Bizarre Hard X-Ray Outbursts Of Cygnus X-1

Abstract:  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 (CGRO). 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. Subject headings: accretion, accretion disks -- gamma-rays: observations -- stars: individual (Cygnus X-1) 1. INTRODUCTION Cyg X-1 is a persistent X-ray source believed to be powered by accretion onto a black hole from a massive companion. Most of the time Cyg X-1 spends in the hard state and sometimes it switches to the soft state (see, e.g., Liang & Nolan ...