Millimeter emission from supermassive black hole coronae
In one sentence
Compact millimetre emission from the immediate surroundings of supermassive black holes is consistent with synchrotron radiation from a non-thermal relativistic particle population in their hot coronae — opening a new observational window onto the central engine.
What’s the question?
X-ray observations of active galactic nuclei have long shown evidence for a hot, magnetised “corona” around the supermassive black hole — the region from which the hard X-ray emission of AGN originates. But X-rays only see so deep into dust-buried sources, and the radio and millimetre wavelengths might offer complementary, less-obscured windows. The recent detection of compact mm emission near several SMBHs is a candidate for that window — but is it really coronal?
What did we do?
We built a model of mm emission from a magnetised SMBH corona and compared it against ALMA-detected compact mm cores in a sample of nearby AGN. The mm continuum is consistent with synchrotron emission from a non-thermal relativistic particle population in the corona, with energy densities and field strengths inferred from the modelling. The framework also makes predictions for dust-obscured and high-redshift lensed sources, and connects naturally to the multi-wavelength and neutrino emission expected from the same particle population.
Why does it matter?
Direct millimetre observations of the corona — if the interpretation holds — give us a way to study the immediate vicinity of supermassive black holes in objects where X-ray observations are degraded or impossible. It also links AGN coronal physics to the multi-messenger landscape (high-energy neutrinos and γ-rays).
My role
Co-author. I spent several sessions with the lead author, Santiago del Palacio — an excellent theorist — sharpening and critiquing the model.