THE ABSENCE OF X-RAY FLASHES FROM NEARBY GALAXIES AND THE GAMMA-RAY BURST DISTANCE SCALE

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 detecti on limit of the Burst and Transient Source Experiment (BATSE). If a su bstantial portion of the burst population detected by BATSE originates in a Galactic halo at distances greater than or equal to 150 kpc, exi sting X-ray telescopes will be able to detect GRBs in external galaxie s out to a distance of at least 4.5 Mpc. As reported in Gotthelf, Hami lton, & Helfand, the imaging proportional counter (IPC) on board the E instein Observatory detected 42 transient events with pointlike spatia l characteristics and timescales of less than 10 s. These events are d istributed isotropically on the sky; in particular, they are not conce ntrated 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 gal axies. We see none. We therefore conclude that if the Ginga detections are representative of the population of GRBs sampled by BATSE, GRBs c annot originate in a Galactic halo population with limiting radii betw een 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 ind ependent of whether the flashes we do detect have an astronomical orig in.