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.