STATISTICS OF X-RAY FLUCTUATIONS FROM CYGNUS X-1 - RESERVOIRS IN THE DISK

X-rays from stellar black hole candidates, such as Cygnus X-1, are known to exhibit rapid time variations. Previously, these variations have seemed to occur at random. At a result, the ''shot-noise model,'' which assumes superpositions of randomly occurring identical shots, has often been utilized to interpret the observed time variabilities. Recent Ginga data of Cyg X-1 display evidence, however, which does not support a Poisson distribution in the time intervals between the shots, i.e., a random temporal distribution of the shots. The observations show that the frequencies of big shot events are lower than those expected by random distributions for the first several seconds before and/or after each big event. Moreover, this low-frequency interval, ''waiting time,'' is longer for the shots with larger peaks. We also find an exponential peak-intensity distribution of X-ray shots. These observational features strongly suggest the presence of numerous reservoirs with different capacities for triggering X-ray fluctuations, a key assumption of the model based on the self-organized criticality.