The nature of the coronae of accreting black holes
Accreting black holes are powerful sources of radiation. The conversion of gravitational energy into radiation is thought to take place in an accretion disk-corona system just outside the black hole. In this system, thermal accretion disk photons are upscattered in a corona of hot electrons situated above the accretion disk, producing X-rays. The nature of this Comptonizing corona remains a key open question in astrophysics. I will discuss recent INTEGRAL and NuSTAR results on galactic black hole binary and AGN coronae.