Chandra X-Rays from the Redshift 7.54 Quasar ULAS J1342+0928

Abstract

We present a 45 ks Chandra observation of the quasar ULAS J1342+0928 at z = 7.54. We detect {14.0}<SUB>-3.7</SUB><SUP>+4.8</SUP> 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 { \mathcal H }{ \mathcal R }=-{0.51}<SUB>-0.28</SUB><SUP>+0.26</SUP> 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}<SUB>-0.53</SUB><SUP>+0.55</SUP>. 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}<SUB>2-10</SUB>={11.6}<SUB>-3.5</SUB><SUP>+4.3</SUP>× {10}<SUP>44</SUP> {erg} {{{s}}}<SUP>-1</SUP>. Its X-ray-to-optical power-law slope is {α }<SUB>OX</SUB>}=-{1.67}<SUB>-0.10</SUB><SUP>+0.16</SUP>, consistent with the general trend indicating that the X-ray emission in the most bolometrically powerful quasars is weaker relative to their optical emission.