Chandra/hetgs Spectroscopy of the Galactic Black Hole Gx 339−4: a Relativistic Iron Emission Line and Evidence for a Seyfert-like Warm Absorber

We observed the Galactic black hole GX 339−4 with the Chandra High Energy Transmission Grating Spectrometer (HETGS) for 75 ksec during the decline of its 2002–2003 outburst. The sensitivity of this observation provides an unprecedented glimpse of a Galactic black hole at about a tenth of the luminosity of the outburst peak. The continuum spectrum is well described by a model consisting of multicolor disk blackbody (kT ≃ 0.6 keV) and power-law (Γ ≃ 2.5) components. X-ray reflection models yield improved fits. A strong, relativistic Fe Kα emission line is revealed, indicating that the inner disk extends to the innermost stable circular orbit. The line is not sufficiently broad to strongly require black hole spin. Absorption lines from H-like and He-like O, and He-like Ne and Mg are detected, as well as lines which are likely due to Ne II and Ne III. The measured line properties make it difficult to associate the absorption with the coronal phase of the interstellar medium. A scenario wherein the absorption lines are due to a local AGN-like warm-absorber geometry — perhaps produced by a disk wind in an extended disk-dominated state — may be more viable. We compare our results to Chandra observations of the Galactic black hole candidate XTE J1650−500, and discuss our findings in terms of prominent models for Galactic black hole accretion flows and connections to supermassive black holes.