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+4.8-3.7 counts from the quasar in the observed-frame energy range 0.5-7.0 keV (6-sigma 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 HR = -0.51+0.26-0.28 between the 0.5-2.0 keV and 2.0-7.0 keV ranges and derive a power-law photon index of Gamma = 1.95+0.55-0.53. Assuming a typical value for high-redshift quasars of Gamma = 1.9, ULAS J1342+0928 has a 2-10 keV rest-frame X-ray luminosity of L2-10 = 11.6+4.3-3.5 x 1044 erg/s. Its X-ray-to-optical power-law slope is alphaOX=-1.67+0.16-0.10, 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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