Intense X-Ray Bursts from a Globular Cluster

J. Grindlay, Center for Astrophysics, Harvard and Smithsonian Observatories; and J. Heise, Laboratory for Space Research, Utrecht, report the detection of two intense x-ray bursts from the x-ray source 3U 1820-30 associated with the globular cluster NGC 6624. These events appear to be qualitatively different from short-term variations previously reported in cosmic x-ray sources. The bursts were detected by both the 1-30 keV (HXX) Cambridge detectors and the 1-6 keV (SXX) Utrecht experiments on ANS on Sept. 28 at 1h31m51s and 9h49m40s UT. In the first event, the source intensity increased by a factor of 20 in about 1 s and decayed approximately exponentially over the next 10 s. The second burst was detected in a region of very low and constant background as a factor of 30 increase (less than 0.5-s rise time) in source intensity, lasting about 2 s and followed by a decay-time constant nearly identical with that of the first event. They can exclude the possibility that these were particle-induced events since the background counters showed no detectable increase and the 10' x 3o collimators of the HXX experiment yielded a direction for the bursts within 1' of 3U 1820-30 and the nucleus of NGC 6624. During the total exposure of three hours in pointings on the source between Sept. 27.4 and 30.7, the source intensity was otherwise constant at about 3.5 cts/s or about one-quarter of the similarly constant flux observed during three hours of ANS pointings between Mar. 24.4 and 28.0. No bursts were detected during this first observation period, during which the source x-ray luminosity was near the Uhuru value of about 10**38 erg/s. The peak luminosity during the bursts was thus near l0**39 erg/s. Although the time profile resembles that of gamma bursts the spectra were much softer (~ 10 keV exponentials). Given the detection of two such events in six hours of observing, further monitoring of globular-cluster x-ray sources at x-ray, optical and radio wavelengths is needed.