The radio emission in radio-quiet quasars: the VLBA perspective

TL;DR

This study uses VLBA observations to investigate the origin of radio emission in radio-quiet quasars, revealing compact cores likely from accretion disk coronae and extended outflows, with correlations to X-ray emissions.

Contribution

First detailed VLBA imaging of RQQ cores and extended structures, linking radio emission to accretion disk coronae and outflows, expanding understanding of RQQ radio origins.

Findings

Most RQQ have unresolved flat-spectrum cores coinciding with optical positions.

Extended emission is steep-spectrum, offset from optical core, and likely from outflows.

VLBA core flux correlates with X-ray flux, supporting coronal origin hypothesis.

Abstract

The origin of the radio emission in radio-quiet quasars (RQQ) is not established yet. We present new VLBA observations at 1.6 and 4.9 GHz of ten RQQ (nine detected), which together with published earlier observations of eight RQQ (five detected), forms a representative sample of 18 RQQ drawn from the Palomar-Green sample of low z (< 0.5) AGN. The spectral slope of the integrated emission extends from very steep (alpha < -1.98) to strongly inverted (alpha = +2.18), and the slopes of nine of the 14 objects are flat (alpha > -0.5). Most objects have an unresolved flat-spectrum core, which coincides with the optical Gaia position. The extended emission is generally steep-spectrum, has a low brightness temperature (< 10^7 K), and is displaced from the optical core (the Gaia position) by ~ 5-100 pc. The VLBA core flux is tightly correlated with the X-ray flux, and follows a radio to X-ray luminosity relation of log L_R/L_X = -6, for all objects with a black hole mass log M_BH/M_Sun < 8.5. The flatness of the core emission implies a compact source size (< 0.1 pc), which likely originates from the accretion disk corona. The mas-scale extended emission is optically thin and of clumpy structure, and is likely produced by an outflow from the center. Radio observations at higher frequencies can further test the accretion disk coronal emission interpretation for the core emission in RQQ.