arXiv subjects

Andrei M. Beloborodov

Publications and source records attributed to Andrei M. Beloborodov.

At least 109 records · Page 6Linked to original sources

arXiv2011

Radiative transfer in ultra-relativistic outflows

Analytical and numerical solutions are obtained for the equation of radiative transfer in ultra-relativistic opaque jets. The solution describes the initial trapping of radiation, its adiabatic cooling, and the transition to transparency. Two opposite regimes are examined: (1) Matter-dominated outflow. Surprisingly, radiation develops enormous anisotropy in the fluid frame before decoupling from the fluid. The radiation is strongly polarized. (2) Radiation-dominated outflow. The transfer occurs as if radiation propagated in vacuum, preserving the angular distribution and the blackbody shape of the spectrum. The escaping radiation has a blackbody spectrum if (and only if) the outflow energy is dominated by radiation up to the photospheric radius.

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arXiv2010

Neutrino heating near hyper-accreting black holes

Hyper-accretion discs around black holes emit copious neutrinos and anti-neutrinos. A fraction of the emitted neutrinos convert to electron-positron plasma above the disc through the annihilation reaction $ν\barν\to e^+e^-$. This process may drive relativistic jets associated with GRB explosions. We calculate the efficiency of energy deposition by neutrinos. Our calculation is fully relativistic and based on a geodesic-tracing method. We find that the efficiency of neutrino heating is a well-defined function of (i) accretion rate and (ii) spin of the black hole. It is practically independent of the details of neutrino transport in the opaque zone of the disc. The results help identify accretion discs whose neutrino emission can power GRBs.

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arXiv2010

Activated Magnetospheres of Magnetars

Like the solar corona, the external magnetic field of magnetars is twisted by surface motions of the star. The twist energy is dissipated over time. We discuss the theory of this activity and its observational status. (1) Theory predicts that the magnetosphere tends to untwist in a peculiar way: a bundle of electric currents (the "j-bundle") is formed with a sharp boundary, which shrinks toward the magnetic dipole axis. Recent observations of shrinking hot spots on magnetars are consistent with this behavior. (2) Continual discharge fills the j-bundle with electron-positron plasma, maintaining a nonthermal corona around the neutron star. The corona outside a few stellar radii strongly interacts with the stellar radiation and forms a "radiatively locked" outflow with a high e+- multiplicity. The locked plasma annihilates near the apexes of the closed magnetic field lines. (3) New radiative-transfer simulations suggest a simple mechanism that shapes the observed X-ray spectrum from 0.1 keV to 1 MeV: part of the thermal X-rays emitted by the neutron star are reflected from the outer corona and then upscattered by the inner relativistic outflow in the j-bundle, producing a beam of hard X-rays.

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arXiv2010

White dwarfs stripped by massive black holes: sources of coincident gravitational and electromagnetic radiation

White dwarfs inspiraling into black holes of mass $\MBH\simgt 10^5M_\odot$ are detectable sources of gravitational waves in the LISA band. In many of these events, the white dwarf begins to lose mass during the main observational phase of the inspiral. The mass loss starts gently and can last for thousands of orbits. The white dwarf matter overflows the Roche lobe through the $L_1$ point at each pericenter passage and the mass loss repeats periodically. The process occurs very close to the black hole and the released gas can accrete, creating a bright source of radiation with luminosity close to the Eddington limit, $L\sim 10^{43}$~erg~s$^{-1}$. This class of inspirals offers a promising scenario for dual detections of gravitational waves and electromagnetic radiation.

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arXiv2010

Collisional mechanism for GRB emission

Nuclear and Coulomb collisions in GRB jets create a hot electron-positron plasma. This collisional heating starts when the jet is still opaque and extends to the transparent region. The e+- plasma radiates its energy. As a result, a large fraction of the jet energy is converted to escaping radiation with a well-defined spectrum. The process is simulated in detail using the known rates of collisions and accurate calculations of radiative transfer in the expanding jet. The result reproduces the spectra of observed GRBs that typically peak near 1 MeV and extend to much higher energies with a photon index β~ -2.5. This suggests that collisional heating may be the main mechanism for GRB emission.

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arXiv2009

Untwisting magnetospheres of neutron stars

Magnetospheres of neutron stars are anchored in the rigid crust and can be twisted by sudden crustal motions ("starquakes"). The twisted magnetosphere does not remain static and gradually untwists, dissipating magnetic energy and producing radiation. The equation describing this evolution is derived, and its solutions are presented. Two distinct regions coexist in untwisting magnetospheres: a potential region where curl(B)=0 ("cavity") and a current-carrying bundle of field lines ("j-bundle"). The cavity has a sharp boundary, which expands with time and eventually erases all of the twist. In this process, the electric current of the j-bundle is sucked into the star. Observational appearance of the untwisting process is discussed. A hot spot forms at the footprints of the j-bundle. The spot shrinks with time toward the magnetic dipole axis, and its luminosity and temperature gradually decrease. As the j-bundle shrinks, the amplitude of its twist can grow to the maximum possible value ~ 1. The strong twist near the dipole axis increases the spindown rate of the star and can generate a broad beam of radio emission. The model explains the puzzling behavior of magnetar XTE J1810-197 -- a canonical example of magnetospheric evolution following a starquake. We also discuss implications for other magnetars. The untwisting theory suggests that the nonthermal radiation of magnetars is preferentially generated on a bundle of extended closed field lines near the dipole axis.

