If typical gamma-ray bursts (GRBs) have X-ray counterparts similar to thosedetected by Ginga, then sensitive focusing X-ray telescopes will be able todetect GRBs three orders of magnitude fainter than the detection limit of theBurst and Transient Source Experiment (BATSE). If a substantial portion of theburst population detected by BATSE originates in a Galactic halo at distancesgreater than or equal to 150 kpc, existing X-ray telescopes will be able todetect GRBs in external galaxies out to a distance of at least 4.5 Mpc. Asreported in Gotthelf, Hamilton, & Helfand (1996) the Imaging Proportionalcounter (IPC) on board the Einstein Observatory detected 42 transient eventswith pointlike spatial characteristics and timescales of less than 10 seconds.These events are distributed isotropically on the sky; in particular, they arenot concentrated in the directions of nearby external galaxies. For halo modelsof the BATSE bursts with radii of 150 kpc or greater, we would expect to seeseveral burst events in observations pointed towards nearby galaxies. We seenone. We therefore conclude that if the Ginga detections are representative ofthe population of GRBs sampled by BATSE, GRBs cannot originate in a Galactichalo population with limiting radii between 150 kpc and 400 kpc. Inasmuch ashalos with limiting radii outside of this range have been excluded by the BATSEisotropy measurements, our result indicates that all halo models are excluded.This result is independent of whether the flashes we do detect have anastronomical origin.
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