied ULX counterparts one may suggest that as the original accretion rate decreases (but nevertheless remains supercritical), the optical luminosity of the wind becomes dimmer and the donor star dominates. However, an observational bias may also influence the distribution.

astro-ph.HE↗

arXiv2015

Optical counterparts of two ULXs in NGC5474 and NGC3627 (M66)

We identified two optical counterparts of brightest ultraluminous X-ray Sources (ULXs) in galaxies NGC5474 and NGC3627 (M66). The counterparts in Hubble Space Telescope images are very faint, their V magnitudes are 24.7 ($M_V \approx -4.5$) and 25.9 ($M_V \approx -4.2$), respectively. NGC5474 X-1 changes the X-ray flux more than two orders of magnitude, in its bright state it has $L_X \approx 1.6 \times 10^{40}$ erg/s, the spectrum is best fitted by an absorbed power-law model with a photon index $Γ\approx 0.94$. M66 X-1 varies in X-rays with a factor of ~2.5, its maximal luminosity being $2.0 \times 10^{40}$ erg/s with $Γ\approx 1.7$. Optical spectroscopy of the NGC5474 X-1 has shown a blue spectrum, which however was contaminated by a nearby star of 23 mag, but the counterpart has a redder spectrum. Among other objects captured by the slit are a background emission-line galaxy (z=0.359) and a new young cluster of NGC5474. We find that these two ULXs have largest X-ray-to-optical ratios of $L_X/L_{opt}$ ~ 7000 for NGC5474 X-1 (in its bright state) and 8000 for M66 X-1 both with the faintest optical counterparts ever measured. Probably their optical emission originates from the donor star. If they have super-Eddington accretion discs with stellar-mass black holes, they may also have the lowest mass accretion rates among ULXs such as in M81 X-6 and NGC1313 X-1.

astro-ph.HE↗

arXiv2013

Ultra-luminous X-ray Sources as Supercritical Accretion Disks: Spectral Energy Distributions

We describe a model of spectral energy distribution in supercritical accretion disks (SCAD) based on the conception by Shakura and Sunyaev. We apply this model to five ultra-luminous X-ray sources (ULXs). In this approach, the disk becomes thick at distances to the center less than the spherization radius, and the temperature dependence is T \propto r^{-1/2}. In this region the disk luminosity is L_bol ~ L_Edd ln(Mdot/Mdot_Edd), and strong wind arises forming a wind funnel above the disk. Outside the spherization radius, the disk is thin and its total luminosity is Eddington, L_Edd. The thin disk heats the wind from below. From the inner side of the funnel the wind is heated by the supercritical disk. In this paper we do not consider Comptonization in the inner hot winds which must cover the deep supercritical disk regions. Our model is technically similar to the DISKIR model of Gierlinski et al. The models differ in disk type (standard - supercritical) and irradiation (disk - wind). We propose to distinguish between these two models in the X-ray region, ~0.3 - 1 keV, where the SCAD model has a flat nu F_nu spectrum, and the DISKIR model never has a flat part, as it is based on the standard alpha-disk. An important difference between the models can be found in their resulting black hole masses. In application to the ULX spectra, the DISKIR model yields black hole masses of a few hundred solar masses, whereas the SCAD model produces stellar-mass black holes ~10 M_sun.

astro-ph.HE↗