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Science. 2011 Apr 22;332(6028):438-9. doi: 10.1126/science.1200848. Epub 2011 Mar 24.

Polarized gamma-ray emission from the galactic black hole Cygnus X-1.

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Astroparticules et Cosmologie (APC), Commissariat à l'Energie Atomique et aux Énergies Alternatives, Institut de Recherche sur les Lois Fondamentales de l'Univers (CEA/IRFU), 10, rue Alice Domon et Léonie Duquet, 75205, Paris Cedex 13, France.


Because of their inherently high flux allowing the detection of clear signals, black hole x-ray binaries are interesting candidates for polarization studies, even if no polarization signals have been observed from them before. Such measurements would provide further detailed insight into these sources' emission mechanisms. We measured the polarization of the gamma-ray emission from the black hole binary system Cygnus X-1 with the International Gamma-Ray Astrophysics Laboratory Imager on Board the Integral Satellite (INTEGRAL/IBIS) telescope. Spectral modeling of the data reveals two emission mechanisms: The 250- to 400-keV (kilo-electron volt) data are consistent with emission dominated by Compton scattering on thermal electrons and are weakly polarized. The second spectral component seen in the 400-keV to 2-MeV band is by contrast strongly polarized, revealing that the MeV emission is probably related to the jet first detected in the radio band.

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