Soft X-ray emission lines from photoionized accretion discs: constraints on their strength and width

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.