The Powerful Jet and Gamma-Ray Flare of the Quasar PKS 0438$-$436

Abstract

PKS 0438$-$436 at a redshift of $z=2.856$ has been previously recognized as possessing perhaps the most luminous known synchrotron jet. Little is known about this source since the maximum elevation above the horizon is low for the Very Large Array (VLA). We present the first VLA radio image that detects the radio lobes. We use both the 151 MHz luminosity, as a surrogate for the isotropic radio lobe luminosity, and the lobe flux density from the radio image to estimate a long term, time averaged, jet power, $\overline{Q} =1.5\pm 0.7 \times 10^{47} \rm{ergs~s^{-1}}$. We analyze two deep optical spectra with strong broad emission lines and estimate the thermal bolometric luminosity of the accretion flow, $L_{\rm{bol}} = 6.7 \pm 3.0 \times 10^{46} \rm{ergs~s^{-1}}$. The ratio, $\overline{Q}/L_{\rm{bol}} = 3.3 \pm 2.6 $, is at the limit of this empirical metric of jet dominance seen in radio loud quasars and this is the most luminous accretion flow to have this limiting behavior. Despite being a very luminous blazar, it previously had no $\gamma$-ray detections (EGRET, AGILE or FERMI) until December 11 - 13 2016 (54 hours) when FERMI detected a flare that we analyze here. The isotropic apparent luminosity from 100 MeV - 100 GeV rivals the most luminous detected blazar flares (averaged over 18 hours), $\sim 5-6 \times 10^{49} \rm{ergs~s^{-1}}$. The $\gamma$-ray luminosity varies over time by two orders of magnitude, highlighting the extreme role of Doppler abberation and geometric alignment in producing the inverse Compton emission.