Detection of millimetre-wave coronal emission in a quasar at cosmological distance using microlensing

¹ Leiden Observatory, Leiden University, P.O. Box 9513 2300 RA Leiden, The Netherlands
² Faculty of Electrical Engineering, Mathematics and Computer Science, Delft University of Technology, Mekelweg 4, 2628 CD Delft, The Netherlands
³ SRON – Netherlands Institute for Space Research, Niels Bohrweg 4, 2333 CA Leiden, The Netherlands
⁴ STAR Institute, University of Liège, Quartier Agora, Allée du Six Aout 19c, 4000 Liège, Belgium
⁵ Sterrenkundig Observatorium, Universiteit Gent, Krijgslaan 281 S9, B-9000 Gent, Belgium
⁶ Kavli Institute for Particle Astrophysics and Cosmology and Department of Physics, Stanford University, Stanford, CA 94305, USA
⁷ Institute for Particle Physics and Astrophysics, ETH Zurich, Wolfgang-Pauli-Strasse 27, CH-8093 Zurich, Switzerland
⁸ Physics Department, Technion, Haifa 32000, Israel
⁹ ICC, University of Barcelona, Marti i Franques 1, 08028 Barcelona, Spain
¹⁰ ICREA, Pg. Lluis Companys 23, Barcelona 08010, Spain
¹¹ Kapteyn Astronomical Institute, University of Groningen, Postbus 800, NL-9700 AV Groningen, The Netherlands
¹² South African Radio Astronomy Observatory (SARAO), P.O. Box 443 Krugersdorp 1740, South Africa
¹³ Department of Physics, University of Pretoria, Lynnwood Road, Hatfield, Pretoria 0083, South Africa
¹⁴ European Southern Observatory, Karl-Schwarzschild-Straße 2, D-85748 Garching bei München, Germany

^⋆ Corresponding author: mrybak@strw.leidenuniv.nl

Received: 17 March 2025
Accepted: 31 July 2025

Abstract

Determining the nature of emission processes at the heart of quasars is critical for understanding environments of supermassive black holes. One of the key open questions is the origin of centimetre- to millimetre-wave emission from radio-quiet quasars. The proposed mechanisms range from central star formation to dusty torus, low-power jets, or emission from the accretion-disc corona. Distinguishing between these scenarios requires probing spatial scales of ≤0.01 pc, beyond the reach of any current millimetre-wave telescope. Fortunately, in gravitationally lensed quasars, compact millimetre-wave emission might be microlensed by stars in the foreground galaxy, providing strong constraints on the source size. We report a striking change in rest-frame 1.3 mm flux ratios in RXJ1131−1231, a quadruply lensed quasar at z = 0.658 observed by the Atacama Large Millimeter/submillimeter Array (ALMA) in 2015 and 2020. Over this period, the flux ratios between the three quasar images, A, B, and C, changed by a factor of 1.6 (A/B) and 3.0 (A/C). The observed flux-ratio variability is consistent with the microlensing of a compact source with a half-light radius of ≤50 astronomical units. The compactness of the source leaves coronal emission as the most likely scenario. Furthermore, the inferred millimetre-wave and X-ray luminosities follow the Güdel-Benz relationship for stellar coronae. These observations represent the first unambiguous evidence that coronae are the dominant mechanism for centimetre- to millimetre-wave emission in radio-quiet quasars.