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

Abstract

We have investigated the jet-linked 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 Chandra observations for six objects and archival XMM–Newton and Swift observations for the other nine. We focused on testing the apparent enhancement of jet-linked 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 ≈ 1–2 with matched UV/optical and radio luminosity, at a 4.0–4.6 σ 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 inverse-Compton/cosmic microwave background (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.