Chandra X-rays from the redshift 7.54 quasar ULAS J1342+0928
  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 L_{2-10} = 11.6^{+4.3}_{-3.5} x 10^{44} erg/s. Its
X-ray-to-optical power-law slope is alpha_{OX}=-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.
