Implication of the discovery of the proper motion of the optical counterpart of GRB 970228 for the models of gamma-ray bursts

Abstract

The accurate position determination of GRB 970228 by the Beppo/SAX Satellite led to the discovery of a fading X-ray and optical counterparts of the burst. About a month after the GRB event the proper motion of the ejecta was detected and the extended optical source has faded below the Keck detection level. We analyze these observations in the framework of the most popular gamma ray burst models. We find a lower limit on the distance to GRB 970228 d>6.6 pc. We estimate the amount of energy required to explain the motion of a point source and the variability of the extended source. We find that the cosmological models are ruled out. The Galactic corona models suffer difficulties if the distance to the burst is > 100 kpc or the variability of the extended source is directly connected with the burst No constraints are found for GRBs originating between 6.6 pc and $\sim 10$ kpc.