Minimum X-ray source size of the on-axis corona in AGN

The lamppost model is often used to describe the X-ray source geometry in AGN, where an infinitesimal point source is located on the black hole spin axis. This is especially invoked for narrow line Seyfert 1 (NLS1) galaxies, where an extremely broad iron line seen in episodes of low X-ray flux can both be explained by extremely strong relativistic effects as the source approaches the black hole horizon. The most extreme spectrum seen from the NLS1 1H0707-495 requires that the source is less than 1GM/c(2) above the event horizon in this geometry. However, the source must also be large enough to intercept sufficient seed photons from the disc to make the hard X-ray Compton continuum which produces the observed iron line/reflected spectrum. We use a fully relativistic ray tracing code to show that this implies that the source must be substantially larger than 1GM/c(2) in 1H0707-495 if the disc is the source of seed photons. Hence the source cannot fit as close as 1GM/c(2) to the horizon, so the observed spectrum and variability are not formed purely by effects of strong gravity but probably also by changes in corona and inner accretion flow geometry. (C) 2016 WILEY-VCH Verlag GmbH&Co. KGaA, Weinheim