X-ray Insights into the Nature of Weak Emission-Line Quasars at High Redshift

TL;DR

This study uses Chandra X-ray observations to analyze high-redshift WLQs, revealing they are X-ray and radio weak compared to BL Lacs and challenging the high accretion rate hypothesis.

Contribution

It provides the strongest constraints on the X-ray properties of WLQs and compares their characteristics to BL Lacs and typical quasars, exploring their nature.

Findings

WLQs are weaker in X-ray and radio than low-redshift BL Lacs.

X-ray spectra of WLQs show typical photon indices, not indicating high accretion rates.

The data challenge the idea that WLQs are simply high-accretion-rate quasars.

Abstract

We present Chandra observations of nine high-redshift quasars (z=2.7-5.9) discovered by the Sloan Digital Sky Survey with weak or undetectable high-ionization emission lines in their UV spectra (WLQs). Adding archival X-ray observations of six additional sources of this class has enabled us to place the strongest constraints yet on the X-ray properties of this remarkable class of AGNs. Although our data cannot rule out the possibility that the emission lines are overwhelmed by a relativistically boosted continuum, as manifested by BL Lac objects, we find that WLQs are considerably weaker in the X-ray and radio bands than the majority of BL Lacs found at much lower redshifts. If WLQs are high-redshift BL Lacs, then it is difficult to explain the lack of a large parent population of X-ray and radio bright weak-lined sources at high redshift. We also consider the possibility that WLQs are quasars with extreme properties, and in particular that the emission lines are suppressed by high accretion rates. Using joint spectral fitting of the X-ray spectra of 11 WLQs we find that the mean photon index in the hard X-ray band is consistent with those observed in typical radio-quiet AGNs with no hint of an unusually steep hard-X-ray spectrum. This result poses a challenge to the hypothesis that WLQs have extremely high accretion rates, and we discuss additional observations required to test this idea.