arXiv subjects

Pavel Abolmasov

Publications and source records attributed to Pavel Abolmasov.

30 records · Page 2Linked to original sources

arXiv2016

Gamma-ray opacity of the anisotropic stratified broad-line regions in blazars

The GeV-range spectra of blazars are shaped not only by non-thermal emission processes internal to the relativistic jet but also by external pair-production absorption on the thermal emission of the accretion disc and the broad-line region (BLR). For the first time, we compute here the pair-production opacities in the GeV range produced by a realistic BLR accounting for the radial stratification and radiation anisotropy. Using photoionization modelling with the CLOUDY code, we calculate a series of BLR models of different sizes, geometries, cloud densities, column densities and metallicities. The strongest emission features in the model BLR are Ly$α$ and HeII Ly$α$. Contribution of recombination continua is smaller, especially for hydrogen, because Ly continuum is efficiently trapped inside the large optical depth BLR clouds and converted to Lyman emission lines and higher-order recombination continua. The largest effects on the gamma-ray opacity are produced by the BLR geometry and localization of the gamma-ray source. We show that when the gamma-ray source moves further from the central source, all the absorption details move to higher energies and the overall level of absorption drops because of decreasing incidence angles between the gamma-rays and BLR photons. The observed positions of the spectral breaks can be used to measure the geometry and the location of the gamma-ray emitting region relative to the BLR. Strong dependence on geometry means that the soft photons dominating the pair-production opacity may be actually produced by a different population of BLR clouds than the bulk of the observed broad line emission.

astro-ph.HE↗

arXiv2015

On the Eddington limit for relativistic accretion discs

Standard accretion disc model relies upon several assumptions, the most important of which is geometrical thinness. Whenever this condition is violated, new physical effects become important such as radial energy advection and mass loss from the disc. These effects are important, for instance, for large mass accretion rates when the disc approaches its local Eddington limit. In this work, we study the upper limits for standard accretion disc approximation and find the corrections to the standard model that should be considered in any model aiming on reproducing the transition to super-Eddington accretion regime. First, we find that for thin accretion disc, taking into account relativistic corrections allows to increase the local Eddington limit by about a factor of two due to stronger gravity in General Relativity (GR). However, violation of the local Eddington limit also means large disc thickness. To consider consequently the disc thickness effects, one should make assumptions upon the two-dimensional rotation law of the disc. For rotation frequency constant on cylinders $r\sinθ=const$, vertical gravity becomes stronger with height on spheres of constant radius. On the other hand, effects of radial flux advection increase the flux density in the inner parts of the disc and lower the Eddington limit. In general, the effects connected to disc thickness tend to increase the local Eddington limit even more. The efficiency of accretion is however decreased by advection effects by about a factor of several.

astro-ph.HE↗

arXiv2014

Black Hole Spin Evolution Affected by Magnetic Field Decay

Black holes are spun up by accreting matter and possibly spun-down by magnetic fields. In our work we consider the effect on black hole rotation of the two electromagnetic processes, Blandford-Znajek and Direct Magnetic Link, that differ in their magnetic field configuration. The efficiency of these processes varies with mass accretion rate and accretion regime and generally result in an equilibrium spin parameter in the range from 0.35 to ~0.98. Magnetic field loses its energy while being accreted that may lead to an increase in equilibrium Kerr parameter for the case of advection-dominated disc. We find magnetic field decay decay can decrease electromagnetic term significantly thus increasing the Kerr parameter. We have performed Monte-Carlo simulations for a supermassive black hole population. Our simulations show broad distributions in Kerr parameter (0.1< a <0.98) with a peak at a~0.6. To explain the high observational Kerr parameter values of a>0.9, episodes of supercritical accretion are required. This implication does not however take into account black hole mergers (that play an important role for supermassive black hole evolution).

