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

TL;DR

This paper investigates the size constraints of X-ray sources in AGN lamppost models, showing that sources must be larger than the event horizon to produce observed spectra, challenging the assumption of point-like sources near black holes.

Contribution

The study uses relativistic ray tracing to demonstrate that X-ray sources in AGN cannot be infinitesimally small and must be larger than 1Rg, refining the lamppost model assumptions.

Findings

Sources must be larger than 1Rg to produce observed spectra.

Pure strong gravity effects cannot explain the spectra without considering source size.

The geometry of the corona and accretion flow influences observed X-ray features.

Abstract

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 1Rg above the event horizon in this geometry. However, the source must also be large enough to intercept sufficient seed photons from the disk 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 1Rg in 1H0707-495 if the disk is the source of seed photons. Hence the source cannot fit as close as 1Rg 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.