Light on the corona of supermassive black holes
For the first time, researchers have direct evidence that the millimetre light emitted by a distant quasar comes from a compact area called "corona". This unprecedented result was made possible by the phenomenon of gravitational lensing, which acted as a magnifying glass, correcting the myopia of telescopes and allowing for a more detailed examination of the heart of a quasar.
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or decades, astrophysicists have wondered about the origin of the light emitted in the millimetre wavelength range (electromagnetic waves with a wavelength between 1 and 10 millimetres) by "radio-quiet" quasars*, active galactic nuclei that do not produce powerful radio jets. Several scenarios have been considered, such as star formation, heated dust, small jets, or the emission of hot plasma surrounding the black hole, known as the corona.
"Distinguishing between these hypotheses requires probing extremely compact regions, much smaller than allowed by current telescopes," explains Dominique Sluse, astrophysicist at ULiège. This is where a natural phenomenon comes into play: gravitational microlensing**. When light from a distant quasar passes through an intervening galaxy, the stars in that galaxy act like tiny lenses, amplifying certain details that would otherwise be invisible." To test their hypothesis, the team, coordinated by Matus Rybak, an astrophysicist at Leiden University, used the ALMA (Atacama Large Millimetre/submillimetre Array) telescope located at ESO in Chile. By analysing observations of the quasar RXJ1131-1231 (located nearly seven billion light-years from Earth) in 2015 and 2020, the researchers were able to detect strong variations in the relative brightness of its multiple images created by gravitational lensing.
"These variations can only be explained if the millimetre emission comes from an extremely compact region, less than 50 astronomical units in size (a few times the size of our solar system). This size rules out dust or star formation and clearly points to the black hole's corona as the source of this light." To confirm this interpretation, the team compared the millimetre emission with the X-ray emission, also produced by the corona. The results fit the model perfectly, reinforcing the conclusion that the corona dominates the millimetre emission in this quasar.
This study provides the first direct geometric evidence of coronal emission in a distant quasar. It also demonstrates that the microlensing effect, combined with the power of ALMA, is a unique tool for exploring the environment of supermassive black holes. Although this discovery has no everyday application, it marks a key milestone for fundamental physics and our understanding of the growth and activity of black holes at the heart of galaxies.
*A quasar is a very bright galactic nucleus powered by a supermassive black hole that swallows matter. As it falls towards the black hole, this matter heats up and shines brightly, sometimes more than the entire galaxy that hosts it. There are two main families of quasars: radio-loud quasars, which emit powerful jets of particles that shine intensely in radio waves, and radio-quiet quasars, which also shine brightly but without large radio jets. Their radio emission is very weak or even absent. The majority of known quasars are radio-quiet (about 90%).
** Gravitational lensing: an astrophysical phenomenon whereby the gravity of a massive object (galaxy, galaxy cluster, star) bends and deflects the light from an object behind it. This "cosmic mirage" can produce multiple images of the same source, luminous arcs or an amplification of its brightness, acting like a natural magnifying glass in the Universe.
Scientific reference
Rybak, D. Sluse, K. K. Gupta, M. Millon, E. Behar, F. Courbin, J. P. McKean and H. R. Stacey, Detection of millimetre-wave coronal emission in a quasar at cosmological distance using microlensing, Astronomy & Astrophysics, 701, A215 (2025). doi.org/10.1051/0004-6361/202554595
