Spectroscopic Implications of Thermal Instability in X-Ray Photoionized Accretion Flows

Details Spectroscopic Implications of Thermal Instability in X-Ray Photoionized Accretion Flows Spectroscopic Implications of Thermal Instability in X-Ray Photoionized Accretion Flows

Jan 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995phdt........18h&link_type=abstract

Thesis (PH.D.)--UNIVERSITY OF CALIFORNIA, BERKELEY, 1995.Source: Dissertation Abstracts International, Volume: 56-10, Section: B

Physics

1

X-Ray Binary, Seyfert Galaxy

Scientific paper

This work explores the implications of an interesting discrepancy between the discrete soft X-ray spectra of some accretion-powered systems and models which simulate the line-emitting accreting gas. In particular, I found that the iron-L ions which produce strong 3s to 2p transitions in the spectra of some low-mass X-ray binaries and Seyfert galaxies should not exist, because they peak in abundance at temperatures where the gas is thermally unstable. I have attempted to reconcile the modeling with the observations by carrying out a detailed study of thermal instability and the assumptions incorporated into the plasma models. In simulations of a thin conduction front bounding two stable gas phases in a multi-phase equilibrium, I found that such a front is unable to reproduce the iron-L emission features with sufficient strength to explain the observations. A careful examination of the mechanism which controls thermal instability followed. I found that the thermal instability is robust to changes in the conditions of a model gas simulating a typical X-ray binary or Seyfert galaxy accretion flow. The instability is extremely sensitive to the abundances of ions which strongly heat and cool the gas, and I found that the onset of instability is controlled by the heating and cooling behavior of iron itself. This rather surprising result underscores the importance of accounting for thermal instability in spectral interpretation and provides a new mechanism for measuring elemental abundances. Because of the importance of iron-L as a contributor to the spectra of low-mass X-ray binaries and Seyfert galaxies, and as the plasma constituent exerting the greatest control over thermal instability, I examined the iron-L atomic rate calculations in the photoionization code used to model the line-emitting gas. Changes implemented where they were most needed, in the calculations of dielectronic recombination rates, produced only minute effects upon thermal stability properties of the model gas.

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