The X-ray properties of typical high-redshift radio-loud quasars

TL;DR

This study presents X-ray spectral, imaging, and variability analyses of high-redshift (z > 4) radio-loud quasars, revealing jet-linked X-ray emission characteristics, absorption features, and extended jet-related X-ray emission, with implications for understanding their high-redshift properties.

Contribution

First detailed X-ray analysis of a representative sample of high-redshift radio-loud quasars, including spectral, imaging, and variability results, expanding knowledge of their high-redshift jet and absorption properties.

Findings

X-ray spectra consistent with jet-linked emission and similar photon indices to lower redshift RLQs.

Detection of extended X-ray emission likely from jets in one quasar.

Evidence of long-term X-ray variability in some high-redshift RLQs.

Abstract

We report spectral, imaging, and variability results from four new XMM-Newton observations and two new Chandra observations of high-redshift (z > 4) radio-loud quasars (RLQs). Our targets span lower, and more representative, values of radio loudness than those of past samples of high-redshift RLQs studied in the X-ray regime. Our spectral analyses show power-law X-ray continua with a mean photon index, \Gamma =1.74 +/- 0.11, that is consistent with measurements of lower redshift RLQs. These continua are likely dominated by jet-linked X-ray emission, and they follow the expected anti-correlation between photon index and radio loudness. We find no evidence of iron K\alpha ~ emission lines or Compton-reflection continua. Our data also constrain intrinsic X-ray absorption in these RLQs. We find evidence for significant absorption (N_H ~ 10^22 cm^-2) in one RLQ of our sample (SDSS J0011+1446); the incidence of X-ray absorption in our sample appears plausibly consistent with that for high-redshift RLQs that have higher values of radio loudness. In the Chandra observation of PMN J221-2719 we detect apparent extended (~ 14 kpc) X-ray emission that is most likely due to a jet; the X-ray luminosity of this putative jet is ~2% that of the core. The analysis of a 4.9 GHz VLA image of PMN J221-2719 reveals a structure that matches the X-ray extension found in this source. We also find evidence for long-term (450-460 days) X-ray variability by 80-100% in two of our targets.