Polarized high-energy emission from the black hole Cygnus X-1
Cygnus X-1 is an X-ray binary composed of a 35 Msun supergiant O star
and a ~10 Msun black hole accreting matter which is either directly
ejected from the companion star, or via an accretion disc. It is
one of the first X-ray binaries discovered in our Galaxy soon after
the advent of X-ray astronomy in the 1960's. Cygnus X-1 is also the
first Galactic source for which optical measurements (in the early
1970's) showed the presence of a black hole.
Radio observations have shown that Cygnus X-1 is a source of powerful
ejections, and is therefore a so-called "micro-quasar", a Galactic
scaled- down version of the very massive black holes powering the
nuclei of active Galaxies. These jets are very important for the
understanding of the accretion-ejection connections, that is the
physical mechanisms at work close to the black hole, since they can
carry large quantities of matter at speeds close to the speed of light.
An international team, led by Philippe Laurent, has very recently,
shown that the jets in Cygnus X-1 could also be responsible of the
most energetic light emitted near the black hole. Using the INTEGRAL
data archive, and using the Compton interactions between the two
detector arrays of IBIS (ISGRI+PICSiT) they were able to measure
polarized gamma-rays emitted by Cygnus X-1. In parallel, by combining
the degree of polarization in different energy bands and the energy
distribution of gamma-ray photons between 250 and 2000 keV, Laurent and
collaborators could attribute the polarization to the "hard tail" emission
visible in the energy spectrum at energies above 400 keV. This allowed
the scientists to attribute this component, whose origin is highly debated,
to the emission from the jet. On the other hand, no polarization was found
for the first component (due to Comptonisation in the accretion flow), at
lower energies, which is consistent with the expectations.
Reference and more details:
"Polarized Gamma-ray Emission from the Galactic Black Hole Cygnus X-1",
P. Laurent, J. Rodriguez, J. Wilms, M. Cadolle Bel, K. Pottschmidt and V. Grinberg
published online in Science Express, 24 March 2011. DOI: 10.1126/science.1200848
