X-ray emission from the most luminous z>4 Palomar Digital Sky Survey   Quasars: the Chandra view
  We present the results obtained from exploratory Chandra observations of nine high-redshift (z=4.09-4.51) quasars, selected from among the optically brightest Palomar Digital Sky Survey quasars known. Their broad-band spectral energy distributions are characterized, on average, by steeper alpha_ox values (<alpha_ox>=-1.81+/-0.03) than those of lower-redshift, lower-luminosity samples of quasars. We find a significant correlation between AB1450(1+z) magnitude and soft X-ray flux, suggesting that the engine powering the UV and X-ray emission is the same. The joint 2-30 keV rest-frame X-ray spectrum is well parameterized by a Gamma=2.0+/-0.2 power-law model with no evidence for intrinsic absorption (N_H<8.8 10^{21} cm^-2). 
C. Vignali, W.N. Brandt, D.P. Schneider, G.P. Garmire (Dept. of   Astronomy & Astrophysics, The Pennsylvania State Universit...