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 F = 1.95-0.26+0.30, and a mean neutral intrinsic absorption column density of NH; 6 × 1022 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 (αox) of optically selected z > 5 RQQs, excluding broad absorption line quasars, is αox = -1.69 ± 0.03, which is consistent with the value predicted from the observed relationship between αox 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.
All Science Journal Classification (ASJC) codes
- Astronomy and Astrophysics
- Space and Planetary Science