E. V. Derishev

Physical parameters and emission mechanism in gamma-ray bursts

Detailed information on the physical parameters in the sources of cosmological Gamma-Ray Bursts (GRBs) is obtained from few plausible assumptions consistent with observations. We consider monoenergetic injection of electrons and let them cool self-consistently, taking into account Klein-Nishina cut-o in electron- photon scattering. The general requirements posed by the assumptions on the emission mechanism in GRBs are formulated. It is found that the observed radiation in the sub-MeV energy range is generated by the synchrotron emission mechanism, though about ten per cent of the total GRB energy should be converted via the inverse Compton (IC) process into the ultra-hard spectral domain (above 100 GeV). We estimate the magnetic eld strength in the emitting region, the Lorentz factor of accelerated electrons, and the typical energy of IC photons. We show that there is a synchrotron-self-Compton constraint which limits the parameter space available for GRBs that are radiatively ecient in the sub-MeV domain. This concept is analogous to the line-of-death relation existing for pulsars and allows us to derive the lower limits on both GRB duration and the timescale of GRB variability. The upper limit on the Lorentz factor of GRB reballs is also found. We demonstrate that steady-state electron distribution consistent with the Compton losses may produce dierent spectral indices, e.g., 3/4 as opposed to the gure 1/2 widely discussed in the literature. It is suggested that the changes in the decline rate observed in the lightcurves of several GRB afterglows may be due to either a transition to ecient IC cooling or the time evolution of Klein-Nishina and/or Compton spectral breaks, which are the general features of self-consistent electron distribution.

Synchrotron emission in the fast cooling regime: which spectra can be explained?

Abstract We consider the synchrotron emission from relativistic shocks assuming that the radiating electrons cool rapidly (either through synchrotron or any other radiation mechanism). It is shown that the theory of synchrotron emission in the fast cooling regime can account for a wide range of spectral shapes. In particular, the magnetic field, which decays behind the shock front, brings enough flexibility to the theory to explain the majority of gamma-ray burst spectra even in the parameter-free fast cooling regime. Also, we discuss whether location of the peak in observed spectral energy distributions of gamma-ray bursts and active galactic nuclei can be made consistent with predictions of diffusive shock acceleration theory, and find that the answer is negative. This result is a strong indication that a particle injection mechanism, other than the standard shock acceleration, works in relativistic shocks.

Formation and dynamics of self-sustained neutron haloes in disk accreting sources

It has been recognized long ago that the presence of hot plasma in the inner accretion disks around black holes could lead to the neutron production via dissociation of helium nuclei. We show that, for a broad range of accretion parameters, neutrons effectively decouple from protons and pile up in the inner disk leading to the formation of self-sustained halo. This means that new neutrons in the halo are supplied mainly by the splitting of helium nuclei in their collisions with existing neutrons. Once formed, such a halo can exist even if the proton temperature is much lower than the energy threshold of helium dissociation. We show that neutron haloes can be the natural source of relativistic electrons and positrons, providing characteristic comptonization spectra and hard spectral tails observed in many black hole candidates, and also giving rise to relativistic outflows. Deuterium gamma-ray line at 2.2 MeV resulting from neutron capture is also expected at a level detectable by future INTEGRAL mission. Furthermore, the presence of a neutron halo strongly affects the dynamics of accretion and leads to the rich variety of transient dynamical regimes.

A NEW MECHANISM FOR PARTICLE ACCELERATION IN RELATIVISTIC JETS

We compare different acceleration mechanisms in application to relativistic jets and show that the converter mechanism, suggested recently, is the least sensitive to the geometry of the magnetic field in accelerators, and can routinely operate up to cosmic-ray energies close to the fundamental limit. The converter mechanism utilizes multiple conversions of charged particles into neutral ones (protons to neutrons and electrons/positrons to photons) and back by means of photon-induced reactions or inelastic nucleon-nucleon collisions. It works efficiently both in relativistic shocks and in shear flows under the conditions typical for active galactic nuclei, gamma-ray bursts, and microquasars, where it often outperforms the standard diffusive shock acceleration. The main advantages of the converter mechanism in such environments are that it greatly diminishes particle losses downstream and avoids the reduction in the energy gain factor, which normally takes place due to the highly collimated distribution of accelerated particles. We also discuss the properties of gamma-ray radiation, which accompanies acceleration of cosmic rays via the converter mechanism and can provide evidence for the latter. In particular, we point out that the opening angle of the radiation beam-pattern is different at different photon energies, which is relevant to the observability of gamma-ray sources as well as to their timing properties.

Magnetic field generation in shock waves and jets

The origin, formation, structure, and long‐term evolution of the self‐consistent magnetic field in collisionless relativistic plasma outflows of astrophysical interest are investigated. The main attention is paid to various models allowing approximate analytical investigation as well as to estimates of expected parameters and scales of the self‐consistent turbulent magnetic field.

