Two X-ray Sources in a z~6.2 Quasar/Galaxy Merger

Thomas Connor, Eduardo Bañados, Daniel Stern, Roberto Decarli, Jan-Torge Schindler, Xiaohui Fan, Emanuele Paolo Farina, Chiara Mazzucchelli, John S. Mulchaey, Fabian Walter

Published 2019-09-18Version 1

Quasars at early redshifts ($z > 6$) with companion galaxies offer unique insights into the growth and evolution of the first supermassive black holes. Here, we report on a 150 ks Chandra observation of PSO J308.0416$-$21.2339, a $z=6.23$ quasar with a merging companion galaxy identified in [C II] and rest-frame UV emission. With $72.3^{+9.6}_{-8.6}$ net counts, we find that PSO J308.0416$-$21.2339 is powerful ($L_X = 2.31^{+1.14}_{-0.76} \times 10^{45}\ \textrm{erg}\,\textrm{s}^{-1}\,\textrm{cm}^{-2}$ in rest frame $2.0-10.0$ keV) yet soft (spectral power-law index $\Gamma=2.39^{+0.37}_{-0.36}$ and optical-to-X-ray slope $\alpha_{\rm OX} = -1.41 \pm 0.11$). In addition, we detect three hard-energy photons 2.0 arcseconds to the west of the main quasar, cospatial with the brightest UV emission of the merging companion. As no soft energy photons are detected in the same area, this is potentially indicative of a highly-obscured source. With conservative assumptions, and accounting for both background fluctuations and the extended wings of the quasar's emission, these photons represent a $2.4\sigma$ detection. If confirmed by deeper observations, this system is the first high redshift quasar and companion individually detected in X-rays and is likely a dual AGN.