arXiv subjects

Andrei M. Beloborodov

Publications and source records attributed to Andrei M. Beloborodov.

134 records · Page 8Linked to original sources

arXiv2000

Small-scale inviscid accretion discs around black holes

Gas falling quasi-spherically onto a black hole forms an inner accretion disc if its specific angular momentum $l$ exceeds $\lmin\sim r_gc$ where $r_g$ is the Schwarzschild radius. The standard disc model assumes $l\gg\lmin$. We argue that, in many black-hole sources, accretion flows have angular momenta just above the threshold for disc formation, $l\simgt\lmin$, and assess the accretion mechanism in this regime. In a range $\lmin<l<\lcr$, a small-scale disc forms in which gas spirals fast into the black hole without any help of horizontal viscous stresses. Such an `inviscid' disc, however, interacts inelastically with the feeding infall. The disc-infall interaction determines the dynamics and luminosity of the accretion flow. The inviscid disc radius can be as large as $14 r_g$, and the energy release peaks at $2r_g$. The disc emits a Comptonized X-ray spectrum with a break at $\sim 100$ keV. This accretion regime is likely to take place in wind-fed X-ray binaries and is also possible in active galactic nuclei.

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arXiv2000

On the Efficiency of Internal Shocks in Gamma-Ray Bursts

The fraction of a fireball kinetic kinetic energy that can be radiated away by internal shocks is sensitive to the amplitude of initial fluctuations in the fireball. We give a simple analytical description for dissipation of modest-amplitude fluctuations and confirm it with direct numerical simulations. At high amplitudes, the dissipation occurs in a non-linear regime with efficiency approaching 100 %. Most of the explosion energy is then radiated away by the prompt GRB and only a fraction remains to be radiated by the afterglow.

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arXiv1999

Power Density Spectra of Gamma-Ray Bursts

Power density spectra (PDSs) of long gamma-ray bursts provide useful information on GRBs, indicating their self-similar temporal structure. The best power-law PDSs are displayed by the longest bursts (T_90 > 100 s) in which the range of self-similar time scales covers more than 2 decades. Shorter bursts have apparent PDS slopes more strongly affected by statistical fluctuations. The underlying power law can then be reproduced with high accuracy by averaging the PDSs for a large sample of bursts. This power-law has a slope alpha\approx -5/3 and a sharp break at 1 Hz. The power-law PDS provides a new sensitive tool for studies of gamma-ray bursts. In particular, we calculate the PDSs of bright bursts in separate energy channels. The PDS flattens in the hard channel (hν> 300 keV) and steepens in the soft channel (hν< 50 keV), while the PDS of bolometric light curves approximately follows the -5/3 law. We then study dim bursts and compare them to the bright ones. We find a strong correlation between the burst brightness and the PDS slope. This correlation shows that the bursts are far from being standard candles and dim bursts should be intrinsically weak. The time dilation of dim bursts is probably related to physical processes occurring in the burst rather than to a cosmological redshift. Finally, we test the internal shock model against the observed PDS. We demonstrate how the model can reproduce the -5/3 power law.

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arXiv1999

Electron-Positron Outflows from Gamma-Ray Emitting Accretion Discs

An electron-positron atmosphere is inevitably created around a black hole accretion disc, the spectrum of which extends to MeV energies. Pairs created in photon-photon collisions outside the disc are blown away by soft radiation (which dominates the bolometric luminosity of the disc) and form a semirelativistic outflow. We simulate numerically the conversion of the MeV radiation into a vertical e+- outflow above a disc-like source. The outflowing e+- plasma becomes optically thick to Thomson scattering if the compactness of the gamma-ray source exceeds 30. The scattering by e+- then collimates the bulk of soft radiation along the disc axis, and the apparent bolometric luminosity of the disc depends strongly on its inclination to the line of sight. The anisotropic central emission may account for the lack of Fe K-alpha lines in the X-ray spectra of bright radio-quiet quasars. The scattering in e+- outflows may also explain the orientation of optical polarization in non-blazar active galactic nuclei.

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arXiv1999

Accretion Disk Models

Models of black hole accretion disks are reviewed, with an emphasis on the theory of hard X-ray production. The following models are considered: i) standard, ii) super-critical, iii) two-temperature, and iv) disk+corona. New developments have recently been made in hydrodynamical models of accretion and in phenomenological radiative models fitting the observed X-ray spectra. Attempts to unify the two approaches are only partly successful.

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arXiv1998

Super-Eddington accretion disc around a Kerr black hole

We calculate the structure of accretion disc around a rapidly rotating black hole with a super-Eddington accretion rate. The luminosity and height of the disc are reduced by the advection effect. In the case of a large viscosity parameter, alpha > 0.03, the accretion flow strongly deviates from thermodynamic equilibrium and overheats in the central region. With increasing accretion rate, the flow temperature steeply increases, reaches a maximum, and then falls off. The maximum is achieved in the advection dominated regime of accretion. The maximum temperature in the disc around a massive black hole, M=10^8 M_sun, with alpha=0.3 is of order 3 x 10^8 K. Discs with large accretion rates can emit X-rays in quasars as well as in galactic black hole candidates.

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arXiv1998

Polarization change due to fast winds from accretion disks

A fraction of the radiation produced by an accretion disk may be Thomson scattered by a wind flowing away from the disk. Employing a simple plane-parallel model of the wind, we calculate the polarization of the scattered radiation and find that its sign depends on the wind velocity, beta=v/c. In the case, 0.12 < beta < 0.78, the polarization is parallel to the disk normal, i.e., it is orthogonal to the standard Chandrasekhar's polarization expected from accretion disks. The velocity of an electron-positron wind is likely to saturate near the equilibrium value beta_* of order 0.5 for which the accelerating radiation pressure is balanced by the Compton drag. Then the change of polarization by the wind is most pronounced. This may help to reconcile the standard accretion disk model with the optical polarimetric observations of non-blazar AGNs.

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arXiv1996

Special relativity and the pseudo-Newtonian potential

A simple re-scaling of velocities calculated from the Paczy{ń}ski \& Wiita (1980) pseudo-Newtonian potential makes them consistent with special relativity and greatly improves the agreement with the exact relativistic calculations. The improvement is relevant in calculations of the Doppler effect, spectra and radiation transfer which all involve effects of special relativity.

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