Observations of Giant Outbursts from Cygnus X-1

Abstract

We present interplanetary network localization, spectral, and time history information for seven episodes of exceptionally intense gamma-ray emission from Cyg X-1. The outbursts occurred between 1995 and 2003, with durations up to ~28,000 s. The observed 15-300 keV peak fluxes and fluences reached 3×10^-7 ergs cm^-2 s^-1 and 8×10^-4 ergs cm^-2, respectively. By combining the triangulations of these outbursts we derive an ~1700 square arcminute (3 σ) error ellipse that contains Cyg X-1 and no other known high-energy sources. The outbursts reported here occurred both when Cyg X-1 was in the hard state as well as in the soft one and at various orbital phases. The spectral data indicate that these outbursts display the same parameters as those of the underlying hard and soft states, suggesting that they represent another manifestation of these states.