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 (GRBs) 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 (less than or similar to 10 s), transient X-ray phenomena not associated a priori with detectable point sources. A total of 1.5 x 10(7) s of exposure time was searched on arcminute spatial scales down to a limiting sensitivity of 10(-11) ergs cm(-2) s(-1) in the 0.2-3.5 keV IPC band. Forty-two highly significant X-ray flashes (Poisson probability less than 10(-7) of being produced by statistical fluctuations) are discovered, of which 18 have spectra consistent with an extragalactic origin and light curves similar to the Ginga-detected X-ray counterparts to GRBs. Great care is taken to identify and exclude instrumental and observational artifacts; we develop a set of tests to cull events that 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 GRBs, 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, we use the results of our search to constrain strongly all halo models for GRBs.