Supermassive black holes don't give off any light themselves, but at those times in their existence when they are actively accreting material, they are encircled by a disk of hot, glowing material. The gravity of a black hole pulls swirling gas in, heating that material and causing it to shine at energies ranging from the ultraviolet to X-rays depending on the mass of the black hole. Another major source of radiation near a black hole is the so-called “corona”. Coronae are made up of highly energetic particles that generate X-ray light, though details about their geometry, location, appearance, and how they form remain uncertain and are driving questions for X-ray astrophysicists. In the artist’s concept shown here, the corona is envisioned as a hot spot of energetic plasma above (and below) the black hole, though other models suggest the corona is a hot atmosphere to the inner accretion disk.

In a new study submitted to the Astrophysical Journal, an international team led by Dr. Xiurui Zhao of the California Institute of Technology systematically studied the X-ray coronae in some of the most distant and luminous accreting supermassive black holes ever observed. These coronae, composed of ultra-hot electrons, produce high-energy X-rays by boosting lower-energy light emitted by the accretion disk. The expansion of the universe then shifts these high-energy emitted photons to lower observed energies, similar to the Doppler shift we hear as an ambulance or police car speeds away from us and the tone of the siren shifts to lower notes. By targeting luminous accreting supermassive black holes in the distant universe, known as quasars, the team leveraged this cosmological shift to bring key spectral features into NuSTAR’s energy range. This enabled direct measurements of the coronal properties which had been out of reach of previous X-ray satellites.

By combining NuSTAR observations with observations by the European Space Agency-led XMM-Newton satellite, which is sensitive to lower energy X-rays, the team constructed the most comprehensive view to date of coronae in active galaxies across cosmic time. The results provide new constraints on how energy is dissipated and radiated near supermassive black holes, offering critical tests for state-of-the-art theoretical models and simulations.

The study reveals that coronae in these powerful, distant quasars are significantly cooler than those found in the nearby universe, where quasars are typically less luminous. This result suggests that the most luminous black holes operate in a fundamentally different physical regime. The findings also indicate that these distant coronae are denser and more efficiently cooled, likely due to intense radiation fields near rapidly accreting black holes. Together, these results challenge standard models of black hole coronae and point toward a more complex interplay between heating, cooling, and particle acceleration in these extreme environments.

In a related study to be presented at the 248^th meeting of the American Astronomical Society later this month, Dr. Zhao and collaborators report on dramatic X-ray variability detected in the corona of the galaxy Mrk 509. Mrk 509 is a relatively nearby galaxy, just shy of 500 million light years away, that hosts an actively accreting supermassive black hole. This new work, relying on observations from the NuSTAR satellite, finds very strong variations in the temperature of the corona, at a level not previously found from either Mrk 509 or similar sources. Both the distant quasar survey and this detailed study of the nearby galaxy Mrk 509 have important implications for understanding the physical processes occurring very close to supermassive black holes.