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Most of the Active Galactic Nuclei (AGN) are radio-quiet (RQ) and,differently from radio-loud (RL) AGN, do not show signature of large-scale andpowerful jets. The physical origin of their radio emission remains then broadlyunclear. The observation of flat/inverted radio spectra at GHz frequenciesseems to support however the presence of an unresolved synchrotronself-absorbed region in the close environment of the supermassive black hole.Its size could be as small as that of the X-ray corona. Since synchrotron selfabsorption decreases strongly with frequency, these sources need to be observedin the millimetric (mm) domain. We report here a 12h simultaneous mm-X-rayobservation of the RQ AGN MCG+08-11-11 by NOEMA and NuSTAR. The mm flux shows aweak but clear increase along the pointing with a fractional variability of$2.0\pm0.1$\%. The 3-10 keV flux of NuSTAR also increases and shows afractional variability of $7.0\pm1.5$\%. A structure function analysis shows alocal maximum in the mm light curve corresponding to 2-3\% of variability ontimescale of $\sim2\times10^4$ seconds (100-300 $R_g$ light crossing time).Assuming an optically thick mm emitting medium, this translates into an upperlimit of its size of $\sim$1300 $R_g$. The observation of fast variability inradio-mm and X-ray wavelengths, as well as a similar variability trend, wellsupport the mm emission to be emitted by a region close, and potentiallyrelated to, the X-ray corona like an outflow/weak jet.

Simultaneous mm/X-ray intraday variability in the radio-quiet AGN MCG+08-11-11