Relativistic distortions in the X-ray spectrum of Cyg X-1

We present the first significant detection of relativistic smearing of the X-ray reflection spectrum from the putative accretion disc in the low/hard state of Cyg X-1. The ionization state and the amount of relativistic smearing are simultaneously constrained by the X-ray spectra, and we conclude that the disc is not strongly ionized, does not generally extend down to the last stable orbit at 3 Schwarzschild radii, and covers rather less than half the sky as seen from the X-ray source. These results are consistent with a geometry where the optically thick disc truncates at a few tens of Schwarzschild radii, with the inner region occupied by the X-ray-hot, optically thin(nish) plasma. Such a geometry is also inferred from previous studies of the reflected spectrum in Galactic black hole transient sources, and from detailed considerations of the overall continuum spectral shape, suggesting that this is a robust feature of low/hard state accretion on to Galactic black holes.