arXiv subjects

E. A. Nikolaeva

Publications and source records attributed to E. A. Nikolaeva.

5 recordsLinked to original sources

arXiv2026

Study of Massive OBA Stars with X-ray Emission

A study of the physical parameters of a sample of 15 OBA-type stars with detected X-ray emission from the Spektr-RG/eROSITA telescope is presented. While X-ray emission from cool stars (spectral types F-G-K-M) originates in their near-surface regions, namely in the chromosphere and corona, the origin of X-ray emission in OBA stars requires case-by-case analysis since isolated OBA stars are not intrinsic X-ray emitters. In this work, we derive the fundamental parameters of the stars in our sample, including the effective temperature $T_{\mathrm{eff}}$ and surface gravity $\log g$, based on spectral energy distribution fitting and optical spectroscopy obtained with the 1.5-m Russian-Turkish telescope RTT-150. An additional analysis of the $\mathrm{H}α$ line profiles allows us to identify possible mechanisms responsible for the observed X-ray emission, including non-stationary stellar winds, interactions in circumstellar material, and coronal emission from hidden cool companions. We find that the X-ray emission in eight stars, with typical luminosities in the range $\log L_{\mathrm{X}} = 28.5$-$30.0$, is most likely associated with hidden late-type companions.

astro-ph.SR↗

arXiv2022

SRGe2149+6736 -- the new candidate to AM~Her type variables discovered by eROSITA telescope on "Spectrum--Roentgen--Gamma" orbital observatory

We present the results of the optical identification, classification, as well as analysis of photometric and spectral observations of the X-ray transient SRGe2149+6736 detected by the eROSITA telescope during SRG all-sky X-ray survey. Photometric observations of the optical companion of SRGe2149+6736 were carried out on 6m telescope BTA SAO RAS, 1.5m Russian-Turkish telescope RTT-150 and 2.5m telescope CMO of Moscow State University. Together with ZTF data they showed that the source is a cataclysmic variable with an orbital period $P=85\pm0.4$~min which demonstrates long-term brightness variability from $23.5$~mag (low state) to $20$~mag (high state). The high-state light curves are consistent with a model of accreting magnetic white dwarf and suggest that SRGe2149+6736 belongs to AM~Her type variables. The optical spectra obtained in the low state are consistent with a spectral energy distribution of a white dwarf with a temperature of ~24000 K.

astro-ph.HE↗

arXiv2021

Optical identification of active galactic nucleus candidates detected by the Mikhail Pavlinsky ART-XC telescope aboard the SRG observatory during the all-sky X-ray survey

We present the results of our identification of eight objects from a preliminary catalogue of X-ray sources detected in the 4-12 keV energy band by the Mikhail Pavlinsky ART-XC telescope aboard the SRG observatory during its first all-sky survey. Three of them (SRGAJ005751.0+210846,SRGAJ014157.0-032915, SRGAJ232446.8+440756) have been discovered by ART-XC, while five were already known previously as X-ray sources, but their nature remained unknown. The last five sources have also been detected in soft X-rays by the eROSITA telescope of the SRG observatory. Our optical observations were carried out at the 1.6-m AZT-33IK telescope of the Sayan Observatory and the 1.5-m Russian-Turkish telescope (RTT-150). All of the investigated objects have turned out to be active galactic nuclei (AGNs) at redshifts from 0.019 to 0.283. Six of them are Seyfert 2 galaxies(including one Seyfert 1.9 galaxy), one (SRGAJ005751.0+210846) is a "hidden" AGN (in an edge-on galaxy), and one (SRGAJ224125.9+760343) is a narrow-line Seyfert 1 galaxy. The latter object is characterized by a high X-ray luminosity (~(2-13)*10^44 erg/s in the 4-12 keV band) and,according to our black hole mass estimate (~2*10^7 M_sun), an accretion rate close to the Eddington limit. All three AGNs discovered by the ART-XC telescope (which are not detected by the eROSITA telescope) are characterized by a high absorption column density (N_H > 3*10^23 cm^-2). The results obtained confirm the expectations that the ART-XC telescope is an efficient instrument for searches of heavily obscured and other interesting AGNs in the nearby (z<0.3) Universe. The SRG all-sky survey will last for more than 3 years more, which will allow many such objects to be discovered.

astro-ph.GA↗

arXiv2017

Spectroscopic study of the optical counterpart to the Fast X-ray Transient IGR J17544-2619 based on observations at the 1.5-m RTT-150 telescope

We present the results of our long-term photometric and spectroscopic observations at the Russian-Turkish RTT-150 telescope for the optical counterpart to one of the best-known sources, representatives of the class of fast X-ray transients, IGR J17544-2619. Based on our optical data, we have determined for the first time the orbital and physical parameters of the binary system by the methods of Doppler spectroscopy. We have computed theoretical spectra of the optical counterpart by applying non-LTE corrections for selected lines and obtained the parameters of the stellar atmosphere and the optical star ($T_{\rm eff}=33\,000$ K, $\log\,g=3.85$, $R=9.5\ R_{\odot},$ and $M=23\ M_{\odot}$). The latter suggest that the optical star is not a supergiant as has been thought previously.

astro-ph.HE↗

arXiv2010

Modeling of the spectral energy distribution of the cataclysmic variable TT Ari and evaluation of the system parameters

The spectral energy distribution (SED) of the TT Ari system, which is well known from published IUE and optical photometric observations, was modeled by a steady-state accretion α-disc around a white dwarf. Parameters of the system were derived from time-resolved optical spectral observations in the bright state that we obtained in Sep. 1998. The radial velocity semi-amplitude of the white dwarf (33.8 +/- 2.5 km/s) and corresponding mass function (f(M) = 5.5 +/- 1.2 *10^{-4}~ M_sun) were derived from the motion of the emission components of Balmer lines. The mass ratio q (\approx 0.315) was evaluated from the fractional period excess of the superhump period over the orbital period ε(\approx 0.085), and a secondary mass range (0.18 - 0.38 M_sun) was estimated from the orbital period. Therefore, the white dwarf mass range is 0.57 - 1.2 M_sun and the inclination angle of the system to the line of sight is 17 - 22.5 degrees. The adopted distance to the system is 335 +/- 50 pc. To fit the observed SED it is necessary to add a thermal spectrum with T \approx 11600 K and luminosity \approx 0.4 L_disk to the accretion disc spectrum. This combined spectrum successfully describes the observed Balmer lines absorption components. Formally the best fit of the HeI 4471 line gives minimum masses of the components (M_RD = 0.18 M_sun and M_WD = 0.57 M_sun), with the corresponding inclination angle i = 22.1 deg and mass-accretion rate \dot M = 2.6 * 10^{17} g/s.

astro-ph.SR↗