Gamma-Ray Bursts from Neutron Star Mergers

Binary neutron stars merger (NS$^2$M) at cosmological distances is probably the only $\gamma$-ray bursts model based on an independently observed phenomenon which is known to be taking place at a comparable rate. We describe this model, its predictions and some open questions.Comment: 4 pages, 1 Figure can be obtained on request by e-mail from the Autho

Reverse Shock Emission as a Probe of GRB Ejecta

We calculate the reverse shock (RS) synchrotron emission in the optical and the radio wavelength bands from electron-positron pair enriched gamma-ray burst ejecta with the goal of determining the pair content of GRBs using early time observations. We take into account an extensive number of physical effects that influence radiation from the reverse-shock heated GRB ejecta. We find that optical/IR flux depends very weakly on the number of pairs in the ejecta, and there is no unique signature of ejecta pair enrichment if observations are confined to a single wavelength band. It may be possible to determine if the number of pairs per proton in the ejecta is > 100 by using observations in optical and radio bands; the ratio of flux in the optical and radio at the peak of each respective reverse-shock light curve is dependent on the number of pairs per proton. We also find that over a large parameter space, RS emission is expected to be very weak; GRB 990123 seems to have been an exceptional burst in that only a very small fraction of the parameter space produces optical flashes this bright. Also, it is often the case that the optical flux from the forward shock is brighter than the reverse shock flux at deceleration. This could be another possible reason for the paucity of prompt optical flashes with a rapidly declining light curve at early times as was seen in 990123 and 021211. Some of these results are a generalization of similar results reported in Nakar & Piran (2004).Comment: 12 pages, 6 figures, 2 tables, accepted to MNRA

Towards understanding gamma-ray bursts

\gamma-ray bursts (GRBs) have puzzled astronomers since their accidental discovery in the sixties. The BATSE detector on COMPTON-GRO satellite has been detecting GRBs for the last four years at a rate of one burst per day. Its findings has revolutionized our ideas about the nature of these objects. In this lecture I show that the simplest, most conventional and practically inevitable, interpretation of the observations is that GRBs form during the conversion of the kinetic energy of ultra-relativistic particles to radiation. The inner ``engine" that accelerates these particles is well hidden from direct observations and its origin might remain mysterious for a long time

Pure and loaded fireballs in SGR giant flares

On December 27, 2004, a giant flare from SGR 1806$-$20 was detected on earth. Its thermal spectrum and temperature suggest that the flare resulted from an energy release of about $10^{47}$ erg/sec close to the surface of a neutron star in the form of radiation and/or pairs. This plasma expanded under its own pressure producing a fireball and the observed gamma-rays escaped once the fireball became optically thin. The giant flare was followed by a bright radio afterglow, with an observable extended size, implying an energetic relativistic outflow. We revisit here the evolution of relativistic fireballs and we calculate the Lorentz factor and energy remaining in relativistic outflow once the radiation escapes. We show that pairs that arise naturally in a pure pairs-radiation fireball do not carry enough energy to account for the observed afterglow. We consider various alternatives and we show that if the relativistic outflow that causes the afterglow is related directly to the prompt flare, then the initial fireball must be loaded by baryons or Poynting flux. While we focus on parameters applicable to the giant flare and the radio afterglow of SGR 1806$-$20 the calculations presented here might be also applicable to GRBs

Neutrinos from the Propagation of a Relativistic Jet Through a Star

We discuss the neutrino signature of a relativistic jet propagating through a stellar envelope, a scenario realized in the collapsar model for Gamma Ray Bursts (GRBs). It is shown that the dramatic slowing of the jet deep within the star is accompanied by inelastic neutron-nucleon collisions and the conversion of a substantial fraction of the jet kinetic energy to neutrinos. These neutrinos have observed energies in the range two to tens of GeV and an estimated detection rate comparable to or larger than the detection rate of GeV neutrinos from other GRB-related processes. The time delay between the arrival of these neutrinos and the GRB photons is tens of seconds. An observation of this delay would provide an indication that the GRB jet originated in a massive star.Comment: To appear in Ap