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

Understand

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.