What sparks the radio-loud phase of nearby quasars?

TL;DR

This study analyzes the properties of nearby quasars to understand why only a subset are radio loud, finding that the radio-loud phase is likely transient and driven by stochastic accretion processes.

Contribution

It provides a detailed comparison of radio-loud and radio-quiet quasars, highlighting the transient and intrinsic nature of the radio-loud phase linked to chaotic accretion.

Findings

22% of quasars are radio loud with mostly compact, weak sources.

Radio-loud quasars have higher bolometric luminosities and ionization powers.

The radio-loud phase appears to be transient and driven by stochastic accretion.

Abstract

To better constrain the hypotheses proposed to explain why only a few quasars are radio loud (RL), we compare the characteristics of 1958 nearby $(z\le 0.3)$ SDSS quasars, covered by the FIRST and NVSS radio surveys. Only 22\% are RL with $\log(L_{1.4{\rm GHz}}) \ge 22.5$ W Hz$^{-1}$, the majority being compact (C), weak radio sources (WRS), with $\log(L_{1.4{\rm GHz}}) < 24.5$ W Hz$^{-1}$. 15\% of the RL quasars have extended radio morphologies: 3\% have a core and a jet (J), 2\% have a core with one lobe (L), and 10\% have a core with two lobes (T), the majority being powerful radio sources (PRS), with $\log(L_{1.4{\rm GHz}}) \ge 24.5$ W Hz$^{-1}$. In general, RL quasars have higher bolometric luminosities and ionisation powers than radio quiet (RQ) quasars. The WRS have comparable black hole (BH) masses as the RQ quasars, but higher accretion rates or radiative efficiencies. The PRS have higher BH masses than the WRS, but comparable accretion rates or radiative efficiencies. The WRS also have higher $FWHM_{\rm [OIII]}$ than the PRS, consistent with a coupling of the spectral characteristics of the quasars with their radio morphologies. Inspecting the SDSS images and applying a neighbour search algorithm reveal no difference between the RQ and RL quasars of their host galaxies, environments, and interaction. Our results prompt the conjecture that the phenomenon that sparks the radio-loud phase in quasars is transient, intrinsic to the AGN, and stochastic, due to the chaotic nature of the accretion process of matter onto the BHs.