On the size and location of the X-ray emitting coronae around black holes

Abstract

The observation of energetic X-ray emission from black holes, inconsistent with thermal emission from an accretion disk, has long indicated the presence of a "corona" around these objects. However, our knowledge of the geometry, composition, and processes within black hole coronae is severely lacking. Basic questions regarding their size and location are still a topic of debate. In this letter, we show that for black holes with luminosities $L\gtrsim10^{-2}L_{Edd}$ -- characteristic of many Seyferts, quasars, and stellar-mass black holes (in their brighter states) -- advanced imaging and timing data strongly favor X-ray emitting regions that are highly compact, and only a few Gravitational radii above the accretion disk. The inclusion of a large number of possible systematics uncertainties does not significantly change this conclusion with our results still suggesting emission from within $\sim20$\rg\ in all cases. This result favors coronal models wherein most of the hard X-ray emission derives from magnetic reconnection in the innermost disk and/or from processes in the compact base of a central, relativistic jet.