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Andrei M. Beloborodov

Publications and source records attributed to Andrei M. Beloborodov.

At least 127 records · Page 7Linked to original sources
arXiv2004

Orbital roulette: a new method of gravity estimation from observed motions

The traditional way of estimating the gravitational field from observed motions of test objects is based on the virial relation between their kinetic and potential energy. We find a more efficient method. It is based on the natural presumption that the objects are observed at a random moment of time and therefore have random orbital time phases. The proposed estimator, which we call "orbital roulette", checks the randomness of the phases. The method has the following advantages: (1) It estimates accurately Keplerian (point-mass) potentials as well as non-Keplerian potentials where the unknown gravitating mass is distributed in space. (2) It is a complete statistical estimator: it checks a trial potential and accepts it or rules it out with a certain significance level; the best-fit measurement is thus supplemented with error bars at any confidence level. (3) It needs no a priori assumptions about the distribution of orbital parameters of the test bodies. We test our estimator with Monte-Carlo-generated motions and demonstrate its efficiency. Useful applications include the Galactic Center, dark-matter halo of the Galaxy, and clusters of stars or galaxies.

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arXiv2004

Early Stages of the GRB Explosion

Physics of GRB blast waves is discussed with a focus on two effects: (1) pair creation in the external medium by the gamma-ray front and (2) decay of neutrons ahead of the decelerating blast wave. Both effects impact the afterglow mechanism at radii up to 10^{17}cm.

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arXiv2004

Theoretical Aspects of Gamma-Ray Bursts

Cosmological GRBs are discussed with an emphasis on their plausible connection with black holes. GRBs can be triggered by collapse of stellar-mass objects that leads to formation of a black hole and a transient debris disk with a huge accretion rate. The disk is believed to produce a relativistic jet (``fireball'') that expands and emits to infinity the observed burst of gamma-rays. This accretion-jet picture is similar to quasars and X-ray binaries, however, there are important differences: the physical conditions and the cooling mechanism in the disk are very different. The observed radiation is emitted when the expanding fireball becomes transparent, at distances much larger than the Schwarzschild radius. The burst is then observed as a powerful relativistic explosion and the transient accretion disk in its center serves as a brief source of energy that drives the explosion. The explosion picture depends on the fireball nuclear composition which is shaped close to the black hole. A large amount of free neutrons survive till the emission phase and link the physics of the central engine to observed radiation.

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arXiv2003

Interaction of GRB Fireballs with Ambient Medium

The customary picture of the fireball deceleration by an external medium neglects two physical agents in the problem that are important at radii R<10^{17} cm: a radiative precursor (the prompt GRB) and a relativistic front of free neutrons. The radiative precursor preaccelerates the medium and loads it with e+- pairs. This provides a new explanation for the early optical flashes in GRBs. The front of free neutrons must form in a standard baryonic fireball and change the mechanism of the fireball deceleration. The neutron effect, however, disappears if the fireball is strongly dominated by the Poynting flux. The neutrons thus provide a unique link between the progenitor physics and the observed external blast wave. The explosion mechanism at R<10^{17} cm is testable with upcoming observations of early afterglows.

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arXiv2003

Stellar disk in the galactic center -- a remnant of a dense accretion disk?

Observations of the galactic center revealed a population of young massive stars within 0.4 pc from Sgr A* -- the presumed location of a supermassive black hole. The origin of these stars is a puzzle as their formation in citu should be suppressed by the black hole's tidal field. We find that out of 13 stars whose 3-dimensional velocities have been measured by Genzel et. al. (2000), 10 lie in a thin disk. The half-opening angle of the disk is consistent with zero within the measurement errors, and does not exceed 10 degrees. We propose that a recent burst of star formation has occurred in a dense gaseous disk around Sgr A*. Such a disk is no longer present because, most likely, it has been accreted by the central black hole. The three-dimensional orbit of S2, the young star closest to Sgr A*, has been recently mapped out with high precision. It is inclined to the stellar disk by 75 degrees. We find that the orbit should undergo Lense-Thirring precession with the period of (5/a) Myr, where a<1 is the dimensionless spin of the black hole. Therefore it is possible that originally S2 orbit lay in the disk plane. If so, we can constrain the black hole spin $a$ be greater than 0.2(t_{S2}/5 Myr), where t_{S2} is the age of S2.

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arXiv2003

Fireballs with a Neutron Component

Standard GRB fireballs must carry free neutrons. This crucially changes the mechanism of fireball deceleration by an external medium. As the ion fireball decelerates, the coasting neutrons form a leading front. They gradually decay, leaving behind a relativistic trail of decay products mixed with the ambient medium. The ion fireball sweeps up the trail and drives a shock wave in it. Thus, observed afterglow emission is produced in the neutron trail. The impact of neutrons turns off at 10^{17}cm from the explosion center, and here a spectral transition is expected in GRB afterglows. Absence of neutron signatures would point to absence of baryons and a dominant Poynting flux in the fireballs.

