2001ApJ...555..829S

Very high activity in Cygnus X-1 during 1999 April 19-21 was recorded by the BATSE Large Area Detectors on board the Compton Gamma Ray Observatory. The peak luminosity was an order of magnitude higher than the normal luminosity of Cyg X-1. This fact can be critical for models of the hard state of Cyg X-1. The longest outburst lasted ∼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 ∼100 keV) and highly variable, while the other is hard (extending above 100 keV) with much slower variability.