On the origin of radio core emission in radio-quiet quasars

TL;DR

This paper proposes that thermal bremsstrahlung from disk winds can explain radio core emission in radio-quiet quasars, challenging jet-based models and implying high accretion rates and significant AGN feedback.

Contribution

It introduces a thermal emission model for radio cores in quasars, emphasizing disk winds over jets, supported by observations and theoretical considerations.

Findings

Thermal bremsstrahlung can produce flat-spectrum radio cores.

Disk winds contribute significantly to radio emission in quasars.

High accretion rates are required for this mechanism.

Abstract

We present a model for the radio emission from radio-quiet quasar nuclei. We show that a thermal origin for the high brightness temperature, flat spectrum point sources (known as radio ``cores'') is possible provided the emitting region is hot and optically-thin. We hence demonstrate that optically-thin bremsstrahlung from a slow, dense disk wind can make a significant contribution to the observed levels of radio core emission. This is a much more satisfactory explanation, particularly for sources where there is no evidence of a jet, than a sequence of self-absorbed synchrotron components which collectively conspire to give a flat spectrum. Furthermore, such core phenomena are already observed directly via milli-arcsecond radio imaging of the Galactic microquasar SS433 and the active galaxy NGC1068. We contend that radio-emitting disk winds must be operating at some level in radio-loud quasars and radio galaxies as well (although in these cases, observations of the radio cores are frequently contaminated/dominated by synchrotron emission from jet knots). This interpretation of radio core emission mandates mass accretion rates that are substantially higher than Eddington. Moreover, acknowledgment of this mass-loss mechanism as an AGN feedback process has important implications for the input of energy and hot gas into the inter-galactic medium (IGM) since it is considerably less directional than that from jets.