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 K \alpha , K \beta 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 \pm 1 \times 10 ^ { 13 } cm from Mg xi ) , temperature ( kT = 7 \pm 3 eV from Ne ix ) , spatial distribution , elemental composition , and kinematics ( \Delta v \lesssim 260 { \thinspace km } { \thinspace s } ^ { -1 } ) 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 \times 10 ^ { 10 } \lesssim r \lesssim 1 \times 10 ^ { 11 } 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 \langle N \rangle of Ly \alpha and Ly \beta line photons from H-like ions .
We derive \langle N \rangle \lesssim 69 for O viii Ly \alpha _ { 1 } , which rules out the presence of a mechanism modeled by Sako ( 2003 ) 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 over-abundance 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 .