On bending angles by gravitational lenses in motion

The bending of lightrays by the gravitational field of a ``lens'' that is moving relative to the observer is calculated within the approximation of weak fields, small angles and thin lenses. Up to first order in $v/c$ -- and assuming the acceleration to be much smaller than $v/c$ -- the bending angle, time delay and redshift of the images are found to be affected by the component of the speed of the deflector along the line of sight. The correction takes the form of an overall factor of $1+v/c$ accompanying the mass of the deflector, leading to an indeterminacy of the order of $v/c$ in the mass of the lens inferred on the basis of the separation of multiple images. The consequent correction to the microlensing lightcurve is pointed out, as well as scenarios where the correction is potentially relevant.Comment: 6 pages, to appear in MNRA

Gamma-ray Burst Models: General Requirements and Predictions

Whatever the ultimate energy source of gamma-ray bursts turns out to be, the resulting sequence of physical events is likely to lead to a fairly generic, almost unavoidable scenario: a relativistic fireball that dissipates its energy after it has become optically thin. This is expected both for cosmological and halo distances. Here we explore the observational motivation of this scenario, and the consequences of the resulting models for the photon production in different wavebands, the energetics and the time structure of classical gamma-ray bursters.Comment: 5 pages, plain TeX, no figures; text of invited review at 17th Texas Conf. Relativistic Astrophysics, GRB Miniworkshop, Munich, Dec. 1994

Gamma-Ray Bursts as the Death Throes of Massive Binary Stars

It is proposed that gamma-ray bursts are created in the mergers of double neutron star binaries and black hole neutron star binaries at cosmological distances. Bursts with complex profiles and relatively long durations are the result of magnetic flares generated by the Parker instability in a post-merger differentially-rotating disk. Some bursts may also be produced through neutrino-antineutrino annihilation into electrons and positrons. In both cases, an optically thick fireball of size \sles\ 100 km is initially created, which expands ultrarelativistically to large radii before radiating. Several previous objections to the cosmological merger model are eliminated. It is predicted that $\gamma$-ray bursts will be accompanied by a burst of gravitational radiation from the spiraling-in binary which could be detected by LIGO.Comment: 14 page

On the nature of gamma-ray burst time dilations

The recent discovery that faint gamma-ray bursts are stretched in time relative to bright ones has been interpreted as support for cosmological distances: faint bursts have their durations redshifted relative to bright ones. It was pointed out, however, that the relative time stretching can also be produced by an intrinsic correlation between duration and luminosity of gamma-ray bursts in a nearby, bounded distribution. While both models can explain the average amount of time stretching, we find a generic difference between them in the way the duration distribution of faint bursts deviates from that of bright ones. This allows us to distinguish between these two broad classes of model on the basis of the duration distributions of gamma-ray bursts, leading perhaps to an unambiguous determination of the distance scale of gamma-ray bursts. We apply our proposed test to the second BATSE catalog and conclude, with some caution, that the data favor a cosmological interpretation of the time dilation.Comment: 9 pages uuencoded compressed postscript including 2 figures, Princeton University Observatory preprint POP-567. Submitted to Astrophysical Journal Letters, 2 June 199

One Parameter Solution of Spherically Symmetric Accretion in Various Pseudo-Schwarzschild Potentials

In this paper we have solved the hydrodynamic equations governing the spherically symmetric isothermal accretion (wind) onto (away from) compact objects using various pseudo-Schwarzschild potentials.These solutions are essentially one parameter solutions in a sense that all relevant dynamical as well as thermodynamic quantities for such a flow could be obtained (with the assumption of a one-temperature fluid) if {\it only one} flow parameter (temperature of the flow $T$) is given. Also we have investigated the transonic behaviour of such a flow and showed that for a given $T$, transitions from subsonic to the supersonic branch of accretion (wind) takes place at different locations depending on the potentials used to study the flow and we have identified these transition zones for flows in various such potentials.Comment: 9 pages, 3 black and white post-script figures. Published in the International Journal of Modern Physics D (IJMPD

