arXiv subjects

G. S. Bisnovatyi-Kogan

Publications and source records attributed to G. S. Bisnovatyi-Kogan.

At least 91 records · Page 5Linked to original sources

arXiv2007

Cosmic gamma-ray bursts: observations and modeling

The present common view about GRB origin is related to cosmology, what is based on statistical analysis, and on measurements of the redshifts in the GRB optical afterglows of long GRB. Models of radiation, and models of the central machine for GRB are critically analyzed. Most of these models cannot produce the amount of energy required for the cosmological GRB. Collimation is needed to reduce GRB energy output. No correlation is found between redshifts, GRB spectrum, and total GRB fluence. Comparison of KONUS and BATSE data about statistics and hard X-ray lines is done, and some differences are noted. Hard gamma-ray afterglows, prompt optical spectra, hard X-ray lines measurements could be important for farther insight into GRB origin. Two bright optical afterglows had been detected, indicating that initial bright optical emission is directly connected with the GRB itself, and has a different origin from the subsequent faint optical emission. Results of these observations are represented. Observations indicate that GRB originate in star forming regions of galaxies with a high matter density. Interaction of the mighty gamma ray flux from the cosmological GRB with the dense surrounding molecular cloud lead to long optical afterglow, lasting up to 10 years, due to reradiation of the cloud. Results of numerical simulations of such reradiation for different forms of clouds are represented. Possible connection of short GRB with soft gamma repeaters is discussed.

astro-ph↗

arXiv2006

Dynamic confinement of jets by magneto-torsional oscillations

Many quasars and active galactic nuclei (AGN) appear in radio, optical, and X-ray maps, as a bright nuclear sources from which emerge single or double long, thin jets (Thomson et al., 1993). When observed with high angular resolution these jets show structure with bright knots separated by relatively dark regions. High percentages of polarization, sometimes more then 50% in some objects, indicates the nonthermal nature of the radiation which is well explained as the synchrotron radiation of the relativistic electrons in an ordered magnetic field. A strong collimation of jets is most probably connected with ordered magnetic fields. The mechanism of magnetic collimation, first suggested by Bisnovatyi-Kogan et al. (1969), was based on the initial charge separation, leading to creation of oscillating electrical current, which produces azimuthal magnetic field, preventing jet expansion and disappearance. Here we consider magnetic collimation, connected with torsional oscillations of a cylinder with elongated magnetic field. Instead of initial blobs with charge separation, we consider a cylinder with a periodically distributed initial rotation around the cylinder axis. The stabilizing azimuthal magnetic field is created here by torsional oscillations, where charge separation is not necessary. Approximate simplified model is developed. Ordinary differential equation is derived, and solved numerically, what gives a possibility to estimate quantitatively the range of parameters where jets may be stabilized by torsional oscillations.

astro-ph↗

arXiv2006

Observational Constraints on the Angular and Spectral Distributions of Photons in Gamma-Ray Burst Sources

The typical spectra of gamma-ray bursts (GRBs) are discussed in the context of the compactness problem for GRB sources and how it is resolved in the popular fireball model. In particular, observational (model-independent) constraints on the collimation of the gamma-rays and the dependence of the collimation angle on the photon energy are considered. The fact that the threshold for the creation of $e^{-}e^{+}$ pairs depends on the angle between the momenta of the annihilating photons in the GRB source provides an alternative solution to the compactness problem. A new approach to explaining GRBs, taking into account the angular dependence for pair creation, is proposed, and the main features of a scenario describing a GRB source with a total (photon) energy smaller or of the order of $10^{49}$ erg are laid out. Thus, we are dealing with an alternative to an ultra-relativistic fireball, if it turns out (as follows from observations) that all "long" GRBs are associated with normal (not peculiar) core-collapse supernovae. The effects of radiation pressure and the formation of jets as a consequence of even a small amount of anisotropy in the total radiation field in a (compact) GRB source are examined in this alternative model. Possible energy release mechanisms acting in regions smaller or of the order of $10^{8}$ cm in size (a compact model for a GRB) are discussed. New observational evidence for such compact energy release in the burst source is considered.

