Coronal origin of the polarization of the high-energy emission of Cygnus X-1
Abstract
Context. Cygnus X-1 is the candidate with the highest probability of
containing a black hole among the X-ray binary systems in the Galaxy. It is also by far
the most often studied of these objects. Recently, the International Gamma-Ray
Astrophysics Laboratory Imager on board the Integral satellite (INTEGRAL/IBIS) detected
strong polarization in the high-energy radiation of this source, between 400 keV and 2
MeV. This radiation has been attributed to a jet launched by the black hole.Aims. We consider whether the corona around the black hole might be the
site of production of the polarized emission instead of the jet. Methods. We studied self-consistently the injection of nonthermal
particles in the hot, magnetized plasma around the black hole.Results. We show that both the high-energy spectrum and polarization of
Cygnus X-1 in the low-hard state can originate in the corona, without needing to invoke a
jet. We estimate the degree of polarization in the intermediate state, where there is no
jet, to provide a tool to test our model.Conclusions. Contrary to the commonly accepted view, the jet might not
be the source of the MeV polarized tail in the spectrum of Cygnus X-1.
Origin : Publication funded by an institution
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