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

TL;DR

This study presents simultaneous millimeter and X-ray observations of the radio-quiet AGN MCG+08-11-11, revealing correlated rapid variability that suggests the mm emission originates near the X-ray corona, possibly from an outflow or weak jet.

Contribution

First simultaneous mm-X-ray observation of a radio-quiet AGN demonstrating correlated rapid variability, indicating the mm emission region is close to the X-ray corona.

Findings

Detected 2-3% variability in mm flux on 2×10^4 s timescale

Observed 7% variability in X-ray flux on similar timescale

Variability suggests mm emission originates near the X-ray corona

Abstract

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 and powerful jets. The physical origin of their radio emission remains then broadly unclear. The observation of flat/inverted radio spectra at GHz frequencies seems to support however the presence of an unresolved synchrotron self-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 self absorption decreases strongly with frequency, these sources need to be observed in the millimetric (mm) domain. We report here a 12h simultaneous mm-X-ray observation of the RQ AGN MCG+08-11-11 by NOEMA and NuSTAR. The mm flux shows a weak 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 a fractional variability of $7.0\pm1.5$\%. A structure function analysis shows a local maximum in the mm light curve corresponding to 2-3\% of variability on timescale of $\sim2\times10^4$ seconds (100-300 $R_g$ light crossing time). Assuming an optically thick mm emitting medium, this translates into an upper limit of its size of $\sim$1300 $R_g$. The observation of fast variability in radio-mm and X-ray wavelengths, as well as a similar variability trend, well support the mm emission to be emitted by a region close, and potentially related to, the X-ray corona like an outflow/weak jet.