X-Ray Observations of a [CII]-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 (similar to 8 proper kpc) companion galaxy. The Atacama Large Millimeter/submillimeter Array observed both the quasar and companion to be bright in [CII], and the system has significant extended Ly alpha 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 HR = -0.60(-0.27)(+0.17), power-law index of Gamma = 2.6(-0.9)(+1.0)), which results in a rest-frame X-ray luminosity comparable to other bright quasars L-2.10 = 1.09(-.070)(+2.20) x 10(45) ers s(-1)) despite the faint observed X-ray flux. We highlight two possible interpretations of this result: the quasar has a steep value of Gamma-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 (Gamma similar to 2) but is therefore less X-ray luminous (L2-10 similar to 0.6 x 1(04) erg s(-1)).