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

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 long-wavelength emission from radio-quiet
quasars. The proposed mechanisms span a broad range, from central star
formation to dusty torus, low-power jets, or coronal emission from the
innermost accretion disk. Distinguishing between these scenarios requires
probing spatial scales $\leq$0.01 pc, beyond the reach of any current
millimetre-wave telescope. Fortunately, in gravitationally lensed quasars,
compact mm-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. The observed flux-ratio variability is
consistent with microlensing of a very compact source with a half-light radius
$\leq$50 astronomical units. The compactness of the source leaves coronal
emission as the most likely scenario. Furthermore, the inferred mm-wave and
X-ray luminosities follow the characteristic G\"udel-Benz relationship for
coronal emission. These observations represent the first unambiguous evidence
for coronae as the dominant mechanism for long-wavelength emission in
radio-quiet quasars.