GRB990123, The Optical Flash and The Fireball Model

We compare the ongoing observations of the remarkable burst GRB990123, the mother of all bursts, with the predictions of the afterglow theory. We show that the observations agree with the recent prediction that a reverse shock propagating into the ejecta would produce a very strong prompt optical flash. This reverse shock has also produced the 8.46GHz radio signal, observed after one day. The forward shock, which propagates into the ISM is the origin of the classical afterglow. It has produced the prompt X-ray signal as well as the late optical and IR emission. It would most likely produce a radio emission within the next few weeks. The observations suggest that the initial Lorentz factor of the ejecta was $\sim 200$. Within factors of order unity, this crude model explains all current observations of GRB990123.Comment: 14 pages including 2 figure

Neutron Stars with a Stable, Light Supersymmetric Baryon

If a light gluino exists, the lightest gluino-containing baryon, the \OSO, is a possible candidate for self-interacting dark matter. In this scenario, the simplest explanation for the observed ratio $\Omega_{dm}/\Omega_b \approx 6-10$ is that $m_{S^0} \sim 900$\MeVcs; this is not at present excluded by particle physics. Such an \OSO could be present in neutron stars, with hyperon formation serving as an intermediate stage. We calculate equilibrium compositions and equation of state for high density matter with the \OSO, and find that for a wide range of parameters the properties of neutron stars with the \OSO are consistent with observations. In particular, the maximum mass of a nonrotating star is $1.7-1.8 M_\odot$, and the presence of the \OSO is helpful in reconciling observed cooling rates with hyperon formation.Comment: ApJL submitted, 4 pages, using emulateapj (very very minor changes to match published versio

The Expected Duration of Gamma-Ray Bursts in the Impulsive Hydrodynamic Models

Depending upon the various models and assumptions, the existing literature on Gamma Ray Bursts (GRBs) mentions that the gross theoretical value of the duration of the burst in the hydrodynamical models is tau~r^2/(eta^2 c), where r is the radius at which the blastwave associated with the fireball (FB) becomes radiative and sufficiently strong. Here eta = E/Mc^2, c is the speed of light, E is initial lab frame energy of the FB, and M is the baryonic mass of the same (Rees and Meszaros 1992). However, within the same basic framework, some authors (like Katz and Piran) have given tau ~ r^2 /(eta c). We intend to remove this confusion by considering this problem at a level deeper than what has been considered so far. Our analysis shows that none of the previously quoted expressions are exactly correct and in case the FB is produced impulsively and the radiative processes responsible for the generation of the GRB are sufficiently fast, its expected duration would be tau ~ar^2/(eta^2 c), where a~O(10^1). We further discuss the probable change, if any, of this expression, in case the FB propagates in an anisotropic fashion. We also discuss some associated points in the context of the Meszaros and Rees scenario.Comment: 21 pages, LATEX (AAMS4.STY -enclosed), 1 ps. Fig. Accepted in Astrophysical Journa