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For refcode 1996ApJ...466..795H:
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1996ApJ...466..795H THE ABSENCE OF X-RAY FLASHES FROM NEARBY GALAXIES AND THE GAMMA-RAY BURST DISTANCE SCALE T. T. HAMILTON Department of Astronomy, California Institute of Technology, Pasadena, CA 91125 E. V. GOTTHELF Laboratory for High Energy Astrophysics, NASA/Goddard Space Flight Center, Greenbelt, MD 20771 AND D. J. HELFAND Department of Astronomy and Columbia Astrophysics Laboratory, Columbia University, 538 West 120th Street, New York, NY 10027 Received 1995 March 17; accepted 1996 February 8 ABSTRACT If typical gamma-ray bursts (GRBs) have X-ray counterparts similar to those detected by Ginga, then sensitive-focusing X-ray telescopes will be able to detect GRBs 3 orders of magnitude fainter than the detection limit of the Burst and Transient Source Experiment (BATSE). If a substantial portion of the burst population detected by BATSE originates in a Galactic halo at distances greater than or equal to 150 kpc, existing X-ray telescopes will be able to detect GRBs in external galaxies out to a distance of at least 4.5 Mpc. As reported in Gotthelf, Hamilton, & Helfand, the imaging proportional counter (IPC) on board the Einstein Observatory detected 42 transient events with pointlike spatial characteristics and timescales of less than 10 s. These events are distributed isotropically on the sky; in particular, they are not concentrated in the directions of nearby external galaxies. For halo models of the BATSE bursts with radii of 150 kpc or greater, we would expect to see several burst events in observations pointed toward nearby galaxies. We see none. We therefore conclude that if the Ginga detections are representative of the population of GRBs sampled by BATSE, GRBs cannot originate in a Galactic halo population with limiting radii between 150 and 400 kpc. Inasmuch as halos with limiting radii outside of this range have been excluded by the BATSE isotropy measurements, our result indicates that all halo models are excluded. This result is independent of whether the flashes we do detect have an astronomical origin. Subject headings: gamma rays: bursts - surveys - X-rays: bursts
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