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 \approx 1–2 with matched UV/optical and radio luminosity , at a 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 .