New limit on the size distribution of γ-ray bursts

Abstract

SINCE the discovery of γ-ray bursts of non-solar origin in 1973 (ref. 1), about 42 events have been confirmed. All have been observed by X-ray and γ-ray detectors on various satellites, though a number of unconfirmed bursts have also been observed by balloon-borne instruments². The lack of confirmation in the latter cases is not surprising, because of short exposure time and differing sensitivities. The detected flux ranges between about 3 × 10⁻⁶ erg cm⁻² for the smallest burst to 5 × 10⁻⁴ erg cm⁻² for the largest observed burst. The integral size distribution of these events above 10⁻⁴ erg cm⁻² follows an S^−1.5 law, suggesting an isotropic distribution of the sources. For events with energy of less than 10⁻⁴ erg cm⁻², however, the observed distribution seems to flatten out, and the choice of number index (α=0.5,1.0 or 1.5) in this region is not strongly supported by experimental results.