2017MNRAS.468L..72P

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, L_bol, provides a strong constraint on the fundamental physics of relativistic jet formation. Since quasars have high values of L_bol by definition, only ∼10 kinetically dominated quasars (with /L_ bol_>1) have been found, where  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 ~5.5 ±1.3 ×10⁴⁶  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 /L_ bol_ ~4.8 ±3.1, making 3C 418 one of the most kinetically dominated quasars found to date. It is shown that this maximal /L_ 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).