astro-ph↗

arXiv2006

Magnetorotational supernovae with jets

We present results of 2D simulation of magnetorotational (MR) supernova accompanied by jet formation in the core collapse supernova explosion. Initial magnetic field used in the simulations has dipole-like symmetry. Contrary to the simulations of MR supernova with initial quadrupole-like magnetic field, where the matter was ejected mainly near the equatorial plane, in presence of the dipole-like initial magnetic field the supernova explosion is developing preferably along a rotational axis, and leads to formation of a protojet. We expect that protojet propagation through the envelope of the star will be accompanied by its collimation. The magnetorotational instability (MRI) was found in simulations, similar to the earlier considered case of the quadrupole-like initial magnetic field. Our estimations show that the characteristic time for the reconnection of the magnetic field is much larger than the MRI development time. The supernova explosion energy for the dipole-like field is about $0.61\cdot 10^{51}$erg, and about $0.13M_\odot$ of mass was ejected during the explosion.

astro-ph↗

arXiv2006

Collimated high-energy photons and others possible observational effects of the photon angular and spectral distribution in gamma-ray bursts sources

Typical observational gamma-ray burst (GRB) spectra are discussed and, in this connection, what is the origin of the compactness problem and how it was solved at first. If the threshold for $e^{-}e^{+}$ pair production depends on an angle between photon momenta, then another solution of the compactness problem is possible. We discuss a possibility of the $γ$-rays collimation and the dependence of photon beaming on photon energies. The list of basic assumptions of the scenario describing the GRB source with energy $< 10^{49}$ ergs are adduced: the matter is about an alternative to the ultrarelativistic fireball if {\it all} long-duration GRBs are related or physically connected with {\it normal/unpeculiar} core-collapse supernovae (SNe). Namely, we consider the questions about radiation pressure and how the jet arises on account of even small asymmetry of the radiation field in a compact GRB source. The possibility of a new approach to explanation of GRB phenomenon is shown. Possible mechanisms of their generation in regions of size $< 10^8$cm are discussed (a compact model of GRBs). Observational consequences of the compact GRB energy release are considered.

astro-ph↗

arXiv2006

Checking the variability of the gravitational constant with binary pulsars

The most precise measurements are done at present by timing of radiopulsars in binary systems with two neutron stars. The timing measurements of the Taylor-Hulse pulsar B1913+16 gave the most precise results on testing of general relativity (GR), finding implicit proof of existence of gravitational waves. We show that available results of existing measurements, obtained to the year 1993, in combination with the results of the Mariner 10 in (1992), give the boundaries for the variation of the gravitational constant ${\dot G}/{G}$ inside the limits $(-0.6 ÷+2)\cdot 10^{-12}$ year{$^{-1}$}.

gr-qc↗

arXiv2005

Approximate dynamics of dark matter ellipsoids

The collapse of non-collisional dark matter and the formation of pancake structures in the Universe are investigated approximately. Collapse is described by a system of ordinary differential equations, in the model of a uniformly rotating, three-axis, uniform density ellipsoid. Violent relaxation, mass and angular momentum losses are taken into account phenomenologically. The formation of the equilibrium configuration, secular instability and the transition from a spheroid to a three-axis ellipsoid are investigated numerically and analytically in this dynamical model.

