paper

The EINSTEIN Observatory Detection of Faint X-Ray Flashes

arXiv:astro-ph/9607170 · doi:10.1086/177552

Abstract

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.

14 pages, LaTex (aas2pp4.sty), 14 postscript figures. To appear in the Astrophysical Journal

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