Observation of a hard X-ray flare in Cyg X-1

Abstract

During a balloon flight carried out from Hyderabad on the night of April 6, 1971, we monitored the intensity of Cyg X-1 in the hard X-ray band 20-154 keV for about 3 h. We have reported1 variations in the intensity of X-rays from Cyg X-1 recorded in this flight in periods of the order of several minutes. Very recently time variation studies of Cyg X-1 have assumed enormous importance because of the variety of intensity modulations that have been reported from rocket and satellite experiments and also because of the identification of the X-ray source with an optical binary and the theoretical models that have been proposed to explain some of the X-ray emission features. We have therefore reanalysed the data of April 6, 1971, on a minute-to-minute basis to examine further possible structures in time variation in hard X-rays (the data are incapable of analysis in shorter intervals because of intensity considerations). This analysis has revealed the occurrence of a "flare" in Cyg X-1 which is similar in characteristics to those recorded in hard X-rays in Sco X-1. We present here details of this flare. Experimental details and data reduction procedures have been previously described.