astro-ph↗

arXiv2005

Core collapse supernovae: magnetorotational explosion

Core-collapse supernovae are connected with formation of neutron stars. Part of the gravitation energy is transformed into the energy of the explosion, observed in SN II, SN Ib,c type supernovae. The mechanism of transformation is not simple, because the overwhelming majority of the energy is going into weakly interacting neutrino. The attempts to use this energy for the explosion were not successful during about 40 years of investigation. We consider the explosion mechanism in which the source of energy is the rotation, and magnetic field serves for the transformation of the rotation energy into the energy of explosion. 2-D MHD simulations of this mechanism were performed. After the collapse the core consists of a rapidly rotating proto-neutron star with a differentially rotating envelope. The toroidal part of the magnetic energy generated by the differential rotation grows as quadratic function with time at the initial stage of the evolution of the magnetic field. The linear growth of the toroidal magnetic field is terminated by the development of magnetohydrodynamic instability, when the twisted toroidal component strongly exceeds the poloidal field, leading to a drastic acceleration in the growth of magnetic energy. At the moment when the magnetic pressure becomes comparable to the gas pressure at the periphery of the proto-neutron star the MHD compression wave appears and goes through the envelope of the collapsed core. It transforms into the fast MHD shock and produces a supernova explosion. Our simulations give the energy of the explosion $0.6\cdot 10^{51}$ ergs. The amount of the mass ejected by the explosion is $\sim 0.14M_\odot$. The implicit numerical method, based on the Lagrangian triangular grid of variable structure, was used for the simulations.

astro-ph↗

arXiv2005

Screening of the Magnetic Field of Disk Accreting Stars

An analytical model is developed for the screening of the external magnetic field of a rotating, axisymmetric neutron star due to the accretion of plasma from a disk. The decrease of the field occurs due to the electric current in the infalling plasma. The deposition of this current carrying plasma on the star's surface creates an induced magnetic moment with a sign opposite to that of the original magnetic dipole. The field decreases independent of whether the star spins-up or spins-down. The time-scale for an appreciable decrease (factor of $>100$) of the field is found to be $\sim 1.6 \times 10^7$ yr, for a mass accretion rate $\dot{M}=10^{-9} M_\odot/$yr and an initial magnetic moment $μ_i = 10^{30}{\rm G cm}^3$ which corresponds to a surface field of $10^{12} $G if the star's radius is $10^6$ cm. The time-scale varies approximately as $μ_i^{3.8}/\dot{M}^{1.9}$. The decrease of the magnetic field does not have a simple relation to the accreted mass. Once the accretion stops the field leaks out on an Ohmic diffusion time scale which is estimated to be $ > 10^9$ yr.

astro-ph↗

arXiv2005

Infrared afterglow of GRB041219 as a result of reradiation on dust in a circumstellar cloud

Observations of gamma ray bursts (GRB) afterglows in different spectral bands provide a most valuable information about their nature, as well as about properties of surrounding medium. Powerful infrared afterglow was observed from the strong GRB041219. Here we explain the observed IR afterglow in the model of a dust reradiation of the main GRB signal in the envelope surrounding the GRB source. In this model we do not expect appearance of the prompt optical emission which should be absorbed in the dust envelope. We estimate the collimation angle of the gamma ray emission, and obtain restrictions on the redshift (distance to GRB source), by fitting the model parameters to the observational data.

astro-ph↗

arXiv2005

Magnetorotational supernovae

We present the results of 2D simulations of the magnetorotational model of a supernova explosion. After the core collapse the core consists of rapidly a rotating proto-neutron star and a differentially rotating envelope. The toroidal part of the magnetic energy generated by the differential rotation grows linearly with time at the initial stage of the evolution of the magnetic field. The linear growth of the toroidal magnetic field is terminated by the development of magnetohydrodynamic instability, leading to drastic acceleration in the growth of magnetic energy. At the moment when the magnetic pressure becomes comparable with the gas pressure at the periphery of the proto-neutron star $\sim 10-15$km from the star centre the MHD compression wave appears and goes through the envelope of the collapsed iron core. It transforms soon to the fast MHD shock and produces a supernova explosion. Our simulations give the energy of the explosion $0.6\cdot 10^{51}$ ergs. The amount of the mass ejected by the explosion is $\sim 0.14M_\odot$. The implicit numerical method, based on the Lagrangian triangular grid of variable structure, was used for the simulations.

