Cygnus X-1: shedding light on the spectral variability and polarization of black holes
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Date
2011-06-30
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Abstract
We present an analysis of extensive recent monitoring observations
of the black hole HMXB Cygnus X-1 obtained as part of the 2007
to 2011 Key Programme (KP) 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.
After spending 3 years in the hardest regime of its parameter space,
the source displayed a softening and flaring episode in mid 2009, entered a soft state in early 2010 June and entered a transitional phase
in April 2011. While the hard X-ray spectrum of Cyg X-1 is one of
the best studied examples of its kind, e.g. through our monitoring campaign with RXTE and coordinated radio observations with
AMI (formerly: Ryle) telescope, the INTEGRAL monitoring allows
us to study the spectral evolution from about half an hour over a
few days to weeks, timescales that have been only sparsely sampled
so far. The parameter ranges for the hard and soft states as well as
the transitional phases are constrained. These measurements are of
special importance for understanding the physics of the hot plasma
of the jet base and/or the corona, but typically difficult to obtain
for transient source. Additionally, we report on polarization studies
which have been conducted using a larger INTEGRAL/IBIS dataset
and led to the discovery of gamma-ray polarization in Cyg X-1.