  We report on Chandra and XMM-Newton observations of a sample of 13 quasars at
z~4.7-5.4 mostly taken from the Sloan Digital Sky Survey (SDSS). The present
sample complements previous X-ray studies of z>4 quasars, in which the majority
of the objects are optically more luminous and at lower redshifts. All but two
of our quasars have been detected in the X-ray band, thus doubling the number
of z>4.8 X-ray detected quasars. The two non-detections are likely to be due to
a short exposure time and to the presence of intrinsic absorption. We confirm
and extend to the highest redshifts the presence of a correlation between
AB1450(1+z) magnitude and soft X-ray flux for z>4 quasars, and the presence of
a steeper optical-to-X-ray spectral energy distribution (parameterized by aox)
for high-luminosity, high-redshift quasars than for lower-luminosity,
lower-redshift quasars. The second effect is likely due to the known
anti-correlation between aox and rest-frame 2500 Angstrom luminosity, whose
significance is confirmed via partial correlation analysis. The joint ~2.5-36
keV rest-frame spectrum of the z>4.8 SDSS quasars observed thus far by Chandra
is well parameterized by a power-law with photon index Gamma=1.84+/-0.30; this
photon index is consistent with those of z~0-3 quasars and that obtained from
joint spectral fitting of z~4.1-4.5 optically luminous Palomar Digital Sky
Survey quasars. No evidence for widespread intrinsic X-ray absorption has been
found (Nh<4.0x10^{22} cm^{-2} on average at 90% confidence). We also obtained
Hobby-Eberly Telescope (HET) photometric observations for eight of our target
quasars. None of these shows significant (>30%) optical variability over the
time interval of a few years (in the observed frame) between the SDSS and HET
observations.
