The X-ray enhancements of radio-loud quasars at high redshift: New results at z = 4 -- 7

Highly radio-loud quasars (HRLQs; log R>2.5) at z 4 show apparent enhanced X-ray emission compared to matched HRLQs at lower redshifts, perhaps due to a redshift-dependent fractional contribution to the X-ray luminosity from inverse-Compton scattering of cosmic microwave background photons (IC/CMB). Using new { Chandra} observations and archival X-ray data, we investigate this phenomenon with an optically flux-limited sample of 41 HRLQs at $z = 4$--5.5 all with sensitive X-ray coverage, the largest sample utilized to date by a wide margin. X-ray enhancements are assessed using X-ray-to-optical flux ratios and spectral energy distributions. We confirm the presence of X-ray enhancements at a 4.9--5.3σ significance level, finding that the median factor of enhancement is ≈ 1.8 at our sample median redshift of z≈ 4.4. Under a fractional IC/CMB model, the expected enhancement at lower redshifts is modest; e.g., ≈ 4% at z≈ 1.5. We also investigate a sample of seven radio-loud quasars (RLQs; log R>1) at even higher redshifts of $z=5.6$--6.8, using new and archival X-ray data. These RLQs also show evidence for X-ray enhancements by a median factor of ≈ 2.7 at a 3.7--4.9σ significance level. The X-ray spectral and other properties of these $z=5.6$--6.8 RLQs, however, pose challenges for a straightforward fractional IC/CMB interpretation of their enhancements.