Vol. 554
In section 2. Astrophysical processes

Long term variability of Cygnus X-1. V. State definitions with all sky monitors

by V. Grinberg, N. Hell, K. Pottschmidt, et al., A&A 554, A88

The present "Golden Age" of multiwavelength all-sky monitoring satellites is exploited by the authors to define an empirical set of X-ray activity states for the prototypical black-hole, high-mass X-ray binary Cygnus X-1. They distinguish between hard, intermediate, and soft - depending on the spectral index in the 1 to 50 keV range - with transition and duration probabilities derived using nearly 16 years of X-ray observations. The two extremes show most probable durations longer than one week. The typical duration of the intermediate state, in contrast, is only a few days or less. This presents an interesting renewal problem, with a rapid, telegraph-like transition between the extreme spectral states and lag times between bands that depend on the spectral index in a nonlinear way. This paper will provide the basis for all future comparisons of the variations at high energy and, more important, for comparisons with any future changes in long-term behavior.