Discovery of millimetre-wave excess emission in radio-quiet active galactic nuclei

Abstract

The physical origin of radio emission in Radio Quiet Active Galactic Nuclei (RQ AGN) remains unclear, whether it is a downscaled version of the relativistic jets typical of Radio Loud (RL) AGN, or whether it originates from the accretion disk. The correlation between 5 GHz and X-ray luminosities of RQ AGN, which follows L R = 10 −5 L X observed also in stellar coronae, suggests an association of both X-ray and radio sources with the accretion disk corona. Observing RQ AGN at higher (mm-wave) frequencies, where synchrotron self absorption is diminished, and smaller regions can be probed, is key to exploring this association. Eight RQ AGN, selected based on their high X-ray brightness and variability, were observed at 95 GHz with the CARMA and ATCA telescopes. All targets were detected at the 1 − 10 mJy level. Emission excess at 95 GHz of up to ×7 is found with respect to archival low-frequency steep spectra, suggesting a compact, optically-thick core superimposed on the more extended structures that dominate at low frequencies. Though unresolved, the 95 GHz fluxes imply optically thick source sizes of 10 −4 − 10 −3 pc, or ∼ 10 − 1000 gravitational radii. The present sources lie tightly along an L R (95 GHz) = 10 −4 L X (2-10 keV) correlation, analogous to that of stellar coronae and RQ AGN at 5 GHz, while RL AGN are shown to have higher L R /L X ratios. The present observations argue that simultaneous mm-wave and X-ray monitoring of RQ AGN features a promising method for understanding accretion disk coronal emission.

Milimetre-band variability of the radio-quiet nucleus of NGC 7469

“…This suggests a 95-GHz source size of no more than 4-5 light days, or ∼0.004 pc, which is still ∼10 times larger than the minimal physical size estimated from the luminosity of a self absorbed synchrotron source (i.e., radio-sphere, Behar et al 2015).…”

“…RQ AGN show either flat or inverted spectra at these high radio frequencies, suggesting optically-thick core emission (Doi et al 2005(Doi et al , 2011. We recently found a 95 GHz excess of a factor of up to ∼7 with respect to the observed low-frequency steep spectra, for a sample of X-ray bright RQ AGN (Behar et al 2015).…”

“…The source shows a rapid flux variation of a factor 2 in 4-5 days. If we consider the 95-GHz flux density of 5.0±0.2 mJy from Behar et al (2015) observed with the same CARMA (C) array in November 2013, the source varied by a factor of ∼1.7 in four months. This variability amplitude of NGC 7469 at 95 GHz is larger and on shorter time scales than that reported at 8.4 GHz by Pérez-Torres & et al (2009) for the same AGN with annual monitoring over 8 years.…”

“…NGC 7469 (z = 0.01588, DL = 71.2 Mpc) was drawn from a recent AGN sample selected based on the high X-ray brightness and documented variability (Behar et al 2015). It was chosen for intense monitoring, because it is one of the brightest targets in the sample.…”

“…It was chosen for intense monitoring, because it is one of the brightest targets in the sample. Its 95 GHz flux density is in excess (×1.5) over the steep spectrum at low-radio frequencies (see Behar et al 2015). Pérez-Torres & et al (2009) reported seven 8.4-GHz observations of NGC 7469 over 8 years and its radio core (0.…”