Astronomy Colloquium

The properties and origin of the powerful, hot, X-ray emitting corona at the heart of most AGN has remained an enigma for decades. NuSTAR has now measured the spectrum of a good number of AGN, revealing temperatures of 30-100 keV with a tail to higher values. I discuss how X-ray reflection and reverberation measurements with NuSTAR and XMM have pinned down the size of coronae and  revealed them to be radiatively compact.   Pair production within the corona provides an upper temperature bound and, if the plasma is a hybrid mix of thermal gas with a small nonthermal component, then it is possible for pairs to act as a thermostat determining the coronal temperature, as suggested back in the 1980s.