Discovery of powerful millisecond flares from Cygnus X-1

We have found a large number of very strong flares in the available RXTE /PCA data of Cyg X-1 (also seen in available HEXTE and BATSE data) with 13 flares satisfying our chosen threshold criterion, occurring both in the hard and the soft states. We analyse two of them here in detail. The strongest one took place in the soft state, with the 3-30 keV energy flux increasing 30 times with respect to the preceding 16-s average. The e-folding time is similar to7 ms for the main flare and similar to1 ms for its precursor. The spectrum strongly hardens during the flare. On the other hand, flares in the hard state have generally lower amplitudes and longer e-folding times, and their spectra soften during the flare, with the hardness of the spectrum at the flare peak similar for both types of the flares. The presence of the flares shows unusually dramatic events taking place in the accretion flow of Cyg X-1. The rate of occurrence of hard-state flares shows they may represent a high-flux end of the distribution of shots present in usual light curves of Cyg X-1.