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 between 250 and 1000 erg cm/s show that emission from O VIII Ly\alpha will dominate the soft X-ray emission spectrum. There is also significant emission from C VI, N VII, O VII, as well as Fe XVII--XIX. As the ionization parameter is increased these lines become weaker and are broadened by Compton scattering. Significantly increasing the O abundance primarily strengthens the O VII line, making it as large or larger than the O VIII 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 O VIII has the required EW without substantial emission from O VII. Furthermore, Compton scattering results in the blue wing of the O VIII line to be 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. 
D.R. Ballantyne (1), R.R. Ross (2), A.C. Fabian (1) ((1) IoA,   Cambridge, (2) College of the Holy Cross)