A Chandra HETGS Spectral Study of the Iron K Bandpass in MCG –6-30-15: A Narrow View of the Broad Iron Line

We present a high-resolution X-ray spectrum of the iron K bandpass in MCG -6-30-15 based on a 522 ks observation with Chandra's High Energy Transmission Grating Spectrometer (HETGS). The Chandra spectrum is consistent with the presence of a relativistically broadened, highly redshifted iron Kα emission line with a profile similar to previous observations. A number of narrow features are detected above 2 keV, including a narrow Fe Kα emission line and narrow absorption lines from H- and He-like Fe, H-like S, and H-like Si. This absorption is well described by a photoionized plasma with a column density log NH = 23.2 and an ionization parameter log ξ = 3.6, assuming the iron abundance has the solar value and a velocity dispersion parameter b = 100 km s-1. Applying this absorption model to a high-fidelity XMM-Newton EPIC pn spectrum, we find that a broad iron line is still required with emission extending to within 1.9rg of the black hole. If the iron line comes from an accretion disk truncated at the innermost stable circular orbit, this indicates that the black hole must be spinning rapidly with a > 0.95. Ionized absorption models attempting to explain the 3-6 keV spectral curvature without strong gravity predict absorption lines in the 6.4-6.6 keV range that are inconsistent with the Chandra spectrum. The H- and He-like iron absorption lines in the Chandra spectrum are blueshifted by 2.0 × 103 km s-1 compared to the source frame and may originate in a high-velocity, high-ionization component of the warm absorber outflow. This high-ionization component may dominate the energy budget of the outflow and account for a significant fraction of the outflowing mass. Detailed modeling of the warm absorber below 2 keV will be addressed in a later paper, but our results are robust to the broader details of the warm absorber behavior. The difference spectrum between the high- and low-flux states is well described by a power law, in agreement with previous studies.