X-Ray Observations of a [C II]-bright, z = 6.59 Quasar/Companion System
Thomas Connor, Eduardo Bañados, Chiara Mazzucchelli, Daniel Stern, Roberto Decarli, Xiaohui Fan, Emanuele Paolo Farina, Elisabeta Lusso, Marcel Neeleman, Fabian Walter
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 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).