Spectral redistribution of gyroresonant photons in magnetized atmospheres of isolated compact stars

Aims. We analyze the spectral redistribution of gyroresonant photons in the course of radiation transfer through magnetized plasma atmospheres of isolated compact stars. Methods. We use analytical estimate and Monte Carlo simulations to prove that this redistribution crucially influences the spectral line formation for atmospheric parameters typical of neutron stars and white dwarfs. Results. We point out the importance of the frequency redistribution of the gyroresonant photons to the process of radiation transfer and analyze its main effects in atmospheres of isolated compact stars with strong magnetic fields, where multiple scattering dominates over the absorption of photons. We estimate analytically and numerically the rate of this redistribution and show that photons’ escape from the line center, which in this case is one-dimensional (1D) in origin, is a very pronounced effect despite being strongly inhibited with respect to three-dimensional (3D) photon redistribution, which takes place in the case of atomic or ion spectral lines. The escape of photons from the cyclotron line greatly affects both the line’s profile and the characteristic optical depth, from where the outgoing radiation originates. Through this, the spectral redistribution of gyroresonant photons changes the radiation pressure on the atmospheric plasma, what makes it one of the key phenomena need to be included in studies of cyclotron-driven winds.

The Self‐Consistent keV to TeV Spectra of Gamma‐Ray Bursts Produced by the Synchrotron‐Self‐Compton Emission in Relativistic Shocks

The spectra of Gamma‐Ray Burst (GRB) emission have been investigated theoretically by a number of authors (e.g., [1, 2, 3]) for different versions of a general fireball‐shock scenario [4, 5]. However, the existing models give rather coarse predictions and suffer from many uncertainties. Here we present a refined analytical model of synchrotron‐self‐Compton (SSC) emission. We take an arbitrary injection spectrum of relativistic particles and let them cool self‐consistently, taking into account both synchrotron and inverse Compton (IC) losses and making corrections for the Klein‐Nishina cut‐off. When two‐photon absorption is negligible, the problem is reduced to the single integral equation for the ratio of synchrotron to total losses as a function of particle’s energy. The spectra of synchrotron and IC components, as well as the electron distribution, can be derived from this function in a regular way. The low‐energy portion of the composite synchrotron spectrum in the fast cooling regime may have spectral...

A neutron star collapse induced by a primordial black hole as the source of cosmological γ-ray bursts

A novel scenario is proposed for the origin of cosmological γ-ray bursts relating them with the induced collapse of an isolated neutron star under the action of a primordial black hole inside it. A mechanism is pointed out for black hole capturing into bounded orbits in a contracting protostellar cloud (which further evolves to a neutron star), and it is shown that this mechanism is most efficient in the pregalactic epoch. The qualitative results of neutrino transfer calculations are presented; these neutrinos originate from the quark phase transition in the nucleon matter which takes place in the accretion flow in the interior of the star. The neutrinos and antineutrinos escaping from a dense nucleon matter are degenerate and annihilate in the immediate vicinity of the star surface where an inverse temperature layer in the outstreaming electron-positron wind is produced. This layer acts as a natural barrier against baryon pollution and gives rise to a very high (≈ 103) value of the Lorentz factor in the expanding plasma, in agreement with the observed energy and duration of the process. This makes it possible to explain the main properties of the γ-ray bursts. We also consider other important features of this scenario, including the predominantly extragalactic origin of the bursts, the apparent absence of the cosmological time dilation, the excess drop in the number of bursts—luminosity dependence for z>0.7, and the unlikely corrllation between the burst and the gravitational wave pulse.

Statistics of the frequency redistribution for gyroresonance radiation in the atmospheres of compact stars

We analyze in detail the statistics of the frequency redistribution of photons during the transfer of gyroresonance radiation under conditions typical of compact stars. The probabilities of photon escape from a scattering atmosphere of arbitrary optical thickness in a single scattering have been found. The effects of gyroresonance photon diffusion in space and in frequency have been simulated. We show that when photons escape from a semi-infinite atmosphere with weak absorption, the frequency redistribution effects lead to a considerable increase in the probability of photon escape from large optical depths and, consequently, modify significantly the dynamics of gyroresonance photon transfer for neutron stars and white dwarfs.

Emission of cosmological gamma-ray bursts in the GeV–TeV energy domain

The observed sub-MeV radiation from gamma-ray bursts (GRBs) arises most likely from synchrotron emission of a relativistic shock. We estimate the magnetic field strength and the Lorentz factor of accelerated electrons required by the synchrotron mechanism. We find that these values are furthermore sufficient to cause significant emission due to inverse Compton scattering. The Comptonized photons typically have energies well above 1 TeV, so that they are strongly absorbed by infrared background radiation and cannot be observed from a source at cosmological distance. On the other hand, the ultra-high energy radiation may be absorbed in the close vicinity of the source due to interaction with ultraviolet and soft X-ray quanta originating from the same source and scattered in the ambient medium. In this case, almost all the energy initially contained in ultra-high energy radiation is reprocessed into a softer spectral range corresponding to the two-photon absorption threshold. Both the presence and the absenc...