Observations of GRB 970228 and GRB 970508 and the Neutron Star Merger Model
Livio, Mario; Bond, Howard E.; Kouveliotou, Chryssa; Sahu, Kailash C.; van Paradijs, Jan; Macchetto, F. Duccio; Petro, Larry; Groot, Paul J.; Galama, Titus J.
Abstract
We present the discovery observations for the optical counterpart of the gamma -ray burster GRB 970508 and discuss its light curve in the context of the fireball model. We analyze the HST data for this object and conclude that any underlying galaxy must be fainter than R=25.5 . We also present a detailed analysis of the HST images of GRB 970228, from which a proper motion of the optical counterpart was claimed by Caraveo et al., and conclude that, within the uncertainties, there is no proper motion. We examine several aspects of the neutron star merger model for gamma -ray bursts. In particular, we use this model to predict the redshift distribution of gamma -ray bursters, and, adopting a recent determination of the cosmic star formation history, we show that the predicted {log}N--{log}P distribution is consistent with that observed for GRBs.