STRUCTURE OF X-RAY SHOTS OF CYGNUS X-1 IN ITS LOW STATE

X-rays from Cygnus X-1 in its low-intensity state show rapid and chaot ic time variability. This variability is believed to consist of many r andomly occurring shots. We analyzed data from a Ginga observation of Cygnus X-1 by superposing many X-ray shots through aligning their peak s and obtained detailed structure of the shots. We found that the shot s had nearly symmetric rise and decay lasting a few seconds, each of w hich was roughly represented by a sum of two exponentials with the tim e constants about 0.1 and 1 s. Moreover, the energy spectrum of the sh ots before the peak intensity is found to be softer than the average s pectrum of the source, and after the peak the energy spectrum becomes harder suddenly, but remaining slightly softer than the average spectr um. The description of these features is more definite than that of an y previous shot model.