Searcharxiv⌕ Search

arXiv subjects

V. V. Usov

Publications and source records attributed to V. V. Usov.

25 records · Page 2Linked to original sources
arXiv1999

Non-thermal Radiation of Cosmological gamma-ray Bursters

We use 1+1/2 dimensional particle-in-cell plasma simulations to study the interaction of a relativistic, strongly magnetized wind with an ambient medium. Such an interaction is a plausible mechanism which leads to generation of cosmological gamma-ray bursts. We confirm the idea of Meszaros and Rees (1992) that an essential part (about 20%) of the energy that is lost by the wind in the process of its deceleration may be transferred to high-energy electrons and then to high-frequency (X-ray and gamma-ray) emission. We show that in the wind frame the spectrum of electrons which are accelerated at the wind front and move ahead of the front is nearly a two-dimensional relativistic Maxwellian with a relativistic temperature $T=6*10^9Γ_T$ K, where $Γ_T=200Γ_0$ with the accuracy of ~20%, and $Γ_0$ is the Lorentz factor of the wind, $Γ_0>100$ for winds outflowing from cosmological gamma-ray bursters. Our simulations point to an existence of a high-energy tail of accelerated electrons with a Lorentz factor of more than $700Γ_0$. Large-amplitude electromagnetic waves are generated by the oscillating currents at the wind front. The mean field of these waves ahead of the wind front is an order of magnitude less than the magnetic field of the wind. High-energy electrons which are accelerated at the wind front and injected into the region ahead of the front generate synchro-Compton radiation in the fields of large-amplitude electromagnetic waves. This radiation closely resembles synchrotron radiation and can reproduce the non-thermal radiation of gamma-ray bursts observed in the Ginga and BATSE ranges (from a few keV to a few MeV).

astro-ph↗
arXiv1998

Particle acceleration and high-frequency (X-ray and $γ$-ray) emission in the jets of active galactic nuclei

It is suggested that the outflowing plasma in the jets of active galactic nuclei (AGNs) is inhomogeneous and consists of separate clouds. These clouds are strongly magnetized and move away from the central engine at relativistic speeds. The clouds interact with an ambient medium which is assumed to be at rest. In the process of this interaction, particles of the ambient medium are accelerated to high energies at the cloud front and flow ahead of the front. It is shown that the radiation of the accelerated particles may be responsible for the X-ray and $γ$-ray emission from AGN jets. TeV $γ$-ray emission is generated in the inner parts of AGN jets where the Lorentz factor of the cloud fronts is $Γ_0\geq 30$, while GeV $γ$-ray emission emanates from the outer parts of AGN jets where $Γ_0$ is $\sim 10$.

astro-ph↗
arXiv1997

Bare Quark Matter Surfaces of Strange Stars and $e^+e^-$ Emission

We show that the Coulomb barrier at the quark surface of a hot strange star may be a powerful source of $e^+e^-$ pairs which are created in an extremely strong electric field of the barrier and flow away from the star. The luminosity in the outflowing pair plasma depends on the surface temperature $T_{_S}$ and may be very high, up to $\sim 3\times 10^{51} ergs/s$ at $T_{_S}\sim 10^{11}$ K. The effect of pair creation by the Coulomb barrier may be a good observational signature of strange stars which can give an answer to the question of whether a compact object is a neutron or strange star.

astro-ph↗
arXiv1997

Wide ultrarelativistic plasma beam -- magnetic barrier collision and astrophysical applications

The interaction between a wide ultrarelativistic fully-ionized plasma beam and a magnetic barrier is studied numerically. It is assumed that the plasma beam is initially homogeneous and impacts with the Lorentz factor $Γ_0\gg 1$ on the barrier. The magnetic field of the barrier $B_0$ is uniform and transverse to the beam velocity. When the energy densities of the beam and the magnetic field are comparable, $α= 8πn_0m_pc^2(Γ_0-1)/B^2_0\sim 1$, the process of the beam -- barrier interaction is strongly nonstationary, and the density of reversed protons is modulated in space by a factor of 10 or so. The modulation of reversed protons decreases with decrease of $α$. The beam is found to penetrate deep into the barrier provided that $α> α_{cr}$, where $α_{cr}$ is about 0.4. The speed of such a penetration is subrelativistic and depends on $α$. Strong electric fields are generated near the front of the barrier, and electrons are accelerated in these fields up to the mean energy of protons, i.e. up to $\sim m_pc^2Γ_0$. The synchrotron radiation of high-energy electrons from the front vicinity is calculated. Stationary solutions for the beam -- barrier collision are considered. It is shown that such a solution may be only at $α\lesssim 0.2 - 0.5$ depending on the boundary conditions for the electric field in the region of the beam -- barrier interaction. Some astrophysical applications of these results are briefly discussed.

astro-ph↗
arXiv1996

On the persistent X-ray emission from the soft gamma-ray repeaters

It is suggested that the persistent X-ray emission from the soft gamma-ray repeaters is the thermal radiation of neutron stars which is enhanced by a factor of 10 or more due to the effect of a very strong magnetic field on the thermal structure of the neutron star envelope. For the thermal luminosity to be consistent with the persistent X-ray luminosity, the field strength at the neutron star surface has to be of the order of $10^{15}$ G. If it is confirmed that the soft gamma-ray repeaters are neutron stars with negligible accretion, then the presence of such a strong magnetic field is inevitable.

astro-ph↗
arXiv1995

PULSARS WITH STRONG MAGNETIC FIELDS: POLAR GAPS, BOUND PAIR CREATION AND NONTHERMAL LUMINOSITIES

Modifications to polar-gap models for pulsars are discussed for the case where the surface magnetic field, $B_§$, of the neutron star is strong. For $B\ga4\times10^8\rm\,T$, the curvature $γ$-quanta emitted tangentially to the curved force lines of the magnetic field are captured near the threshold of bound pair creation and are channelled along the magnetic field as bound electron-positron pairs (positronium). The stability of such bound pairs against ionization by the parallel electric field, $E_\parallel$, in the polar cap, and against photoionization is discussed. Unlike free pairs, bound pairs do not screen $E_\parallel$ near the neutron star. As a consequence, the energy flux in highly relativistic particles and high-frequency (X-ray and/or $γ$-ray) radiation from the polar gaps can be much greater than in the absence of positronium formation. We discuss this enhancement for (a) Arons-type models, in which particles flow freely from the surface, and find any enhancement to be modest, and (b) Ruderman-Sutherland-type models, in which particles are tightly bound to the surface, and find that the enhancement can be substantial. In the latter case we argue for a self-consistent, time-independent model in which partial screening of $E_\parallel$ maintains it close to the threshold value for field ionization of the bound pairs, and in which a reverse flux of accelerated particles maintains the polar cap at a temperature such that thermionic emission supplies the particles needed for this screening. This model applies only in a restricted range of periods, $P_2<P<P_1$, and it implies an energy flux in high-energy particles that can correspond to a substantial fraction of the spin-down power of the pulsar. Nonthermal, high-frequency radiation has been observed from six radio pulsars

astro-ph↗