Gamma-ray burst measurements at low fluxes

To obtain more data at the low flux end of the size spectrum and in particular to both lower the size limit of burst detection and look for a possible excess flux from the galactic centre region, large-area detector flights were undertaken from Australia. The apparatus consisted of plastic scintillators, 5 cm thick and 6.3 m2 in effective area, arranged in six trays, two of which were horizontal and four of which were inclined at 30? to the horizontal so as to make up the four sloping sides of a box. Laboratory calibration gave a mean efficiency for the detection of 50 keV to 2 MeV photons of 0.404 when translated to 4 g cm-2 atmospheric depth and for all bursts arriving up to a cut-off zenith angle of 60?. Thirty-three hours of good data were obtained in total during flights in November 1977 and November 1978. Gamma-burst searches were made by demanding three successive 2o increases in at least three of the six trays with a sampling interval of 1/16s. Two bursts were detected, one at 1978, 15 November, 22h14, in coincidence with Venera spacecraft observations of an event, and the second at 1978, 16 November, 13h40 at a flux level of 9.2 x 10-8 erg cm-2. Directional determination for this second burst with the inclined tray rates gave I- 82.5?, b = - 71.4?, with an error circle of 12'. A Venera event occurred on 19 November 1978, 24' away from this position. Correction of the flight results to an isotropic flux, missed short bursts, the flat detector response and missing energy being taken into account, on the assumption of the Apollo 16 (Mezger et al. I974) spectrum, yielded a rate (7+10) x 103 year-1 at sizes greater than or equal to 8.47 x 10-9 erg cm-2. This point is plotted