Investigating the X-ray enhancements of highly radio-loud quasars at z > 4

TL;DR

This study confirms that highly radio-loud quasars at redshift above 4 exhibit significantly enhanced X-ray emission linked to their jets, compared to similar quasars at lower redshifts, with implications for jet emission models.

Contribution

The paper provides the first robust statistical confirmation of X-ray enhancement in high-redshift HRLQs and constrains jet emission models, especially the IC/CMB contribution, at early cosmic times.

Findings

HRLQs at z>4 have 1.9 times higher X-ray emission than at lower redshifts.

X-ray enhancement is statistically significant at 4.0–4.6 sigma.

A fractional IC/CMB model can explain the results, but a dominant IC/CMB model cannot.

Abstract

We have investigated the jet-linked \mbox{X-ray} emission from highly radio-loud quasars (HRLQs; $\log R>2.5$) at high redshift. We studied the X-ray properties of 15 HRLQs at $z>4$, using new {\it Chandra} observations for six objects and archival {\it XMM-Newton} and {\it Swift} observations for the other nine. We focused on testing the apparent enhancement of jet-linked \mbox{X-ray} emission from HRLQs at $z>4$. Utilizing an enlarged (24 objects) optically flux-limited sample with complete X-ray coverage, we confirmed that HRLQs at $z>4$ have enhanced X-ray emission relative to that of HRLQs at $z\approx$ 1--2 with matched UV/optical and radio luminosity, at a \mbox{4.0--4.6}~$\sigma$ level; the X-ray enhancements are confirmed considering both two-point spectral indices and inspection of broad-band spectral energy distributions. The typical factor of enhancement is revised to $1.9^{+0.5}_{-0.4}$, which is smaller than but consistent with previous results. A fractional IC/CMB model can still explain our results at high redshift, which puts tighter constraints on the fraction of IC/CMB X-rays at lower redshifts, assuming the physical properties of quasar jets do not have a strong redshift dependence. A dominant IC/CMB model is inconsistent with our data.