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; 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 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.
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