An X-ray and multiwavelength survey of highly radio-loud quasars at z>4: jet-linked emission in the brightest radio beacons of the early Universe

We present a systematic study of the X-ray and multiwavelength properties of a sample of 17 highly radio-loud quasars (HRLQs) at z > 4 with sensitive X-ray coverage from new Chandra and archival Chandra, XMM-Newton, and Swift observations. Eight of the new and archival observations are reported in this work for the first time. New Chandra observations of two moderately radio-loud and highly optically luminous quasars at z >~ 4 are also reported. Our HRLQ sample represents the top ~5% of radio-loud quasars (RLQs) in terms of radio loudness. We found that our HRLQs have an X-ray emission enhancement over HRLQs at lower redshifts (by a typical factor of ≈3), and this effect, after controlling for several factors which may introduce biases, has been solidly estimated to be significant at the 3σ-4σ level. HRLQs at z = 3-4 are also found to have a similar X-ray emission enhancement over z < 3 HRLQs, which further supports the robustness of our results. We discuss models for the X-ray enhancement's origin including a fractional contribution from inverse Compton scattering of cosmic microwave background photons. No strong correlations are found between the relative X-ray brightness and optical/UV emission-line rest-frame equivalent widths (REWs) for RLQs. However, the line REWs are positively correlated with radio loudness, which suggests that relativistic jets make a negligible contribution to the optical/UV continua of these HRLQs (contrary to the case where the emission lines are diluted by the relativistically boosted continuum). Our HRLQs are generally consistent with the known anti-correlation between radio loudness and X-ray power-law photon index. We also found that the two moderately radio-loud quasars appear to have the hardest X-ray spectra among our objects, suggesting that intrinsic X-ray absorption (NH ~ 10^23 cm–2) may be present. Our z > 4 HRLQs generally have higher X-ray luminosities than those for the composite broadband spectral energy distributions of HRLQs at lower redshift, which further illustrates and supports the X-ray emission enhancement of z > 4 HRLQs. Some of our HRLQs also show an excess of mid-infrared emission which may originate from the synchrotron emission of the relativistic jets. None of our z > 4 HRLQs is detected by the Fermi-LAT two-year survey, which provides constraints on jet-emission models.