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We observed the Galactic black hole Cygnus X-1 with the Chandra High EnergyTransmission Grating Spectrometer for 30 kiloseconds on 4 January, 2001. Thesource was in an intermediate state, with a flux that was approximately twicethat commonly observed in its persistent low/hard state. Our best-fit model forthe X-ray spectrum includes narrow Gaussian emission line (E = 6.415 +/- 0.007keV, FWHM = 80 (+28, -19) eV, W = 16 (+3, -2) eV) and broad line (E = 5.82(+0.06, -0.07) keV, FWHM = 1.9 (+0.5, -0.3) keV, W = 140 (+70, -40) eV)components, and a smeared edge at 7.3 +/- 0.2 keV (tau ~ 1.0). The broad lineprofile is not as strongly skewed as those observed in some Seyfert galaxies.We interpret these features in terms of an accretion disk with irradiation ofthe inner disk producing a broad Fe K-alpha emission line and edge, andirradiation of the outer disk producing a narrow Fe K-alpha emission line. Thebroad line is likely shaped predominantly by Doppler shifts and gravitationaleffects, and to a lesser degree by Compton scattering due to reflection. Wediscuss the underlying continuum X-ray spectrum and these line features in thecontext of diagnosing the accretion flow geometry in Cygnus X-1 and otherGalactic black holes.Comment: Accepted for publication in Ap

Resolving the Composite Fe Kα Emission Line in the Galactic Black Hole Cygnus X‐1 withChandra