Broad iron emission lines in Seyfert Galaxies - re-condensation of gas onto an inner disk below the ADAF

TL;DR

This paper explores whether broad iron emission lines in Seyfert 1 AGN originate from an inner cool disk re-condensed from an ADAF, similar to phenomena observed in stellar-mass black hole binaries, supported by accretion rate analysis.

Contribution

It demonstrates that many Seyfert AGN with broad iron lines can be explained by a re-condensation model involving an inner cool disk beneath an ADAF, based on accretion rate calculations.

Findings

Over half of the studied Seyfert galaxies have accretion rates below 0.1 Eddington, supporting the re-condensation model.

Some Seyfert galaxies with higher accretion rates may be in a 'very high' spectral state similar to LMXBs.

The study links observed iron lines to accretion flow geometry consistent with the re-condensation scenario.

Abstract

Recent observations of Seyfert 1 AGN with Chandra, XMM-Newton and Suzaku revealed broad iron K_alpha emission lines, some relativistically blurred. For galactic black hole X-ray binaries XMM-Newton spectra during hard state also reveal the presence of a relativistic iron emission line and a thermal component, interpreted as an indication for a weak inner cool accretion disk underneath a hot corona. These thermal components were found after the transition from soft to hard spectral state and can be understood as sustained by re-condensation of gas from an advection-dominated flow (ADAF) onto the disk. In view of the similarity of accretion flows around stellar mass and supermassive black holes we discuss whether the broad iron emission lines in Seyfert 1 AGN can be understood as arising from a similar accretion flow geometry. We derive Eddington-scaled accretion rates for Seyfert galaxies with strong lines in samples of Miller (2007) and Nandra et al. (2007). For the evaluation we use the observed X-ray luminosity, bolometric corrections and black hole masses from literature, most values taken from Fabian and Vasudevan (2009). Rates derived are less than 0.1 of the Eddington rate for more than half of the sources. For 10^7 to 10^8 solar mass black holes in Seyfert 1 AGN this limit corresponds to 0.01 to 0.2 solar masses per year. Our investigation shows that for quite a number of Seyfert AGN in hard spectral state iron emission lines can arise from an inner weak disk surrounded by an ADAF as predicted by the re-condensation model. Some of the remaining sources with higher accretion rates may be in a spectral state comparable to the "very high" state of LMXBs. (abridged)