Millimeter emission from supermassive black hole coronae

¹ Department of Space, Earth and Environment, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden
² Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 43992 Onsala, Sweden
³ Instituto de Estudios Astrofísicos, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Santiago, 8370191 Chile
⁴ Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing, 100871 China
⁵ Theoretical Physics IV: Plasma-Astroparticle Physics, Faculty for Physics & Astronomy, Ruhr University Bochum, 44780 Bochum, Germany
⁶ Ruhr Astroparticle And Plasma Physics Center (RAPP Center), Ruhr-Universität Bochum, 44780 Bochum, Germany
⁷ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA, 22903 USA
⁸ National Astronomical Observatory of Japan, 2-21-1, Osawa, Mitaka, Tokyo, 181-8588 Japan
⁹ Observatorio Astronómico Nacional (OAN-IGN)-Observatorio de Madrid, Alfonso XII, 3, 28014 Madrid, Spain
¹⁰ Observatoire de Paris, LUX, Collège de France, CNRS, PSL University, Sorbonne University, 75014 Paris, France
¹¹ Steward Observatory, University of Arizona, 933 N Cherry Avenue, Tucson, AZ, 85721 USA
¹² Jodrell Bank Centre for Astrophysics, The University of Manchester, M13 9PL, UK
¹³ Leiden Observatory, Leiden University, PO Box 9513 2300 RA Leiden, The Netherlands
¹⁴ George P. and Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy, Texas A&M University, College Station, TX, 77845 USA
¹⁵ CSIRO Space and Astronomy, ATNF, PO Box 1130 Bentley, WA, 6102 Australia
¹⁶ Institute of Astronomy, The University of Tokyo, 2-21-1, Osawa, Mitaka, Tokyo, 181-0015 Japan
¹⁷ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA, 22903 USA
¹⁸ Department of Astronomy, University of Virginia, 530 McCormick Road, Charlottesville, VA, 22903 USA
¹⁹ Centro de Astrobiología (CAB), CSIC-INTA, Camino Bajo del Castillo s/n, E-28692 Villanueva de la Cañada, Madrid, Spain
²⁰ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo, 181-8588 Japan
²¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn, Germany
²² Xinjiang Astronomical Observatory, Chinese Academy of Sciences, 830011 Urumqi, PR China
²³ Instituto de Física Fundamental, CSIC, Calle Serrano 123, E-28006 Madrid, Spain
²⁴ European Southern Observatory, Alonso de Córdova, 3107, Vitacura, Santiago, 763-0355 Chile
²⁵ Joint ALMA Observatory, Alonso de Córdova, 3107, Vitacura, Santiago, 763-0355 Chile
²⁶ Instituto de Física Fundamental, CSIC, Calle Serrano 123, 28006 Madrid, Spain
²⁷ Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA) Northwestern University, Evanston, IL, 60208 USA

^⋆ Corresponding author: santiago.delpalacio@chalmers.se

Received: 1 April 2025
Accepted: 26 June 2025

Abstract

Context. Active galactic nuclei (AGNs) host accreting supermassive black holes (SMBHs). The accretion process can lead to the formation of a hot, X-ray emitting corona close to the SMBH that can accelerate relativistic electrons. Observations in the millimeter band can probe its synchrotron emission.

Aims. We intend to provide a framework to derive physical information of SMBH coronae by modelling their spectral energy distribution (SED) from radio to far-infrared frequencies. We also explore the possibilities of deriving additional information from millimeter observations, such as the SMBH mass, and studying high-redshift lensed sources.

Methods. We introduce a corona emission model based on a one-zone spherical region with a hybrid thermal and non-thermal plasma. We investigated the dependence of the corona SED on different parameters such as size, opacity, and magnetic field strength. Other galactic emission components from dust, ionised gas, and diffuse relativistic electrons were also included in the SED fitting scheme. We applied our code consistently to a sample of radio-quiet AGNs with strong indications of a coronal component in the millimeter.

Results. The detected millimeter emission from SMBH coronae is consistent with a non-thermal relativistic particle population with an energy density that is ≈0.5–10% of that in the thermal plasma. This requires magnetic energy densities close to equipartition with the thermal gas and corona sizes of 60–250 gravitational radii. The model can also reproduce the observed correlation between millimeter emission and SMBH mass when we accounted for the uncertainties in the corona size.

Conclusions. The millimeter band offers a unique window into the physics of SMBH coronae, enabling the study of highly dust-obscured sources and high-redshift lensed quasars. Gaining a deeper understanding of the relativistic particle population in SMBH coronae can provide key insights into their potential multiwavelength and neutrino emission.