Constraints on γ-ray bursters from soft X-ray transients

Abstract

Perhaps the most enigmatic and elusive of the short time scale astrophysical phenomena observed so far are the γ-ray bursts discovered in the Vela satellite data in 19731. After a decade of observational and theoretical work, there is still no agreement on a model for the radiation detected or for the type of astronomical system responsible. We have conducted a search of the Columbia Astrophysics Laboratory's Einstein imaging proportional counter (IPC) database for possible X-ray counterparts to these transient events. The limiting sensitivity was ∼10−10 erg cm−2 with a total exposure of ∼3×106s divided amongst ∼103 1°×1° fields. Four events with a pointlike spatial distribution were detected but their extremely soft spectra (kT ≤ 0.1 keV) makes them unlikely candidates for γ-ray burst counterparts. We use our results here to place limits on the X-ray and γ-ray luminosity ratio and log (number) N–log (flux) S relation for γ-ray bursts, and comment on the constraints placed on models for their origin.