Shot profiles of Cygnus X-1.

The X-ray variability of black hole candidates in the hard (low) state is characterized by flare-like events, X-ray shots. Using Ginga data of Cyg X-1, average properties of the shots have been obtained by superposing a number of shots. The shots have nearly symmetrical profiles with long wings before and after the sharp peaks. The energy spectrum is softer than the average spectrum of this source, and dramatically hardens at the peak intensity. The rise profiles of the shots independent of the peak intensity and the spectral change at the peak intensity prefer a model that the shots arise from density fluctuations of accretion matter drifting to the inner region of the disk around a black hole. The advection-dominated disk theory supports this hypothesis.