Bizarre Hard X-ray Outbursts of Cygnus X-1
  A very high activity of Cygnus X-1 on 1999 April 19-21 was recorded by BATSE Large Area Detectors onboard the Compton Gamma-Ray Observatory. The peak luminosity was one 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, and the other one is hard (extending above 100 keV), with much slower variability. 
Boris E. Stern, Andrei M. Beloborodov, and Juri Poutanen (Stockholm   Observatory)