arXiv subjects

Tsvi Piran

Publications and source records attributed to Tsvi Piran.

336 records · Page 19Linked to original sources

arXiv1994

$γ$-Ray Bursts and Neutron Star Mergers

$γ$-ray bursts have baffled theorists ever since their accidental discovery at the sixties. We suggest that these bursts originate in merger of neutron star binaries, taking place at cosmological distances. These mergers release $\approx 10^{54}ergs$, in what are possibly the strongest explosions in the Universe. If even a small fraction of this energy is channeled to an electromagnetic signal it will be detected as a grbs. We examine the virtues and limitations of this model and compare it with the recent Compton \g-ray observatory results. We also discuss the potential application of grbs to study cosmology and show that these burst might lead to a new and independent determination of both $H_0$ and $Ω$.

astro-ph↗

arXiv1994

Fireballs and Gamma-Ray Bursts

The sudden release of copious $\g$-ray photons into a compact region creates an opaque photon--lepton fireball due to the prolific production of electron--positron pairs. The photons that we observe in the bursts emerge only at the end of the fireball phase after it expanded sufficiently to become optically thin or after it converted its energy to the kinetic energy of relativistic baryons which convert it, in turn, to electromagnetic pulse via the interaction with interstellar matter. It is essential, therefore, to analyze the evolution of a fireball in order to comprehend the observed features of $γ$-ray bursts. We discuss various aspects of fireball hydrodynamics and the resulting emitted spectra.

astro-ph↗

arXiv1993

The Quantum Propagator for a Nonrelativistic Particle in the Vicinity of a Time Machine

We study the propagator of a non-relativistic, non-interacting particle in any non-relativistic ``time-machine'' spacetime of the type shown in Fig.~1: an external, flat spacetime in which two spatial regions, $V_-$ at time $t_-$ and $V_+$ at time $t_+$, are connected by two temporal wormholes, one leading from the past side of $V_-$ to t the future side of $V_+$ and the other from the past side of $V_+$ to the future side of $V_-$. We express the propagator explicitly in terms of those for ordinary, flat spacetime and for the two wormholes; and from that expression we show that the propagator satisfies completeness and unitarity in the initial and final ``chronal regions'' (regions without closed timelike curves) and its propagation from the initial region to the final region is unitary. However, within the time machine it satisfies neither completeness nor unitarity. We also give an alternative proof of initial-region-to-final-region unitarity based on a conserved current and Gauss's theorem. This proof can be carried over without change to most any non-relativistic time-machine spacetime; it is the non-relativistic version of a theorem by Friedman, Papastamatiou and Simon, which says that for a free scalar field, quantum mechanical unitarity follows from the fact that the classical evolution preserves the Klein-Gordon inner product.

gr-qc↗

arXiv1993

Do Gamma-Ray Burst Sources Repeat

Following the discovery by Quashnock and Lamb (1993) of an apparent excess of $γ$-ray burst pairs with small angular separations, we reanalyze the angular distribution of the bursts in the BATSE catalogue. We find that in addition to an excess of close pairs, there is also a comparable excess of antipodal bursts, i.e pairs of bursts separated by about 180 degrees in the sky. Both excesses have only modest statistical significance. We reject the hypothesis put forward by Quashnock and Lamb that burst sources are repeaters, since it is obvious that this hypothesis does not predict an excess of antipodal coincidences. Lacking any physical model of bursts that can explain the antipodal pairs, we suggest that the two excesses seen in the data are either due to an unusual statistical fluctuation or caused by some unknown selection effect.

astro-ph↗

arXiv1993

Gravitational Waves and $γ$-Ray Bursts

Coalescing binaries in distant galaxies are one of the most promising sources of gravitational waves detectable by the LIGO project.$^{[1-5]}$ They are also a copious source of neutrinos,$^{[1]}$ however these neutrino pulses are far too weak to be detected on earth. Several years ago Eichler \etal$\,$$^{[6]}$ suggested that they are also sources of $γ$-ray bursts (GRBs). Recently it was found$^{[7]}$ that GRBs are likely to be cosmological in origin, and coalescing binary systems$^{[6,8-11]}$ are probably the most promising cosmological sources. The current estimates of the burst and LIGO rates from a cosmologically distributed population are based on the systems observed in our galaxy.$^{[12-14]}$ These estimates are based on only three binarys so there are large statistical uncertainties in the coalescence rate. If we accept the cosmological/coalescing binary hypothesis for GRBs, then we get a more accurate estimate of the rate at which binaries coalesce and hence of the predicted LIGO signal. The association between GRBs and binaries can significantly improve the performance of LIGO. The detection of gravitational radiation from a GRB source not only confirms the coalescing binary model, but it will also provide information on the geometry and energy generation mechanism of the burst.