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arXiv2003

Neutron-Fed Afterglows of Gamma-Ray Bursts

Fireballs of gamma-ray bursts are partially made of free neutrons. Their presence crucially changes the mechanism of the fireball deceleration by an external medium. At late stages of the explosion, neutrons fully decouple from ions and coast with a constant Lorentz factor Γ_n. As the ion fireball decelerates, the neutrons form a leading front. This front gradually decays, leaving behind a trail of decay products mixed with the ambient medium. The kinetic energy of the decay products far exceeds the medium rest-mass energy, and the trail has a high Lorentz factor at radii R < R_{trail} = 10R_β\approx 10^{17} cm where R_β= 10^{16}(Γ_n/300) cm is the mean radius of neutron decay. The ion fireball sweeps up the trail material and drives a shock wave in it. Thus, observed afterglow emission is produced in the neutron trail. It can naturally re-brighten or display a spectral transition at R \approx R_{trail} where the impact of neutrons turns off. Absence of any neutron signatures would point to an extremely low baryon loading of the fireballs and a strong dominance of a Poynting flux.

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arXiv2003

Nuclear Composition of Gamma-Ray Burst Fireballs

We study three processes that shape the nuclear composition of GRB fireballs: (1) neutronization in the central engine, (2) nucleosynthesis in the fireball as it expands and cools, and (3) spallation of nuclei in subsequent internal shocks. The fireballs are found to have a neutron excess and a marginally successful nucleosynthesis. They are composed of free nucleons, alpha-particles, and deuterium. A robust result is the survival of a significant neutron component, which has important implications. First, as shown in previous works, neutrons can lead to observable multi-GeV neutrino emission. Second, as we show in an accompanying paper, neutrons impact the explosion dynamics at radii up to 10^{17} cm and change the mechanism of the GRB afterglow emission.

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arXiv2002

Early GRB Afterglows Revisited

The standard afterglow model neglects the presence of the GRB prompt radiation ahead of the blast wave. In fact, the blast wave is dramatically influenced by the leading gamma-ray front which preaccelerates the ambient medium and loads it with electron-positron pairs. The front sweeps the medium outward with a high Lorentz factor and results in a spectacular effect: the GRB ejecta moves freely in a cavity behind its own radiation front. When the front expands sufficiently and gets diluted, a blast wave develops, and it does it differently from the standard model used before. The afterglow should initially appear as a steep rise of soft emission (from infrared to soft X-rays) at a time comparable to the prompt GRB duration and then the emission should quickly evolve to a normal X-ray afterglow. This may explain the prompt optical flash observed in GRB 990123 and allows one to infer the ejecta Lorentz factor in this burst: Γ=200. The effect of the gamma-ray front on the afterglow emission is especially pronounced if the GRB has a massive progenitor and the blast wave propagates in the progenitor wind. We emphasize the importance of early afterglow observations in soft bands, as they will allow one to test different progenitor models.

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arXiv2002

Modes of Accretion in X-ray Sources

Three classical modes of accretion are briefly discussed: wind-fed, spherical, and disk. The three modes are illustrated with the mass transfer onto black holes in high-mass X-ray binaries. Then a new regime of mini-disk accretion is described and it is argued that observed wind-fed X-ray sources are likely to accrete in this regime. Switching from one accretion mode to another can cause the observed spectral state transitio ns.

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arXiv2002

Gravitational Bending of Light Near Compact Objects

A photon emitted near a compact object at an angle $α$ with respect to the radial direction escapes to infinity at a different angle $ψ>α$. This bending of light is caused by a strong gravitational field. We show that, in a Schwarzschild metric, the effect is described by $1-\cosα=(1-\cosψ)(1-r_g/R)$ where $R/r_g$ is the emission radius in Schwarzschild units. The formula is approximate and it applies at $R\geq 2r_g$ only, however at these radii it has amazing accuracy, fully sufficient in many applications. As one application we develop a new formulation for the light bending effects in pulsars. It reveals the simple character of these effects and gives their quantitative description with practically no losses of accuracy (for the typical radius of a neutron star $R=3r_g$ the error is 1%). The visible fraction of a star surface is shown to be $S_v/4πR^2=[2(1-r_g/R)]^{-1}$ which is 3/4 for $R=3r_g$. The instantaneous flux of a pulsar comes from one or two antipodal polar caps that rotate in the visible zone. The pulse produced by one blackbody cap is found to be sinusoidal (light bending impacts the pulse amplitude but not its shape). When both caps are visible, the pulse shows a plateau: the variable parts of the antipodal emissions precisely cancel each other. The pulsed fraction of blackbody emission with antipodal symmetry has an upper limit $A_{max}=(R-2r_g)/(R+2r_g)$. Pulsars with $A>A_{max}$ must be asymmetric.

