Identification of an Extended Accretion Disk Corona in the Hercules X-1 Low State: Moderate Optical Depth, Precise Density Determination, and Verification of CNO Abundances

We identify an accretion disk atmosphere and corona from the high resolution X-ray spectrum of Hercules X-1, and we determine its detailed physical properties. More than two dozen recombination emission lines (from Fe XXVI at 1.78 Å to N VI at 29.08 Å) and Fe Kα,Kβ fluorescence lines were detected in a 50 ks observation with the Chandra High-Energy Transmission Grating Spectrometer (HETGS). They allow us to measure the density (ne = 2±1×10 13 cm from Mg XI), temperature (kT = 7± 3 eV from Ne IX), spatial distribution, elemental composition, and kinematics (∆v . 260 km s ) of the plasma. We exclude HZ Her as the source of the recombination emission. We compare accretion disk model atmospheres with the observed spectrum in order to constrain the stratification of density and ionization, elemental composition, energetics, and thermal stability. The derived disk atmosphere and corona radii are 8× 10 . r . 1× 10 cm, in agreement with previously measured eclipse ingress light curves. The atmospheric spectrum observed during the low state is photoionized by the main-on X-ray continuum, indicating that the disk is observed edge-on during the low state. We infer the mean number of scatterings 〈N〉 of Lyα and Lyβ line photons from H-like ions. We derive 〈N〉 . 69 for O VIII Lyα1, which rules out the presence of a mechanism modeled by Sako (2003) to enhance N VII emission via