Interpreting Soft X-ray Spectra of Accretion-Powered Sources by Accounting for Thermal Instability

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Session 65 -- Interacting Binaries: Compact Stars
Oral presentation, Thursday, January 13, 2:15-3:45, Salon V Room (Crystal Gateway)

[65.02] Interpreting Soft X-ray Spectra of Accretion-Powered Sources by Accounting for Thermal Instability

C. J. Hess, S. M. Kahn, F. B. Paerels (Department of Physics and Space Sciences Laboratory, University of California, Berkeley)

Recent spectral observations of accretion-powered X-ray sources, primarily low-mass X-ray binaries and active galactic nuclei, show features in the soft X-ray band (0.5-1.5 keV) which are not predicted by standard nebular models. In particular, a photoionized plasma with cosmic abundances should show very strong H-like and He-like oxygen-K lines overpowering any iron-L features. In fact, several observations have confirmed that iron-L features occur in accretion-powered sources with no observable oxygen-K lines. Models of X-ray photoionized nebulae show that the thermal balance and therefore the spectrum of a line-emitting region are extremely sensitive to initial elemental abundances and the shape of the ionizing spectrum. We have found that for plausible values of incident spectrum energy, there are temperatures where a line-emitting region is thermally unstable. Because of the interdependence of thermal balance and ionization balance, the behavior of an instability varies strongly with changing abundances. We have determined which cooling sources drive the onset and termination of instability, and we can predict the thermal balance of a model photoionized plasma. We show that modifying the elemental composition of a line-emitting region could place the oxygen-K ions in a thermally unstable region and thus eliminate the oxygen-K lines. We also show that predictions about ion abundances in accretion-powered sources can be made from line spectra, but

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