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arXiv2009

Mini-discs around spinning black holes

Accretion onto black holes in wind-fed binaries and in collapsars forms small rotating discs with peculiar properties. Such ``mini-discs'' accrete on the free-fall time without help of viscosity and nevertheless can have a high radiative efficiency. The inviscid mini-disc model was previously constructed for a non-rotating black hole. We extend the model to the case of a spinning black hole, calculate the structure and radiative efficiency of the disc and find their dependence on the black hole spin. If the angular momenta of the disc and the black hole are anti-aligned, a hydrodynamic analog of Penrose process takes place.

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arXiv2008

Efficiency of Neutrino Annihilation around Spinning Black Holes

A fraction of neutrino emission from GRB accretion disks annihilates above the disk, creating e+- plasma that can drive GRB explosions. We calculate the efficiency of this annihilation using the recent detailed model of hyper-accretion disks around Kerr black holes. Our calculation is fully relativistic and based on a geodesic-tracing method. We find that the efficiency is a well-defined function of (1) accretion rate and (2) spin of the black hole. It is practically independent of the details of neutrino transport in the opaque zone of the disk. The results help identify the accretion disks whose neutrino emission can power GRBs.

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arXiv2008

Hyper-accreting black holes

Hyper-accretion disks are short-lived, powerful sources of neutrinos and magnetized jets. Such disks are plausible sources of gamma-ray bursts. This review describes the disk structure, the neutrino conversion to electron-positron plasma around the disk, and the post-burst evolution.

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arXiv2008

Polar-cap accelerator and radio emission from pulsars

Electric currents j flow along the open magnetic field lines from the polar caps of neutron stars. Activity of a polar cap depends on the ratio α=j/cρ_GJ, where ρ_GJ is the corotation charge density. The customary assumption α\approx 1 is not supported by recent simulations of pulsar magnetospheres and we study polar caps with arbitrary α. We argue that no significant activity is generated on field lines with 0<α<1. Charges are extracted from the star and flow along such field lines with low energies. By contrast, if α>1 or α<0, a high voltage is generated, leading to unsteady e^+- discharge on a scale-height smaller than the size of the polar cap. The discharge can power observed pulsars. Voltage fluctuations in the discharge imply unsteady twisting of the open flux tube and generation of Alfven waves. These waves are ducted along the tube and converted to electromagnetic waves, providing a new mechanism for pulsar radiation.

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arXiv2007

On the Mechanism of Gamma-Ray Burst Afterglows

The standard model of afterglow production by the forward shock wave is not supported by recent observations. We propose a model in which the forward shock is invisible and afterglow is emitted by a long-lived reverse shock in the burst ejecta. It explains observed optical and X-ray light curves, including the plateau at 10^3-10^4 s with a peculiar chromatic break, and the second break that was previously associated with a beaming angle of the explosion. The plateau forms following a temporary drop of the reverse-shock pressure much below the forward-shock pressure. A simplest formalism that can describe such blast waves is the ``mechanical'' model (Beloborodov, Uhm 2006); we use it in our calculations.

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arXiv2006

Pulse profiles of millisecond pulsars and their Fourier amplitudes

Approximate analytical formulae are derived for the pulse profile produced by small hot spots on a rapidly rotating neutron star. Its Fourier amplitudes and phases are calculated. The proposed formalism takes into account gravitational bending of light, Doppler effect, anisotropy of emission, and time delays. Its accuracy is checked with exact numerical calculations.

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arXiv2006

Corona of Magnetars

We develop a theoretical model that explains the formation of hot coronae around strongly magnetized neutron stars -- magnetars. The starquakes of a magnetar shear its external magnetic field, which becomes non-potential and is threaded by an electric current. Once twisted, the magnetosphere cannot untwist immediately because of its self-induction. The self-induction electric field lifts particles from the stellar surface, accelerates them, and initiates avalanches of pair creation in the magnetosphere. The created plasma corona maintains the electric current demanded by curl(B) and regulates the self-induction e.m.f. by screening. This corona persists in dynamic equilibrium: it is continually lost to the stellar surface on the light-crossing time of 10^{-4} s and replenished with new particles. In essence, the twisted magnetosphere acts as an accelerator that converts the toroidal field energy to particle kinetic energy. Using a direct numerical experiment, we show that the corona self-organizes quickly (on a millisecond timescale) into a quasi-steady state, with voltage ~1 GeV along the magnetic lines. The heating rate of the corona is ~10^{36} erg/s, in agreement with the observed persistent, high-energy output of magnetars. We deduce that a static twist that is suddenly implanted into the magnetosphere will decay on a timescale of 1-10 yrs. The particles accelerated in the corona impact the solid crust, knock out protons, and regulate the column density of the hydrostatic atmosphere of the star. The transition layer between the atmosphere and the corona is the likely source of the observed 100-keV emission from magnetars. The corona emits curvature radiation and can supply the observed IR-optical luminosity. (Abridged)

