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

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 Lyα 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 13.3-3.7+4.8 net counts are observed. From a basic spectral analysis, the X-ray spectrum of the quasar is soft (hardness ratio of H R = -0.60-0.27+0.17, power-law index of Γ = 2.6-0.9+1.0), which results in a rest-frame X-ray luminosity comparable to other bright quasars (L2- 10 = 1.09-0.70+2.20 × 1045 erg s-1) 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 (L2 10 ∼ 0.6 × 1045 erg s-1).