astro-ph.HE↗

arXiv2013

Black hole spin-down by truncated disc emission

The influence of disc radiation capture upon black hole rotational evolution is negligible for radiatively inefficient discs. For the standard thin disc model it is a slight but potentially important effect leading to the equilibrium spin parameter value of about 0.998. For optically thin discs, the fraction of disc radiation captured by the black hole is however about two times larger. In some disc radiation models, inner parts of the accretion flow are optically thin, advection-dominated flows, and the thin disc ends at some transition radius R_{tr}. The thermal energy of the disc stored in trapped radiation is released at this radius. Angular distribution of the radiation released at this radial photosphere facilitates its capture by the black hole. For accretion rates close to critical and disc truncation radius of (2..4) GM/c^2, radiation capture is most efficient in spinning the black hole down that may lead to a_{eq} ~ 0.996..0.997 or less depending on the mass accretion rate. For an accretion flow radiating some constant fraction εof dissipated energy, the equilibrium Kerr parameter is shown to obey the relation 1-a_{eq} \propto ε^{3/2} as long as 1-a_{eq} << 1. Deviations from Keplerian law near the last stable orbit dominate over the radiation capture effect if they exceed 1..2%.

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arXiv2012

Microlensing evidence for super-Eddington disk accretion in quasars

Gravitational microlensing by the stellar population of lensing galaxies provides an important opportunity to spatially resolve the accretion disk structure in strongly lensed quasars. Some of the objects (like Einstein's cross) are reasonably consistent with the predictions of the standard accretion disk model. In other cases, the size of the emitting region is larger than predicted by the standard thin disk theory and practically independent on wavelength. This may be interpreted as an observational manifestation of an optically-thick scattering envelope possibly related to super-Eddington accretion with outflows.

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arXiv2011

Modelling the optical spectrum of Romano's star

We consider the luminous blue variable (LBV) star V532 in M33, also known as Romano's star, in two different spectral states: in the optical minimum of 2007/2008 and during a local brightening in 2005. Optical spectra of low and moderate resolution are modelled using the non-LTE model atmosphere code CMFGEN. All the observed properties of the object in the minimum are well described by a late nitrogen-sequence Wolf-Rayet (WN) star model with relatively high hydrogen abundance (H/He=1.9), while the spectrum during the outburst corresponds to the spectral class WN11 and is similar to the spectrum of P Cyg. The atmosphere is enriched in nitrogen by about a factor of 6 in both states. Most of the heavy element abundances are consistent with the chemical composition of M33. Bolometric luminosity is shown to vary between the two states by a factor of ~1.5. This makes V532 another example of an LBV that shows variations in its bolometric luminosity during an outburst.

astro-ph.SR↗

arXiv2011

Spectral Study of the WNL Star FSZ35 in the M33 Galaxy

We study and analyse low-resolution spectra of the unordinary late WN star FSZ35 in M33. We classify the object as a hydrogen-rich WN8 star. Using the radiative transfer code CMFGEN, we determine the physical parameters of this object and compare them to the parameters of other WN8 stars including the LBV star Romano's star during the minimum of brightness. Unlike Romano's star, the object is fairly stable both spectrally and photometrically, that may be attributed to its more advanced evolutionary stage or lower luminosity. FSZ35 is shown to possess a compact nebula producing faint but detectable [OIII]emission. Location of this object at a large distance (about 100 pc) from the nearest association suggests the object may be one more example of a massive runaway star.

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arXiv2011

Stochastic Variability of Luminous Blue Variables

Using the archives of the American Association of Variable Stars Observers and our own data, we analyse the long-term variability of several well-studied Luminous Blue Variables (LBVs) aiming on a general picture of stochastic variability of these objects. The power density spectra of all the selected objects may be generally described by a single power law contaminated by observational noise at higher frequencies. The slopes of the power-law component are close to p=2 (where PDS ~ f^{-p}, and f is frequency) for strongly variable flaring objects like AG Car and significantly smaller (p~1.3) for P Cyg where brightness variation amplitude is <~ 1 mag and dominated by slow low-amplitude variability. The slope holds for about two orders of magnitude in the frequency domain, though peaks and curvatures are present at f ~ 10^{-2}.. 10^{-3} d^{-1}. We show that pseudo-photosphere approach to variability may explain the power-law shape of the variability spectrum at higher frequencies. However, the observed spectra are actually rather "red" than "brown": flux variations are correlated up to tens of years that is much longer than the characteristic refreshment time scales of the pseudo-photosphere. We propose that several stochastic noise components produce the power spectra of LBV stars.

