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Register a new userHeating in an Extended Accretion Disk Corona along the Z-Pattern in Cyg X-2
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We observed at very high spectral resolution the prototype Z-source Cyg x-2 twice along its entire X-ray spectral variation pattern. In this preliminary analysis we find an extended accretion disk corona exhibiting Lyman alpha emissions from various H-like ions, as well as emissions from He-like ions of Fe and Al, and Li-like ions of Fe. The brightest lines show a range of line broadening: H-like lines are very broad with Doppler velocities between 1100 and 2700 km/s, while some others are narrower with widths of a few hundred km/s. Line diagnostics allow us for the first time to determine coronal parameters. The line properties are consistent with a stationary, extended up to 10^10 cm, dense (1x10^15 cm^-3), and hot (log xi > 3; T > 10^6 K) accretion disk corona. We find ongoing heating of the corona along the Z-track and determine that heating luminosities change from about 0.4 L_Edd on the horizontal to about 1.4 L_Edd on the flaring branch.
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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 A to N VI at 29.08 A) 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, temperature, spatial distribution, elemental composition, and kinematics 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, disk atmosphere area, elemental composition, and energetics. 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 Ly-alpha and Ly-beta line photons from H-like ions. We derive N < 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.
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