Multiwavelength Observations of Radio-quiet Quasars with Weak Emission Lines

TL;DR

This study investigates a subset of SDSS BL Lac candidates with weak emission lines, revealing many are radio-quiet AGN possibly related to weak line quasars, challenging the typical radio-loud nature of BL Lac objects.

Contribution

It provides the first systematic analysis of low-redshift, radio-quiet BL Lac candidates, suggesting they may be intrinsically weak-lined quasars and expanding the understanding of AGN diversity.

Findings

Half of the candidates are stars, galaxies, or absorbed quasars.

Ten of the 13 remaining are definitively radio-quiet.

Some candidates are low-redshift analogs to weak line quasars.

Abstract

We present radio and X-ray observations, as well as optical light curves, for a subset of 26 BL Lac candidates from the Sloan Digital Sky Survey (SDSS) lacking strong radio emission and with z<2.2. Half of these 26 objects are shown to be stars, galaxies, or absorbed quasars. We conclude that the other 13 objects are Active Galactic Nuclei (AGN) with abnormally weak emission features; ten of those 13 are definitively radio-quiet, and, for those with available optical light curves, their level of optical flux variability is consistent with radio-quiet quasars. We cannot exclude the possibility that some of these 13 AGN lie on the extremely radio-faint tail of the BL Lac distribution, but our study generally supports the notion that all BL Lac objects are radio-loud. These radio-quiet AGN appear to have intrinsically weak or absent broad emission line regions, and, based on their X-ray properties, we argue that some are low-redshift analogs to weak line quasars (WLQs). SDSS BL Lac searches are so far the only systematic surveys of the SDSS database capable of recovering such exotic low-redshift WLQs. There are 71 more z<2.2 radio-quiet BL Lac candidates already identified in the SDSS not considered here, and many of those might be best unified with WLQs as well. Future studies combining low- and high-redshift WLQ samples will yield new insight on our understanding of the structure and formation of AGN broad emission line regions.