Andrei Beloborodov

Research Summary

Andrei M. Beloborodov specializes in high energy phenomena in the Universe: X-ray binaries, active galactic nuclei, and cosmological gamma-ray bursts (GRBs). His major interests include physics of black holes and neutron stars, mechanism of relativistic explosions, and related radiative processes.

Research Projects:
(September 2002 - August 2003)

Young stellar disk in the galactic center

Andrei Beloborodov and Yuri Levin analyzed the three-dimensional velocities of young stars in the inner 0.1 pc region of our galaxy and discovered that 10 out of total 13 stars in the sample have velocities lying in a single plane. The 10 stars also have common (clockwise) sense rotation on the sky. Levin and Beloborodov concluded that a thin stellar disk exists in the galactic center. This sheds light on the mysterious origin of young stars in this region where normal star formation is inhibited by the strong tidal field of the central black hole. Levin and Beloborodov proposed that the stars are remnants of a dense gaseous disk around the black hole that existed several million years ago and clumped into stars by gravitational instability.

Mass estimation in gravitating systems

Andrei Beloborodov and Yuri Levin developed a new method of ``cosmic roulette'' for mass estimation in gravitating systems with known instantaneous positions and velocities of test bodies. The method is based on a novel statistical approach to this classical astronomical problem. It allows one to calculate consistently both mean expectation (best-fit) value and error bars at a given confidence level, which was not possible with traditional methods based on the virial theorem. As a test, the method was applied to many Monte-Carlo realizations of N random satellites around a known mass, and showed a high performance. The method was applied to the galactic center and a new independent estimate was obtained for the mass of the central black hole

Neutron-fed blast waves in GRBs

Andrei Beloborodov made detailed calculations of the nuclear composition of GRB fireballs and showed that a significant part of ejected baryons are free neutrons with a Lorentz factor 100-1000. Their lifetime is increased by 100-1000 due to relativistic time dilation, and neutrons survive till the decelerating stage of the explosion. The neutrons overtake the decelerating blast wave and deposit mass, momentum, and heat into the external medium via beta-decay. This qualitatively changes the mechanism of GRB afterglow emission at radii up to ten times the mean radius of beta-decay, which covers the main peak of blast wave emission and a part of its subsequent fall-off tail. The exponentially decaying neutron precursor changes the standard picture of self-similar blast waves in GRBs, and this can be tested by observations of early afterglows by the upcoming Swift mission.

Beta-decay heating of GRB fireballs

Elena Rossi (IoA, Cambridge), Andrei Beloborodov, and Martin Rees (IoA, Cambridge) investigated the impact of neutron component on the intrinsic dynamics of GRB fireballs and their thermal history. Beta-decay of a small fraction of neutrons at early times of the fireball expansion was found to result in significant heating of the fireball material. A popular model of GRB emission envisions a fireball with a strongly inhomogeneous Lorentz factor, which leads to internal shocks. The decaying neutron component serves as a decelerating background for the fastest shells in the fireball and reduces the contrast of Lorentz factors. As a result, neutrons reduce the amplitude of internal shocks or suppress them completely. This may change the view of internal shocks as a dominant mechanism of gamma-ray emission in GRBs.

Currents in twisted magnetospheres of neutron stars

Andrei Beloborodov and Chris Thompson investigated the formation of currents in twisted magnetospheres of strongly magnetized neutron stars. The current is dictated by the twist and carried by ions and electrons which are pulled out of the star by an electric field. Charges go up to the magnetosphere and, following the closed magnetic line, fall back to the star. It was shown that the presence of the gravitational potential barrier between cathode and anode excludes a steady current. To undestand the time-dependent behavior of the system a numerical simulation has been done. It gave a surprising result: particles carrying the current develop enormous energies in the magnetosphere, much higher than needed to overcome the potential barrier. If true, neutron-star magnetospheres could not have significant twists because they dissipate quickly. The simulation, however, neglects the coupling of currents on neighboring field lines, which can change the result. This is a subject of current investigation.

[ Back to CITA People | CITA Research 2003 | CITA Annual Report 2003 ]

Questions and Comments <frolov@cita.utoronto.ca>