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 (<~ 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.
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Gotthelf et al. (1996) studied this question.