Cygnus X-1: shedding light on the spectral variability of the hard state of black holes

Abstract

We present an analysis of extensive recent monitoring observations of the black hole X-ray binary Cygnus X-1 obtained as part of the 2007 to 2010 Cygnus Region Key Programme observations of the INTEGRAL mission. Cyg X-1 is one of only three persistent black hole binaries in our galaxy that spend most of their time in the hard spectral state. We concentrate on constraining the parameter range of the hard spectrum, a measurement that is typically difficult to obtain with high accuracy for transient sources, but which is important to know in order to understand the physics of the hot plasma of the jet base and/or the corona. While the hard X-ray spectrum of Cyg X-1 is one of the best studied examples of its kind, e.g., through our years long monitoring campaign with RXTE, the INTEGRAL monitoring allows us to study the spectral evolution from about half an hour over a few days to a few weeks, timescales that have been in part only sparsely sampled so far. After spending ∼ 3 years in the hardest regime of its parameter space, the source displayed a softening and flaring episode in mid 2009 and entered a soft state in early 2010 June. We compare X-ray broad band spectra (RXTE, INTEGRAL) of these two emission states. Furthermore, we use INTEGRAL/IBIS to extend the timing analysis with a resolution of up to 0.1 s to energies above 20 keV.