Resolving the composite Fe Kα emission line in the Galactic black hole Cygnus X-1 with Chandra

We observed the Galactic black hole Cyg X-1 with the Chandra High Energy Transmission Grating Spectrometer for 30 ks on 2001 January 4. The source was in an intermediate state, with a flux that was approximately twice that commonly observed in its persistent low/hard state. Our best-fit model for the X-ray spectrum includes narrow Gaussian emission line (E = 6.415 +/- 0.007 keV, FWHM = 80(-19)(+28) eV, W = 16(-2)(+3) eV) and broad-line (E = 5.82(-0.07)(+0.06) keV, FWHM = 1.9(-0.3)(+0.5) keV, W = 140(-40)(+70) eV) components, and a smeared edge at 7.3 +/- 0.2 keV (tau similar to 1.0). The broad-line profile is not as strongly skewed as those observed in some Seyfert galaxies. We interpret these features in terms of an accretion disk with irradiation of the inner disk producing a broad Fe Kalpha emission line and edge, and irradiation of the outer disk producing a narrow Fe Kalpha emission line. The broad line is likely shaped predominantly by Doppler shifts and gravitational effects, and to a lesser degree by Compton scattering due to reflection. We discuss the underlying continuum X-ray spectrum and these line features in the context of diagnosing the accretion flow geometry in Cyg X-1 and other Galactic black holes.