Bizarre hard X-ray outbursts of Cygnus X-1

Very high activity in Cygnus X-1 during 1999 April 19-21 was recorded by th e BATSE Large Area Detectors on board the Compton Gamma Ray Observatory. Th e peak luminosity was an order of magnitude higher than the normal luminosi ty of Cyg X-1. This fact can be critical for models of the hard state of Cy g X-1. The longest outburst lasted similar to 1000 s and demonstrated very unusual temporal and spectral behavior, which indicates the presence of two emission components. One component is relatively soft (with a cutoff below similar to 100 keV) and highly variable, while the other is hard (extendin g above 100 keV) with much slower variability.