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Black-hole binary (BHB) systems comprise a stellar-mass black hole and aclosely orbiting companion star. Matter is transferred from the companion tothe black hole, forming an accretion disk, corona and jet structures. Theresulting release of gravitational energy leads to emission of X-rays. Theradiation is affected by special/general relativistic effects, and can serve asa probe of the properties of the black hole and surrounding environment, if theaccretion geometry is properly identified. Two competing models describe thedisk-corona geometry for the hard spectral state of BHBs, based on spectral andtiming measurements. Measuring the polarization of hard X-rays reflected fromthe disk allows the geometry to be determined. The extent of the corona differsbetween the two models, affecting the strength of relativistic effects (e.g.,enhancement of polarization fraction and rotation of polarization angle). Here,we report observational results on linear polarization of hard X-ray (19-181keV) emission from a BHB, Cygnus X-1, in the hard state. The low polarizationfraction, <8.6% (upper limit at 90% confidence level), and the alignment of thepolarization angle with the jet axis show that the dominant emission is notinfluenced by strong gravity. When considered together with existing spectraland timing data, our result reveals that the accretion corona is either anextended structure, or is located far from the black hole in the hard state ofCygnus X-1.

Accretion geometry of the black-hole binary Cygnus X-1 from X-ray polarimetry