Search for γ-ray bursts with coincident balloon flights

Abstract

WE report on an experiment to search for cosmic γ-ray bursts of the type discovered by Klebesadel et al.1, but of smaller size and of sufficient frequency of occurrence to be detected during a 1-d observation program. Two similar detectors, successfully balloon-borne from launch sites in South Dakota and Texas, achieved about 20 h of simultaneous operation at several millibars atmospheric depth, with continuous separation of over 1,500km on 10–11 May 1975. Fluctuations of the counting rates of >150keV photons with temporal structures from microseconds to several minutes were compared in order to detect coincident or associated responses from the two instruments. The results of this experiment can be summarised as follows. First, no coincident γ-ray burst events were detected. Second, the resulting integral size spectrum of small bursts, from this and from all other searches, remains a spectrum of upper limits, consistent with an extrapolation of the size spectrum of the largest known bursts, fitting a power law of index –1.5. Third, a curious effect was found which can be described as consisting of associated, but not coincident, counting-rate increases of photons of energy >150keV. This phenomenon undoubtedly presents a background, event-confusion problem for any single γ-ray burst instrument, placed either on balloons or on satellites which orbit the Earth beneath the trapped radiation.