Abstract
Abstract We present a 45 ks Chandra observation of the quasar ULAS J1342+0928 at z = 7.54. We detect 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 between the 0.5–2.0 keV and 2.0–7.0 keV ranges and derive a power-law photon index of . 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 . Its X-ray-to-optical power-law slope is , consistent with the general trend indicating that the X-ray emission in the most bolometrically powerful quasars is weaker relative to their optical emission.
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