The kinetically dominated quasar 3C 418

Abstract

Abstract The existence of quasars that are kinetically dominated, where the jet kinetic luminosity, Q, is larger than the total (infrared to X-ray) thermal luminosity of the accretion flow, Lbol, provides a strong constraint on the fundamental physics of relativistic jet formation. Since quasars have high values of Lbol by definition, only ∼10 kinetically dominated quasars (with $\overline{Q}/L_{\rm {bol}}>1$) have been found, where $\overline{Q}$ is the long-term time-averaged jet power. We use low-frequency (151 MHz–1.66 GHz) observations of the quasar 3C 418 to determine $\overline{Q}\approx 5.5 \pm 1.3 \times 10^{46}\, \rm {erg\,s^{-1}}$. Analysis of the rest-frame ultraviolet spectrum indicates that this equates to 0.57 ± 0.28 times the Eddington luminosity of the central supermassive black hole and $\overline{Q}/L_{\rm {bol}} \approx 4.8 \pm 3.1$, making 3C 418 one of the most kinetically dominated quasars found to date. It is shown that this maximal $\overline{Q}/L_{\rm {bol}}$ is consistent with models of magnetically arrested accretion of jet production in which the jet production reproduces the observed trend of a decrement in the extreme ultraviolet continuum as the jet power increases. This maximal condition corresponds to an almost complete saturation of the inner accretion flow with vertical large-scale magnetic flux (maximum saturation).