X-ray Enhancements of High-redshift Highly Radio-loud Quasars

Quasars can be divided into radio-loud quasars (RLQs; R≥10) and radio-quiet quasars (RQQs; R<10) based on radio-loudness parameter R, which is defined as R = f_5GHz / f_4400Å. Highly radio-loud quasars (HRLQs) are a subpopulation of RLQs with R ≳ 300. Previous studies revealed that unlike RQQs and moderately radio-loud quasars, HRLQs at high redshifts show substantial X-ray enhancements compared to matched low-redshift HRLQs by a factor of ≈3. In the classic IC/CMB model, X-ray production was thought to be dominated by cosmic microwave background (CMB) photons that are inverse Compton scattered to the X-ray band. Previous studies challenged this model and proposed a fractional IC/CMB model. This project analyzes X-ray data of 9 HRLQs at 4.0<z<5.5. We combine these 9 sources investigated in this project with 32 previously studied sources to form a sample of 41 sources. We define a two-point spectral index α_ox, the spectral index of a power-law relation connecting rest-frame 2500 angstrom and 2 keV. We compare α_ox values of selected sources with predicted α_ox of low redshift sourses to investigate X-ray enhancements of HRLQs at high redshift. A difference in α_ox implies an X-ray excess or deficit. We also examine X-ray enhancements of HRLQs by constructing broad-band spectral energy distributions (SEDs) using multiwavelength data from various surveys. Our investigation on the enlarged sample further constrains the X-ray enhancements and revises the fractional IC/CMB model.