2024/02/27 by Zihao Zuo, Zuo, Zihao, Shifu Zhu +11 · 1 citation
Physics and Astronomy · #Astrophysics and Cosmic Phenomena #Astrophysics of Galaxies (astro-ph.GA) #FOS: Physical sciences #Galaxies: Formation, Evolution, Phenomena #High Energy Astrophysical Phenomena (astro-ph.HE) #Radio Astronomy Observations and Technology
paper · pdf · doi:10.48550/arxiv.2402.17828
openalex publication_date 2024/02/27 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/01
Highly radio-loud quasars (HRLQs; log R>2.5) at z\gtrsim 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 \it 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.