ChandraObservations of Radio-Loud Quasars atz > 4: X-Rays from the Radio Beacons of the Early Universe

Abstract

We present the results of Chandra observations of six radio-loud quasars (RLQs) and one optically bright radioquiet quasar (RQQ) at z 4.1‐4.4. These observations cover a representative sample of RLQs with moderate radio-loudness (R 40‐400), filling the X-ray observational gap between optically selected radio-quiet quasars (predominantly R 2‐10) and the five known blazars at z 4 (R 800‐27000), where R= f5 GHz f4400 ˚ A (rest frame). We study the relationship between X-ray luminosity and radio-loudness for quasars at high redshift and constrain RLQ X-ray continuum emission and absorption. From a joint spectral fit of nine moderate-R RLQs observed by Chandra, we find tentative evidence for absorption above the Galactic NH , with a best-fit neutral intrinsic column density of NH 2 4 2 0 1 8 10 22 cm 2 , consistent with earlier claims of increased absorption toward high-redshift RLQs. We also search for evidence of an enhanced jet-linked component in the X-ray emission due to the increased energy density of the cosmic microwave background (CMB) at high redshift, but we find neither spatial detections of X-ray jets nor a significant enhancement in the X-ray emission relative to comparable RLQs at low-to-moderate redshifts. Overall, the z 4‐5 RLQs have basic X-ray properties consistent with comparable RLQs in the local universe, suggesting that the accretion/jet mechanisms of these objects are similar as well. Subject headings: galaxies: active — galaxies: high-redshift — galaxies: jets — galaxies: nuclei — quasars: general