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arXiv2010

Understanding ULX Nebulae in the Framework of Supercritical Accretion

For a long time, the well-known supercritically accreting binary SS433 is being proposed as a prototype for a class of hypothetical bright X-ray sources that may be identified with the so-called Ultraluminous X-ray sources (ULXs) in nearby galaxies or at least with part of them. Like SS433, these objects should be associated with optical nebulae, powered by both radiation of the central source and its wind or jet activity. Indeed, around many ULXs, bright optical nebulae (ULX Nebulae, ULXNe) are found. Here, we use SS433 as a prototype for the power source creating the nebulae around ULXs. Though many factors are important such as the structure of the host star-forming region and the possible supernova remnant formed together with the accreting compact object, we show that most of the properties of ULXNe may be explained by an SS433-like system evolving for up to about one million years in a constant density environment. The basic stages of evolution of a ULX Nebula include a non-spherical HII-region with a central cavity created by non-radiative shock waves, an elongated or bipolar shock-powered nebula created by jet activity and a large-scale quasi-spherical bubble.

astro-ph.HE↗

arXiv2008

Optical Spectroscopy of the ULX-Associated Nebula MF16

We present the results of optical panoramic and long-slit spectroscopy of the nebula MF16 associated with the Ultraluminous X-ray Source NGC6946 ULX-1. More than 20 new emission lines are identified in the spectra. Using characteristic line ratios we find the electron density n_e ~ 600cm^{-3}, electron temperature in the range from ~9000K to ~20 000K (for different diagnostic lines) and the total emitting gas mass M ~ 900 Msolar. We also estimate the interstellar extinction towards the nebula as A_V = 1.m54 somewhat higher than the Galactic absorption. Observed line luminosities and ratios appear to be inconsistent with excitation and ionization by shock waves so we propose the central object responsible for powering the nebula. We estimate the parameters of the ionizing source using photon number estimates and Cloudy modelling. Required EUV luminosity ($\sim 10^{40}$\ergl) is high even if compared with the X-ray luminosity. We argue that independently of their physical nature ULXs are likely to be bright UV and EUV sources. It is shown that the UV flux expected in the GALEX spectral range (1000-3000Angstroms) is quite reachable for UV photometry. Measuring the luminosities and spectral slopes in the UV range may help to distinguish between the two most popular ULX models.

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arXiv2008

Ultraluminous X-ray Sources and Their Nebulae

One of the interesting features of Ultraluminous X-ray sources is that many of them are surrounded by luminous nebulae exhibiting diverse observational properties. In different cases the nebulae are photoionized or shock-powered. Generally, the two energy sources appear to coexist. ULX bubble nebulae may be considered a new class of shock-powered nebulae similar to upscaled versions of stellar wind bubbles. Their expansion rates support constant energy influx rather than single powerful events like Hypernova explosions.

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arXiv2007

Super-critically accreting stellar-mass black holes as ultraluminous X-ray sources

We derive the luminosity-temperature relation for the super-critically accreting black holes (BHs) and compare it to the data on ultraluminous X-ray sources (ULXs). At super-Eddington accretion rates, an outflow forms within the spherization radius. We construct the accretion disc model accounting for the advection and the outflow, and compute characteristic disc temperatures. The bolometric luminosity exceeds the Eddington luminosity L_Edd by a logarithmic factor 1+0.6 ln mdot (where mdot is the accretion rate in Eddington units) and the wind kinetic luminosity is close to L_Edd. The apparent luminosity for the face-on observer is 2-7 times higher because of geometrical beaming. Such an observer has a direct view of the inner hot accretion disc, which has a peak temperature T_max of a few keV in stellar-mass BHs. The emitted spectrum extends as a power-law F_E ~ E**{-1} down to the temperature at the spherization radius T_sp ~ mdot**(-1/2) keV. We associate T_max with a few keV spectral components and T_sp with the soft, 0.1-0.2 keV components observed in ULXs. An edge-on observer sees only the soft emission from the extended envelope, with the photosphere radius exceeding the spherization radius by orders of magnitude. The dependence of the photosphere temperature on luminosity is consistent with that observed in the super-Eddington accreting BHs SS 433 and V4641 Sgr. Strong outflows combined with the large intrinsic X-ray luminosity of the central BH explain naturally the presence of the photoionized nebulae around ULXs. An excellent agreement between the model and the observational data strongly argues in favour of ULXs being super-critically accreting, stellar-mass BHs similar to SS 433, but viewed close to the symmetry axis.

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