AGN coronal emission models – I. The predicted radio emission

Abstract

Accretion discs in AGN may be associated with coronal gas, as suggested by their X-ray emission. Stellar coronal emission includes radio emission, and AGN corona may also be a significant source for radio emission in radio quiet (RQ) AGN. We calculate the coronal properties required to produce the observed radio emission in RQ AGN, either from synchrotron emission of power-law (PL) electrons, or from cyclo-synchrotron emission of hot mildly-relativistic thermal electrons. We find that a flat spectrum, as observed in about half of RQ AGN, can be produced by corona with a disc or a spherical configuration, which extends from the innermost regions out to a pc scale. A spectral break to an optically thin power-law emission is expected around 300-1000 GHz, as the innermost corona becomes optically thin. In case of thermal electrons, a sharp spectral cutoff is expected above the break. The position of the break can be measured with VLBI observations, which exclude the cold dust emission, and it can be used to probe the properties of the innermost corona. Assuming equipartition of the coronal thermal energy density, the PL electrons energy density, and the magnetic field, we find that the energy density in a disc corona should scale as ∼ R −1.3 , to get a flat spectrum. In the spherical case the energy density scales as ∼ R −2 , and is ∼ 4 × 10 −4 of the AGN radiation energy density. In paper II we derive additional constraints on the coronal parameters from the Gudel-Benz relation, L radio /L X−ray ∼ 10 −5 , which RQ AGN follow.

Evidence that the AGN dominates the radio emission in z ∼ 1 radio-quiet quasars

“…Another suggestion, by Laor & Behar (2008), is that synchrotron emission in RQQs could be due to magnetic reconnection events accelerating electrons in the corona above the disc. This is developed further by Raginski & Laor (2016), who argue that the spectral index and variability of RQQs favours a coronal origin over an AGN-driven wind. Further investigation of the above explanations is beyond the scope of this work, but a combination of these different processes may be the reason for the large variation seen in L 1.5 GHz, acc as a function of M i .…”

The mm-wave compact component of an AGN

“…This excess suggests that the AGN emission is still significant into the FIR. Theoretical works (Inoue & Doi 2014;Raginski & Laor 2016) further highlight the potential of mm observations for detecting coronal magnetic activity on AGN accretion disks. Baldi et al (2015) found that the nearby Seyfert NGC 7469 varies at 95 GHz (confidence level 99.98%) on a time scale of days, while being apparently steady for years at 5 GHz (Pérez-Torres et al, 2009).…”

Galaxy Evolution in the Radio Band: The Role of Star-forming Galaxies and Active Galactic Nuclei

“…Alternatively to the small-scale jet origin, the radio emission of RQ AGNs may be traced back to shock fronts associated with the AGN-driven outflow (e.g., Zakamska & Green 2014;Nims et al 2015), or to winds originated from the outermost portion of the thin accretion disk (see Blundell et al 2001;King et al 2013), or to electron acceleration via magnetic reconnection in the thin disk corona (see Lahor & Behar 2008;Raginski & Laor 2016). Whatever mechanism operates, in this phase of a massive galaxy's evolution the radio emission from the system should be mainly ascribed to the nuclear activity typical of a RQ AGN.…”