Astronomy and Astrophysics – Astronomy
Scientific paper
Dec 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993aas...183.6502h&link_type=abstract
American Astronomical Society, 183rd AAS Meeting, #65.02; Bulletin of the American Astronomical Society, Vol. 25, p.1391
Astronomy and Astrophysics
Astronomy
Scientific paper
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 thermal instability has to be taken into account.
Hess Cynthia Jean
Kahn Steve M.
Paerels Frits B.
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