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arXiv2006

Neutrino-Cooled Accretion Disks around Spinning Black Hole

We calculate the structure of accretion disks around Kerr black holes for accretion rates 0.001 - 10 M_sun/s. Such disks are plausible candidates for the central engine of gamma-ray bursts. Our disk model is fully relativistic and treats accurately microphysics of the accreting matter: neutrino emissivity, opacity, electron degeneracy, and nuclear composition. The neutrino-cooled disk forms above a critical accretion rate that depends on the black hole spin. The disk has the ``ignition'' radius r_ign where neutrino flux rises dramatically, cooling becomes efficient, and the proton-to-nucleon ratio Y_e drops. Other characteristic radii are r_alpha where most of alpha-particles are disintegrated, r_nu where the disk becomes neutrino-opaque, and r_trap where neutrinos get trapped and advected into the black hole. We find r_alpha, r_ign, r_nu, r_trap and show their dependence on the accretion rate. We discuss the qualitative picture of accretion and present sample numerical models of the disk structure. All neutrino-cooled disks regulate themselves to a characteristic state such that: (1) electrons are mildly degenerate, (2) Y_e ~ 0.1, and (3) neutrons dominate the pressure in the disk.

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arXiv2006

Neutrino-cooled Accretion Disks around Spinning Black Holes

We calculate the structure of accretion disk around a spinning black hole for accretion rates 0.01 - 10 M_sun/s. The model is fully relativistic and treats accurately the disk microphysics including neutrino emissivity, opacity, electron degeneracy, and nuclear composition. We find that the accretion flow always regulates itself to a mildly degenerate state with the proton-to-nucleon ratio Y_e ~ 0.1 and becomes very neutron-rich. The disk has a well defined "ignition" radius where neutrino flux raises dramatically, cooling becomes efficient, and Y_e suddenly drops. We also calculate other characteristic radii of the disk, including the neutrino-opaque and neutrino-trapping radii, and show their dependence on the accretion rate. Accretion disks around fast-rotating black holes produce intense neutrino fluxes which may deposit enough energy above the disk to generate a GRB jet.

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arXiv2005

Neutron-loaded outflows in gamma-ray bursts

Relativistic neutron-loaded outflows in gamma-ray bursts are studied at their early stages, before deceleration by a surrounding medium. The outflow has four components: radiation, electrons, protons and neutrons. The components interact with each other and exchange energy as the outflow expands. The presence of neutrons significantly changes the outflow evolution. Before neutrons decouple from protons, friction between the two components increases their temperatures by many orders of magnitude. After the decoupling, the gradual neutron decay inside the outflow has a drag effect on the protons and reduces their final Lorentz factor.

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arXiv2005

Afterglow emission from pair-loaded blast waves in gamma-ray bursts

The MeV radiation front of gamma-ray bursts creates copious e+- pairs as it propagates through an ambient medium. The created pairs enrich the leptonic component of the medium by a large factor at distances R < R_load ~ 10^{16} cm from the burst center. The following blast wave sweeps up the pair-rich medium and then emits the observed afterglow radiation. We find that the afterglow has a "memory" of e+- loading outside R_load. The e+- pairs remain in the swept-up material and slowly cool down by emitting synchrotron radiation. They are likely to dominate the blast-wave emission in IR, optical, and UV bands during the first minutes of the observed afterglow. The expected e+- radiation is described by a simple formula, which is derived analytically and checked by numerical integration of synchrotron emission over the blast material; a suitable Lagrangian formalism is developed for such calculations. The main signature of e+- radiation is its flat ("white") spectrum in a broad range of frequencies from IR to UV and possibly soft X-rays. This radiation can be detected by Swift satellite, which would enable new observational tests for the explosion physics.

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arXiv2004

Optical and GeV-TeV flashes from gamma-ray bursts

The synchrotron optical flash caught in GRB 990123 overlaps with the MeV radiation front, and the optical-emitting electrons must also produce GeV-TeV emission by inverse Compton scattering of MeV photons. The ultra-high-energy flash can be much stronger than its optical counterpart. We also note that Compton cooling by MeV photons immediately terminates the optical emission unless the fireball Lorentz factor exceeds 10^3. Severe Compton losses may explain the non-detections of optical flashes in several long GRBs. Such failed optical flashes should be especially efficient GeV producers and likely to develop e+- cascades. This probably happened in GRB 941017, and its mysterious high-energy component is well explained by Compton upscattering of GRB photons at the fireball deceleration radius. The proposed mechanism of GeV emission should not work for short gamma-ray bursts that early decouple from the fireball and avoid interaction with the electrons in the deceleration flash. Observations by Swift and GLAST will provide an opportunity to test these expectations. The existing data for GRB 990123 already impose interesting constraints on the explosion.

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