2005ApJ...625..931J

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 [n_e=(2±1)x10¹³ cm from Mg XI], temperature (kT=7±3 eV from Ne IX), spatial distribution, elemental composition, and kinematics (Δv≲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 8x10¹⁰cm≲r≲1x10¹¹ 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 to enhance N VII emission via a line overlap with O VIII. The line optical depth diagnostics are consistent with a flattened atmosphere. Our spectral analysis, the disk atmosphere model, and the presence of intense N VII and N VI lines (plus N V in the UV) confirm the overabundance of nitrogen relative to other metals, which was shown to be indicative of CNO cycle processing in a massive progenitor. The spectral signatures of a thermal instability in the photoionized plasma are not evident, but the measured density is in the stable regime of the models.