  Black-hole binary (BHB) systems comprise a stellar-mass black hole and a
closely orbiting companion star. Matter is transferred from the companion to
the black hole, forming an accretion disk, corona and jet structures. The
resulting release of gravitational energy leads to emission of X-rays. The
radiation is affected by special/general relativistic effects, and can serve as
a probe of the properties of the black hole and surrounding environment, if the
accretion geometry is properly identified. Two competing models describe the
disk-corona geometry for the hard spectral state of BHBs, based on spectral and
timing measurements. Measuring the polarization of hard X-rays reflected from
the disk allows the geometry to be determined. The extent of the corona differs
between 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 (19-181
keV) emission from a BHB, Cygnus X-1, in the hard state. The low polarization
fraction, <8.6% (upper limit at 90% confidence level), and the alignment of the
polarization angle with the jet axis show that the dominant emission is not
influenced by strong gravity. When considered together with existing spectral
and timing data, our result reveals that the accretion corona is either an
extended structure, or is located far from the black hole in the hard state of
Cygnus X-1.
