The EINSTEIN Observatory Detection of Faint X-Ray Flashes

We report on the result of an extensive search for X-ray counterparts to Gamma Ray Bursts (GRB) using data acquired with the Imaging Proportional Counter (IPC) on-board the Einstein Observatory. We examine background sky fields from all pointed observations for short timescale (~10 sec) transient X-ray phenomena not associated a-priori with detectable point sources. A total of 1.5 X 10^7 seconds of exposure time was searched on arc-minute spatial scales down to a limiting sensitivity of 10^-11 erg/cm^2/sec in the 0.2-3.5 keV IPC band. Forty-two highly significant X-ray flashes (Poisson probability < 10^-7 of being produced by statistical fluctuations) are discovered, of which eighteen have spectra consistent with an extragalactic origin and lightcurves similar to the Ginga-detected X-ray counterparts to GRB. Great care is taken to identify and exclude instrumental and observational artifacts; we develop a set of tests to cull events which may be associated with spacecraft or near-Earth space backgrounds. The flashes are found to be distributed isotropically on the sky and have an approximately Euclidean number-size relation. They are not associated with any known sources and, in particular, they do not correlate with the nearby galaxy distribution. Whether or not these flashes are astrophysical and/or associated with GRB, the limits imposed by the search described herein produces important constraints on GRB models. In this paper, we discuss possible origins for these flashes; in a companion paper, Hamilton, Gotthelf, and Helfand (1996) we use the results of our search to constrain strongly all halo models for GRB.