The X-ray spectral properties and variability of luminous high-redshift active galactic nuclei

We perform a detailed investigation of moderate- to high- quality X- ray spectra of 10 of the most luminous active galactic nuclei ( AGNs) known at z > 4 ( up to z similar to 6: 28). This study includes five new XMM- Newton observations and five archived X- ray observations ( four by XMM- Newton and one by Chandra). We find that the X- ray powerlaw photon indices of our sample, composed of eight radio- quiet sources and two that are moderately radio- loud, are not significantly different from those of lower redshift AGNs. The upper limits obtained on intrinsic neutral hydrogen column densities, N-H less than or similar to 10(22) 10(23) cm (-2), indicate that these AGNs are not significantly absorbed. A joint fit performed on our eight radio- quiet sources, with a total of similar to 7000 photons, constrains the mean photon index of z > 4 radio- quiet AGNs to Gamma = 1: 97 (+0.06)(-0.04) , with no detectable intrinsic dispersion from source to source. We also obtain a strong constraint on the mean intrinsic column density, N-H less than or similar to 3 x 10(21) cm (-2), showing that optically selected radioquiet AGNs at z > 4 are, on average, not more absorbed than their lower redshift counterparts. All this suggests that the X- ray production mechanism and the central environment in radio- quiet AGNs have not significantly evolved over cosmic time. The mean equivalent width of a putative neutral narrow Fe K alpha line is constrained to be less than or similar to 190 eV, and similarly we place constraints on the mean Compton reflection component ( R P 1: 2). None of the AGNs varied on short (similar to 1 hr) timescales, but on longer timescales ( months to years) strong variability is observed in five of the sources. In particular, the X- ray flux of the z = 5: 41 radio- quiet AGN SDSS 0231 - 0728 dropped by a factor of similar to 4 over a rest- frame period of 73 days. This is the most extreme X- ray variation observed in a luminous z > 4 radioquiet AGN.