arXiv subjects

A. S. Tavleev

Publications and source records attributed to A. S. Tavleev.

3 recordsLinked to original sources

arXiv2025

Fast giant flares in discs around supermassive black holes

We studied the thermal stability of non-self-gravitating turbulent $α$-discs around supermassive black holes (SMBHs) to test a new type of high-amplitude galactic nucleus flares. By calculating the disc structures, we computed the critical points of equilibrium curves for discs around SMBHs, which cover a wide range of accretion rates and resemble the shape $ξ$. We find that a transition of a disc ring from a recombined cold state to a hot, fully ionised, advection dominated, geometrically thick state is possible. Such a transition can trigger a giant flare for SMBHs with masses $\sim 10^6-10^8\, M_\odot$ if the prior geometrically thin and optically thick disc surrounded a central radiatively inefficient accretion flow. An increase in the viscosity parameter $α$ is a necessary condition for this scenario. This increase may be related to the fact that the magnetic Prandtl number increases and exceeds 1 during ionisation. When self-gravity effects in the disc are negligible, the duration and power of the flare exhibit a positive correlation with the prior truncation radius of the geometrically thin disc. According to our estimates, the mass of about $\sim 4-3000\, M_\odot$ can be involved in the giant flare lasting 1 to 400 years if the flare is triggered somewhere between $60$ and $600$ gravitational radii from the SMBH of $10^7\, M_\odot$. The accretion rate on the SMBH peaks about 10 times faster at the potentially super-Eddington level. An optically thick outflow leads to anisotropy of the emission. At the beginning of the giant flare, the region near the truncation radius is heated to $\sim 10^5\,$K, and its UV/optical luminosity is at least $\sim 0.3-4 \,L_\mathrm{Edd}$ depending on the SMBH mass. The sudden heating of a cold disc around a SMBH can trigger a massive outburst, similar in appearance to what is proposed to occur after a tidal disruption event.

astro-ph.HE↗

arXiv2024

Application of hydrostatic local thermodynamic equilibrium atmosphere models to interpretations of supersoft X-ray source spectra

Supersoft X-ray sources (SSSs) are accreting white dwarfs (WDs) with stable or recurrent thermonuclear burning on their surfaces. High-resolution X-ray spectra of such objects are rather complex, often consist of several components, and are difficult to interpret accurately. The main emission source is the hot surface of the WD and the emergent radiation can potentially be described by hot WD model atmospheres. We present a new set of such model atmosphere spectra computed in the effective temperature range from $100\rm\,kK$ to $1000\rm\,kK$, for eight values of surface gravity and three different chemical compositions. These compositions correspond to the solar one as well as to the Large and Small Magellanic Clouds, with decreased heavy element abundances, at one-half and one-tenth of the solar value. The presented model grid covers a broad range of physical parameters and, thus, it can be applied to a wide range of objects. It is also publicly available in XSPEC~format. As an illustration, we applied it here for the interpretation of \textit{Chandra} and XMM grating spectra of two classical SSSs, namely, CAL 83 (RX J0543.5$-$6823) and RX J0513.9$-$6951. The obtained effective temperatures and surface gravities of $T_{\rm eff} \approx 560$ kK, $\log g \approx 8.6-8.7$, and $T_{\rm eff} \approx 630\,{\rm kK}, \log g \approx 8.5-8.6$, respectively, are in a good agreement with previous estimations for both sources. The derived WD~mass estimations are within $1.1-1.4\,M_\odot$ for CAL 83 and $1.15-1.4\,M_\odot$ for RX J0513.9$-$6951. The mass of the WD in CAL $83$ is consistent with the mass predicted from the respective model of recurrent thermonuclear burning.

astro-ph.SR↗

arXiv2023

Analysis of accretion disc structure and stability using open code for vertical structure

Radial structure of accretion discs around compact objects is often described using analytic approximations which are derived from averaging or integrating vertical structure equations. For non-solar chemical composition, partial ionization, or for supermassive black holes, this approach is not accurate. Additionally, radial extension of `analytically-described' disc zones is not evident in many cases. We calculate vertical structure of accretion discs around compact objects, with and without external irradiation, with radiative and convective energy transport taken into account. For this, we introduce a new open Python code, allowing different equations of state (EoS) and opacity laws, including tabular values. As a result, radial structure and stability `S-curves' are calculated for specific disc parameters and chemical composition. In particular, based on more accurate power-law approximations for opacity in the disc, we supply new analytic formulas for the farthest regions of the hot disc around stellar-mass object. On calculating vertical structure of a self-irradiated disc, we calculate a self-consistent value of the irradiation parameter $C_{\rm irr}$ for stationary $α$-disc. We find that, for a fixed shape of the X-ray spectrum, $C_{\rm irr}$ depends weakly on the accretion rate but changes with radius, and the dependence is driven by the conditions in the photosphere and disc opening angle. The hot zone extent depends on the ratio between irradiating and intrinsic flux: corresponding relation for $T_{\rm irr,\, crit}$ is obtained.

astro-ph.HE↗