CHANDRA OBSERVATIONS OF THE HIGHEST REDSHIFT QUASARS FROM THE SLOAN DIGITAL SKY SURVEY

摘要

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 Γ = 1.95_(-0.26)~(+0.30), and a mean neutral intrinsic absorption column density of N_H approx< 6 x 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 (α_(ox)) of optically selected z > 5 RQQs, excluding broad absorption line quasars, is α_(ox) = — 1.69 ± 0.03, which is consistent with the value predicted from the observed relationship between α_(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.