Evidence for higher black hole spin in radio-loud quasars

TL;DR

This study provides observational evidence supporting the hypothesis that radio-loud quasars have higher black hole spins than radio-quiet quasars, based on differences in their spectral energy distributions and radiative efficiencies.

Contribution

It compares the mean radiative efficiencies of matched radio-loud and radio-quiet quasars, revealing higher efficiencies in radio-loud quasars consistent with higher black hole spins.

Findings

Radio-loud quasars show at least 1.5 times stronger [OIII] emission than radio-quiet ones.

Radio-loud quasars exhibit higher bolometric luminosities at fixed accretion rates.

Results support the spin paradigm linking black hole spin to radio loudness.

Abstract

One of the major unsolved questions on the understanding of the AGN population is the origin of the dichotomy between radio-quiet and radio-loud quasars. The most promising explanation is provided by the spin paradigm, which suggests radio-loud quasars have higher black hole spin. However, the measurement of black hole spin remains extremely challenging. We here aim at comparing the mean radiative efficiencies of carefully matched samples of radio-loud and radio-quiet SDSS quasars at 0.3<z<0.8. We use the [OIII] luminosity as an indirect average tracer of the ionizing continuum in the extreme-UV regime where differences in the SED due to black hole spin are most pronounced. We find that the radio-loud sample shows an enhancement in [OIII] line strength by a factor of at least 1.5 compared to a radio-quiet sample matched in redshift, black hole mass and optical continuum luminosity or accretion rate. We argue that this enhancement is caused by differences in the SED, suggesting higher average bolometric luminosities at fixed accretion rate in the radio-loud population. This suggests that the radio-loud quasar population has on average systematically larger radiative efficiencies and therefore higher black hole spin than the radio-quiet population, providing observational support for the black hole spin paradigm.