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

Abstract

We present deep Chandra observations of PSO J231.6576$-$20.8335, a quasar at redshift z=6.59 with a nearby ($\sim 8$ proper kpc) companion galaxy. ALMA observed both the quasar and companion to be bright in [C II], and the system has significant extended Ly$\hat{I}\pm$ 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 $\mathcal{H}\mathcal{R}=-0.60_{-0.27}^{+0.17}$, power-law index of $\hat{I}“=2.6_{-0.9}^{+1.0}$), which results in a rest-frame X-ray luminosity comparable to other bright quasars ($L_{2-10}=1.09_{-0.70}^{+2.20}\times 10^{45}\text{erg}{\text{s}}^{-1}$) despite the faint observed X-ray flux. We highlight two possible interpretations of this result: the quasar has a steep value of $\hat{I}“$ -- 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 ($\hat{I}“\sim 2$) but is therefore less X-ray luminous ($L_{2-10}\sim 0.6\times 10^{45}\text{ erg}{\text{ s}}^{-1}$).

11 pages, 4 figures. Accepted for publication in ApJ