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The lamppost model is often used to describe the X-ray source geometry inAGN, where an infinitesimal point source is located on the black hole spinaxis. 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 bothbe explained by extremely strong relativistic effects as the source approachesthe black hole horizon. The most extreme spectrum seen from the NLS1 1H0707-495requires that the source is less than 1Rg above the event horizon in thisgeometry. However, the source must also be large enough to intercept sufficientseed photons from the disk to make the hard X-ray Compton continuum whichproduces the observed iron line/reflected spectrum. We use a fully relativisticray tracing code to show that this implies that the source must besubstantially larger than 1Rg in 1H0707-495 if the disk is the source of seedphotons. Hence the source cannot fit as close as 1Rg to the horizon, so theobserved spectrum and variability are not formed purely by effects of stronggravity but probably also by changes in corona and inner accretion flowgeometry.

Minimum X-ray source size for a lamp-post corona in light-bending models for AGN