Extracting Energy from Accretion into Kerr Black Hole

The highest efficiency of converting rest mass into energy by accreting matter into a Kerr black hole is ~ 31% (Thorne 1974). We propose a new process in which periods of accretion from a thin disk, and the associated spin-up of the black hole, alternate with the periods of no accretion and magnetic transfer of energy from the black hole to the disk. These cycles can repeat indefinitely, at least in principle, with the black hole mass increasing by ~ 66% per cycle, and up to ~ 43% of accreted rest mass radiated away by the disk.Comment: 4 pages, 1 figur

Pseudo-Newtonian Potentials for Nearly Parabolic Orbits

We describe a pseudo-Newtonian potential which, to within 1% error at all angular momenta, reproduces the precession due to general relativity of particles whose specific orbital energy is small compared to c^2 in the Schwarzschild metric. For bound orbits the constraint of low energy is equivalent to requiring the apoapsis of a particle to be large compared to the Schwarzschild radius. Such low energy orbits are ubiquitous close to supermassive black holes in galactic nuclei, but the potential is relevant in any context containing particles on low energy orbits. Like the more complex post-Newtonian expressions, the potential correctly reproduces the precession in the far-field, but also correctly reproduces the position and magnitude of the logarithmic divergence in precession for low angular momentum orbits. An additional advantage lies in its simplicity, both in computation and implementation. We also provide two simpler, but less accurate potentials, for cases where orbits always remain at large angular momenta, or when the extra accuracy is not needed. In all of the presented cases the accuracy in precession in low energy orbits exceeds that of the well known potential of Paczynski & Wiita (1980), which has ~30% error in the precession at all angular momenta.Comment: 4 pages, 1 figure. Accepted by Ap

Gamma-Ray Bursts as Hypernovae

The energetics of optical and radio afterglows following BeppoSAX and BATSE gamma-ray bursts (GRBs) suggests that gamma-ray emission is not narrowly collimated, but a moderate beaming is possible, so the total energy of a GRB may be in the range 10^{50} - 10^{51} erg. All attempts to generate a fireball powered by neutrino-antineutrino annihilation have failed so far, and a rapid rotation combined with a magnetic field of $\sim 10^{15}$ gauss gains popularity. In this paper a hypernova scenario is described: a collapse of a massive member in a close binary system, similar to the `failed' Type Ib supernova model proposed by Woosley (1993). The collapse may lead to explosion, with energy transmitted from the rapidly spinning hot neutron core or black hole to the envelope by a strong magnetic field, as in a supernova model proposed by Ostriker and Gunn (1971). However, because of a large mass and rapid rotation of the core the explosion of a hypernova may release up to 10^{54} erg of kinetic energy, creating a `dirty' fireball. In this scenario a moderately non-spherical explosion may accelerate a very small fraction of matter to a very large Lorentz factor, and this may give rise to a gamma-ray burst and its afterglow, just like in a conventional fireball model. However, the highest velocity ejecta from a hypernova are followed with matter which expands less rapidly but carries the bulk of kinetic energy, providing a long term power source for the afterglow. If the afterglows remain luminous for a very long time then the proposed hypernova scenario may provide an explanation. Pre-hypernovae, being massive stars, are likely to be located in the young, star forming regions.Comment: 12 pages, latex, uses aaspp4.sty, no figures, submitted to ApJ Letter

Accretion and Structure of Radiating Disks

We studied a steadily accreting, geometrically thick disk model that selfconsistently takes into account selfgravitation of the polytropic gas, its interaction with the radiation and the mass accretion rate. The accreting mass is injected inward in the vicinity of the central $z=0$ plane, where also radiation is assumed to be created. The rest of the disk remains approximately stationary. Only conservation laws are employed and the gas-radiation interaction in the bulk of the disk is described in the thin-gas approximation. We demonstrate that this scheme is numerically viable and yields a structure of the bulk that is influenced by the radiation and (indirectly) by the prescribed mass accretion rate. The obtained disk configurations are typical for environments in Active Galactic Nuclei (AGN), with the central mass of the order of 10^7 M_{\astrosun} to 10^8 M_{\astrosun}, quasi-Keplerian rotation curves, disk masses ranging from about 10^6 M_{\astrosun} to 10^7 M_{\astrosun}, and the luminosity ranging from 10^6 L_{\astrosun} to 10^9 L_{\astrosun}. These luminosities are much lower than the corresponding Eddington limit.Comment: Changes according to the version accepted by Astronomy & Astrophysic