X-Ray Observations of a [C ii]-bright, z = 6.59 Quasar/Companion System

Abstract

We present deep Chandra observations of PSO J231.6576−20.8335, a quasar at redshift z  = 6.59 with a nearby (∼8 proper kpc) companion galaxy. The Atacama Large Millimeter/submillimeter Array observed both the quasar and companion to be bright in [C ii ], and the system has significant extended Ly α emission around the quasar, suggesting that a galaxy merger is ongoing. Unlike previous studies of two similar systems, and despite observing the system with Chandra for 140 ks, we do not detect the companion in X-rays. The quasar itself is detected, but only net counts are observed. From a basic spectral analysis, the X-ray spectrum of the quasar is soft (hardness ratio of , power-law index of ), which results in a rest-frame X-ray luminosity comparable to other bright quasars ( ) despite the faint observed X-ray flux. We highlight two possible interpretations of this result: the quasar has a steep value of Γ—potentially related to observed ongoing Eddington accretion—thereby pushing much of the emission out of our observed band, or the quasar has a more normal spectrum (Γ ∼ 2) but is therefore less X-ray luminous ( ).