Letter to the Editor

Simultaneous millimetric and X-ray intraday variability in the radio-quiet AGN MCG+08-11-11

¹ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
e-mail: pierre-olivier.petrucci@univ-grenoble-alpes.fr
² IRAM, 300 rue de la piscine, 38406 Saint Martin d’Hères, France
³ Department of Physics, Technion, Haifa 32000, Israel
⁴ Dipartimento di Matematica e Fisica, Università degli Studi Roma Tre, via della Vasca Navale 84, 00146 Roma, Italy
⁵ Department of Earth and Space Science, Osaka University, Osaka 560-0043, Japan
⁶ IRAP, Université de toulouse, CNRS, UPS, CNES, Toulouse, France
⁷ ORN, Observatoire de Paris, CNRS, PSL, Université d’Orléans, Nançay, France
⁸ AIM, CEA, CNRS, Université Paris Cité, Université Paris-Saclay, 91191 Gif-sur-Yvette, France

Received: 18 July 2023
Accepted: 4 September 2023

Abstract

Most of the active galactic nuclei (AGN) are radio quiet (RQ) and, differently from radio-loud (RL) AGN, they do not show a signature of large-scale or powerful jets. The physical origin of their radio emission thus remains broadly unclear. The observation of flat and inverted radio spectra at gigahertz 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 12 h simultaneous mm-X-ray observation of the RQ AGN MCG+08-11-11 by NOEMA and NuSTAR, respectively. The mm flux shows a weak but clear increase along the pointing with a fractional variability of 2.0 ± 0.1%. The 3–10 keV flux of NuSTAR also increases and shows a fractional variability of 7.0 ± 1.5%. A structure function analysis shows a local maximum in the mm light curve corresponding to 2–3% of variability on a timescale of ∼2 × 10⁴ s (100–300 R_g light crossing time). Assuming an optically thick mm emitting medium, this translates into an upper limit of its size of ∼1300 R_g. The observation of fast variability in radio-mm and X-ray wavelengths, as well as a similar variability trend, thoroughly support the idea that the mm emission is emitted by a region close to, and potentially related to, the X-ray corona such as an outflow or weak jet.