Dependence of the Radio Emission on the Eddington Ratio of Radio-quiet Quasars

Abstract

Roughly 10% of quasars are “radio-loud,” producing copious radio emission in large jets. The origin of the low-level radio emission seen from the remaining 90% of quasars is unclear. Observing a sample of eight radio-quiet quasars with the Very Long Baseline Array, we discovered that their radio properties depend strongly on their Eddington ratio r Edd ≡ L AGN/L Edd. At lower Eddington ratios r Edd ≲ 0.3, the total radio emission of the AGN predominately originates from an extremely compact region, possibly as small as the accretion disk. At higher Eddington ratios (r Edd ≳ 0.3), the relative contribution of this compact region decreases significantly, and though the total radio power remains about the same, the emission now originates from regions ≳100 pc in size. The change in the physical origin of the radio-emitting plasma region with r Edd is unexpected, as the properties of radio-loud quasars show no dependence with Eddington ratio. Our results suggest that at lower Eddington ratios the magnetized plasma is likely confined by the accretion disk corona and only at higher Eddington ratios escapes to larger scales. Stellar-mass black holes show a similar dependence of their radio properties on the accretion rate, supporting the paradigm that unifies the accretion onto black holes across the mass range.