We present new Chandra observations of 21 z > 4 quasars , including 11 sources at z > 5 .
These observations double the number of X-ray detected quasars at z > 5 , allowing investigation of the X-ray spectral properties of a substantial sample of quasars at the dawn of the modern Universe .
By jointly fitting the spectra of 15 z > 5 radio-quiet quasars ( RQQs ) , including sources from the Chandra archive , with a total of 185 photons , we find a mean X-ray power-law photon index of \Gamma = 1.95 ^ { +0.30 } _ { -0.26 } , and a mean neutral intrinsic absorption column density of N _ { H } \stackrel { < } { \sim } 6 \times 10 ^ { 22 } cm ^ { -2 } .
These results show that quasar X-ray spectral properties have not evolved up to the highest observable redshifts .
We also find that the mean optical–X-ray spectral slope ( \alpha _ { ox } ) of optically-selected z > 5 RQQs , excluding broad absorption line quasars , is \alpha _ { ox } = -1.69 \pm 0.03 , which is consistent with the value predicted from the observed relationship between \alpha _ { ox } and ultraviolet luminosity .
Four of the sources in our sample are members of the rare class of weak emission-line quasars , and we detect two of them in X-rays .
We discuss the implications our X-ray observations have for the nature of these mysterious sources and , in particular , whether their weak-line spectra are a consequence of continuum boosting or a deficit of high-ionization line emitting gas .