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, GBB1508+5714, PMNJ0324-2918, and PKSB1251-407 do not show soft X-ray spectral flattening; the derived upper limits on assumed intrinsic absorption are (3.3 − 17.3) × 10 cm, 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 PMNJ1451-1512 at z=4.76, though the significance is not high. These observations more than double the number of z > 4 radio-loud quasars having 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 the 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 NH. For those with detected X-ray absorption, the derived NH values fall into a very narrow range (around a few times 10 22 cm for ‘cold’ absorption), suggesting a possible common origin of the absorber. This NH 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 NH broadly consistent in general with the lower end of the distribution of the detected NH. Compared to lower-redshift samples at z < 2, there is an extension, or a systematic shift, toward higher values in the intrinsic NH distribution at z > 4, and an increase of the fraction of radio-loud quasars showing X-ray absorption toward 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 1–50keV 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 timescales from a few months to years in the quasar rest-frame, sometimes in both fluxes and spectral slopes.