astro-ph↗

arXiv1993

Numerical Analysis of Black Hole Evaporation

Black hole formation/evaporation in two-dimensional dilaton gravity can be described, in the limit where the number $N$ of matter fields becomes large, by a set of second-order partial differential equations. In this paper we solve these equations numerically. It is shown that, contrary to some previous suggestions, black holes evaporate completely a finite time after formation. A boundary condition is required to evolve the system beyond the naked singularity at the evaporation endpoint. It is argued that this may be naturally chosen so as to restore the system to the vacuum. The analysis also applies to the low-energy scattering of $S$-wave fermions by four-dimensional extremal, magnetic, dilatonic black holes.

hep-th↗

arXiv1993

On the Bimodal Distribution of Gamma-Ray Bursts

Kouveliotou et al. (1993) recently confirmed that gamma-ray bursts are bimodal in duration. In this paper we compute the statistical properties of the short ($\le 2$~s) and long ($>2$~s) bursts using a method of analysis that makes no assumption regarding the location of the bursts, whether in the Galaxy or at a cosmological distance. We find the 64 ms channel on BATSE to be more sensitive to short bursts and the 1024 ms channel is more sensitive to long bursts. We show that all the currently available data are consistent with the simple hypothesis that both short and long bursts have the same spatial distribution and that within each population the sources are standard candles. The rate of short bursts is $\sim 0.4$ of the rate of long bursts. Although the durations of short and long gamma-ray bursts span several orders of magnitude and the total energy of a typical short burst is smaller than that of a typical long burst by a factor of $\sim 20$, surprisingly the peak luminosities of the two kinds of bursts are equal to within a factor of $\sim 2$.

astro-ph↗

arXiv1993

Hydrodynamics of Relativistic Fireballs

We examine analytically and numerically the evolution of a relativistic fireball. We show that, after an early rearrangement phase, most of the matter and energy in the fireball is concentrated within a narrow shell. The shell propagates at nearly the speed of light, with a frozen radial profile and according to a simple set of scaling laws. The spectrum of the escaping radiation is harder at early times and softer later on. The results are applicable to models of $γ$-ray bursts. Submitted to Monthly Notices of the RAS Figures available on request from the authors

astro-ph↗

arXiv1992

Gamma-Ray Bursts and Neutron Star Mergers - Possibly the Strongest Explosions in the Universe

astro-ph↗

arXiv1992

Fireballs in the Galactic Halo and $γ$-Ray Bursts

If gamma ray burst sources are in the galactic halo they inevitably involve a creation of an opaque pair plasma fireball, just like in cosmological sources. We find that the typical physical conditions in a galactic halo fireball are: optical depth $\ap 10^8$, thermal energy $\ap 100 KeV$, maximal relativstic expansion $\G \ap 300 $ and a maximal baryonic load of $\ap 10^{-15} M_\odot$. This does not rule out galctic halo models but it poses an addtional severe constraint on all such sources. A comparison of these conditions with the physical conditions at cosmological fireballs reveal that galactic halo fireballs are less favorable than cosmological ones as sources of gamma ray bursts.

astro-ph↗

arXiv1992

The Loss of Unitarity in the Vicinity of a Time Machine

We construct the propagator of a non-relativistic non-interacting particle in a flat spacetime in which two regions have been identified. This corresponds to the simplest "time machine". We show that while completeness is lost in the vicinity of the time machine it holds before the time machine appears and it is recovered afterwards. Unitarity, however, is not satisfied anywhere. We discuss the implications of these results and their relationship to the loss of unitarity in black hole evaporation.

gr-qc↗

arXiv1992

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 $γ$-ray bursts will be accompanied by a burst of gravitational radiation from the spiraling-in binary which could be detected by LIGO.

astro-ph↗