X‐ray spectral properties of high‐redshift radio‐loud quasars beyond redshift 4 – first results ★

We present the results of X‐ray spectroscopic observations with XMM–Newton for four high‐redshift radio‐loud quasars at z > 4 . Among these, three objects, namely GB B1508+5714, PMN J0324−2918 and PKS B1251−407, do not show soft X‐ray spectral flattening; the derived upper limits on assumed intrinsic absorption are (3.3–17.3) × 10 21  cm −2 , the least of which is among the most stringent limits for z > 4 quasars. There is a tentative indication for soft X‐ray spectral flattening in PMN J1451‐1512 at z = 4.76 , though the significance is not high. These observations more than double the number of z > 4 radio‐loud quasars that have X‐ray spectroscopic data to seven, which compose a significant subset of a flux‐limited sample of z > 4 radio‐loud quasars. Based on this subset, we show, in the second part of this paper, some preliminary results on the overall X‐ray spectral properties of the sample. Soft X‐ray spectral flattening, which is thought to arise from intrinsic X‐ray absorption, was found in about half of the sample (3/7 or 4/7). We give a preliminary distribution of the absorption column density N H . For those with detected X‐ray absorption, the derived N H values fall into a very narrow range (around a few times 10 22  cm −2 for ‘cold’ absorption), suggesting a possible common origin of the absorber. This N H distribution is consistent with that in the redshift range of 2–4, though the data are sparse. Those that do not show X‐ray absorption are constrained to have upper limits on the N H broadly consistent in general with the lower end of the distribution of the detected N H . Compared to lower redshift samples at z < 2 , there is an extension, or a systematic shift, towards higher values in the intrinsic N H distribution at z > 4 , and an increase of the fraction of radio‐loud quasars showing X‐ray absorption towards high redshifts. These results indicate a cosmic evolution effect, which seems to be the strongest at redshifts around 2. There is a tentative tendency that objects showing X‐ray absorption have X‐ray fluxes systematically higher than those showing apparently no absorption. After the spectral flattening is accounted for, the rest frame 1–50 keV continua have photon indices with a mean of 1.64 and a standard deviation of 0.11 (or a mean of 1.67 and a standard deviation of 0.14 for a Gaussian fit). Variability appears to be common on time‐scales from a few months to years in the quasar rest frame, sometimes in both fluxes and spectral slopes.