astro-ph↗

arXiv2004

Evolution of dense stellar clusters

A qualitative study is made of the evolutionary tracks of various dense clusters, with allowance for the evaporation of stars, and for head-on collisions, until relativistic collapse occures or the cluster breaks up.

astro-ph↗

arXiv2004

On chaotic behavior of gravitating stellar shells

Motion of two gravitating spherical stellar shells around a massive central body is considered. Each shell consists of point particles with the same specific angular momenta and energies. In the case when one can neglect the influence of gravitation of one ("light") shell onto another ("heavy") shell ("restricted problem") the structure of the phase space is described. The scaling laws for the measure of the domain of chaotic motion and for the minimal energy of the light shell sufficient for its escape to infinity are obtained.

nlin.CD↗

arXiv2004

A Simplified Model of the Formation of Structures in Dark Matter

The collapse of a rotating 3-axis ellipsoid is approximated by a system of ordinary differential equations. Violent relaxation, mass and angular momentum losses are taken into account phenomenologically. The formation of the equilibrium configuration and different types of instability are investigated.

astro-ph↗

arXiv2004

Magnetorotational supernova simulations

We present 2D results of simulations of the magnetorotational core collapsed supernova. For the first time we obtain strong explosion for the core collapsed supernova. In 2D approximation we show that amplification of the toroidal magnetic field due to the differential rotation leads to the formation of MHD shockwave, which produces supernova explosion. The amounts of the ejected mass $0.1M_\odot$ and energy $\sim 0.5\div0.6 \cdot 10^{51}$ergs can explain the energy output for supernova type II or type Ib/c explosions. The shape of the explosion is qualitatively depends on the initial configuration of the magnetic field, and may form strong ejection neat the equatorial plane, or produce mildly collimated jets. Our simulation show that during the evolution of the magnetic field the magnetorotational instability appears and leads to exponential growth of the magnetic field strength.

astro-ph↗

arXiv2004

Accretion disks around Black Holes with Advection and Optical Depth Transition

We consider the effects of advection and radial gradients of pressure and radial drift velocity on the structure of accretion disks around black holes with proper description of optically thick/thin transitions. We concentrated our efforts on the models with large accretion rate. Contrary to disk models neglecting advection, we find that continuous solutions extending from the outer disk regions to the inner edge exist for all accretion rates we have considered. We show that the sonic point moves outward with increasing accretion rate, and that in the innermost disk region advection acts as a heating process that may even dominate over dissipative heating. Despite the importance of advection on it's structure, the disk remains geometrically thin. Global solutions of advective accretion disks, which describe continuously the transition between optically thick outer region and optically thin inner region are constructed and analyzed.

astro-ph↗

arXiv2004

Chaotic motion and ballistic ejection of gravitating shells

It is shown that during the motion of two initially gravitationally bound spherical shells, consisting of point particles moving along ballistic trajectories, one of the shell may be expelled to infinity at subrelativistic speed $v_{exp}\leq 0.25 c$. The problem is solved in Newtonian gravity. Motion of two intersecting shells in the case when they do not runaway shows a chaotic behaviour. We hope that this toy and oversimplyfied model can give nevertheless a qualitative idea on the nature of the mechanism of matter outbursts from the dense stellar clusters.

astro-ph↗

arXiv2004

Regular particle acceleration in relativistic jets

Exact solution is obtained for electromagnetic field around a conducting cylinder of infinite length and finite radius, with a periodical axial current, when the wave length is much larger than the radius of the cylinder. The solution describes simultaneously the fields in the near zone close to the cylinder, and transition to the wave zone. Proper long-wave oscillations of such cylinder are studied. The electromagnetic energy flux from the cylinder is calculated. These solutions could be applied for description of the electromagnetic field around relativistic jets from active galactic nuclei and quasars and particle acceleration inside jets.

astro-ph↗