2018ApJ...856L..25B

We present a 45 ks Chandra observation of the quasar ULAS J1342+0928 at z = 7.54. We detect 14.0_–3.7^+4.8 counts from the quasar in the observed-frame energy range 0.5-7.0 keV (6σ detection), representing the most distant non-transient astronomical source identified in X-rays to date. The present data are sufficient only to infer rough constraints on the spectral parameters. We find an X-ray hardness ratio of H R =-0.51_–0.28^+0.26 between the 0.5-2.0 keV and 2.0-7.0 keV ranges and derive a power-law photon index of Γ=1.95_–0.53^+0.55. Assuming a typical value for high-redshift quasars of Γ = 1.9, ULAS J1342+0928 has a 2-10 keV rest-frame X-ray luminosity of L_2–10=11.6_–3.5^+4.3×10⁴⁴ergs^–1. Its X-ray-to-optical power-law slope is α_OX=-1.67_–0.10^+0.16, consistent with the general trend indicating that the X-ray emission in the most bolometrically powerful quasars is weaker relative to their optical emission.