The soft γ-ray burst GB790107

Abstract

Nearly all of the known γ-ray bursts (GRBs), when observed over the energy range ∼30 keV to 1 MeV, have intensity spectra that can be described in terms of several-hundred-keV exponential functions. The Venera 11 and Venera 12 KONUS data indicate that, in addition to these ‘normal’ GRBs, there might exist a separate class of short, soft events^1,2. There can be little doubt that bursts with spectra that seem anomalously soft (e-folding energy ≲30 keV) do exist, and that the most convincing known examples—GB790305b (the famous 5 March burst, including its follow-on events), GB790324 (and its follows-ons) and GB790930—generally have atypically short durations of ∼0.1–0.25 s. (Three of the 11 GB790305b follow-on events were longer³.) However, because most GRB instruments were designed primarily for observations above ∼100keV, events with very soft spectra have not been well enough studied to be classified unambiguously. Here we present the results of fortuitous detections of GB790107 (determined in ref. 2 to have a soft spectrum) with instrumentation better suited for the study of such bursts. Above 20 keV the spectrum is indeed much softer than any other cosmic event observed through the apertures of these experiments, and does not appear to belong to the general GRB spectral distribution. However, in the 5–15-keV range the spectrum is very flat. Statistical arguments indicate that soft-spectrum events might belong to a disk population, but this is not a strong conclusion.