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arXiv2001

Radiation Front Sweeping the Ambient Medium of Gamma-Ray Bursts

Gamma-ray bursts (GRBs) are emitted by relativistic ejecta from powerful cosmic explosions. Their light curves suggest that the gamma-ray emission occurs at early stages of the ejecta expansion, well before it decelerates in the ambient medium. If so, the launched gamma-ray front must overtake the ejecta and sweep the ambient medium outward. As a result a gap is opened between the ejecta and the medium that surfs the radiation front ahead. Effectively, the ejecta moves in a cavity until it reaches a radius R_{gap}=10^{16}E_{54}^{1/2} cm where E is the isotropic energy of the GRB. At R=R_{gap} the gap is closed, a blast wave forms and collects the medium swept by radiation. Further development of the blast wave is strongly affected by the leading radiation front: the front plays the role of a precursor where the medium is loaded with e+- pairs and preaccelerated just ahead of the blast. It impacts the emission from the blast at R < R_{load}=5R_{gap} (the early afterglow). A spectacular observational effect results: GRB afterglows should start in optical/UV and evolve fast (< min) to a normal X-ray afterglow. The early optical emission observed in GRB 990123 may be explained in this way. The impact of the front is especially strong if the ambient medium is a wind from a massive progenitor of the GRB. In this case three phenomena are predicted: (1) The ejecta decelerates at R<R_{load} producing a lot of soft radiation. (2) The light curve of soft emission peaks at t_{peak}=40(1+z)E_{54}^{1/2}(Gamma_{ej}/100)^{-2} s where Gamma_{ej} is the Lorentz factor of the ejecta. Given measured redshift z and t_{peak}, one finds Gamma_{ej}. (3) The GRB acquires a spectral break at 5 - 50 MeV because harder photons are absorbed by radiation scattered in the wind.

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arXiv2001

Accretion Disk Models of Luminous Black Holes

Models of X-ray production in black-hole sources are reviewed and compared with recent observations. Possible diagnostics of the hot-disk and disk-corona models are discussed in the light of spectral and temporal data. A new model of a small-scale inviscid accretion disk is presented.

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arXiv2001

Bizarre Hard X-ray Outbursts of Cygnus X-1

A very high activity of Cygnus X-1 on 1999 April 19-21 was recorded by BATSE Large Area Detectors onboard the Compton Gamma-Ray Observatory. The peak luminosity was one order of magnitude higher than the normal luminosity of Cyg X-1. This fact can be critical for models of the hard state of Cyg X-1. The longest outburst lasted ~1000 s and demonstrated very unusual temporal and spectral behavior which indicates the presence of two emission components. One component is relatively soft (with a cutoff below 100 keV) and highly variable, and the other one is hard (extending above 100 keV), with much slower variability.

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arXiv2001

X-ray spectra of accretion discs with dynamic coronae

We compute the X-ray spectra produced by non-static coronae atop accretion discs around black holes and neutron stars. The hot corona is radiatively coupled to the underlying disc (the reflector) and generates an X-ray spectrum which is sensitive to the bulk velocity of the coronal plasma, beta=v/c. We show that an outflowing corona reproduces the hard-state spectrum of Cyg X-1 and similar objects. The dynamic model predicts a correlation between the observed amplitude of reflection R and the X-ray spectrum slope Gamma since both strongly depend on beta. A similar correlation was observed and its shape is well fitted by the dynamic model. The scattering of soft radiation in an outflowing corona can also account for the observed optical-UV polarization pattern in active galactic nuclei.

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arXiv2000

Free-fall accretion and emitting caustics in wind-fed X-ray sources

In wind-fed X-ray binaries the accreting matter is Compton cooled and falls freely onto the compact object. The matter has a modest angular momentum $l$ and accretion is quasi-spherical at large distances from the compact object. Initially small non-radial velocities grow in the converging supersonic flow and become substantial in the vicinity of the accretor. The streamlines with $l>(GMR_*)^{1/2}$ (where $M$ and $R_*$ are the mass and radius of the compact object) intersect outside $R_*$ and form a two-dimensional caustic which emits X-rays. The streamlines with low angular momentum, $l<(GMR_*)^{1/2}$, run into the accretor. If the accretor is a neutron star, a large X-ray luminosity results. We show that the distribution of accretion rate/luminosity over the star surface is sensitive to the angular momentum distribution of the accreting matter. The apparent luminosity depends on the side from which the star is observed and can change periodically with the orbital phase of the binary. The accretor then appears as a `Moon-like' X-ray source.

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