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Title:
Soft X-ray emission lines from photoionized accretion discs: constraints on their strength and width
Authors:
Ballantyne, D. R.; Ross, R. R.; Fabian, A. C.
Affiliation:
AA(Institute of Astronomy, Madingley Road, Cambridge CB3 0HA), AB(Physics Department, College of the Holy Cross, Worcester, MA 01610, USA), AC(Institute of Astronomy, Madingley Road, Cambridge CB3 0HA)
Publication:
Monthly Notice of the Royal Astronomical Society, Volume 336, Issue 3, pp. 867-872. (MNRAS Homepage)
Publication Date:
11/2002
Origin:
MNRAS
MNRAS Keywords:
line: formation, radiative transfer, galaxies: active, galaxies: individual: MCG-6-30-15, X-rays: galaxies, X-rays: general
Abstract Copyright:
(c) RAS
DOI:
10.1046/j.1365-8711.2002.05818.x
Bibliographic Code:
2002MNRAS.336..867B

Abstract

We consider the properties of soft X-ray emission lines in the reprocessed emission from a photoionized accretion disc. Observations of these lines will be important in determining the ionization state and metallicity of the innermost regions of active galaxies. Calculations of reflection from a constant-density disc with an ionization parameter, ξ, between 250 and 1000 erg cm s-1 show that emission from OVIII Lyα will dominate the soft X-ray emission spectrum. There is also significant emission from CVI, NVII, OVII and FeXVII-XIX. As ξ is increased these lines become weaker and are broadened by Compton scattering. Significantly increasing the O abundance primarily strengthens the OVII line, making it as large or larger than the OVIII line. The nitrogen and carbon lines are quite weak with equivalent widths (EWs) <30 eV, even with an increase in the N abundance. A hydrostatic ionized disc model has a more realistic density structure and shows a similar spectrum, but with the lines weaker and broader. This is a result of the hot ionized skin at the surface of the disc. We apply these results to the controversial claim of soft X-ray relativistic lines in the XMM-Newton spectrum of MCG-6-30-15. We are unable to find a situation where OVIII has the required EW without substantial emission from OVII. Furthermore, Compton scattering results in the blue wing of the OVIII line being much broader than the <<10 eV drop observed in the data. We conclude that soft X-ray accretion disc lines will, in general, be weak and broad features and are unlikely to produce sharp edges in the data.

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