Weak-emission-line quasars: A new clue from their optical variability

TL;DR

This study reveals that weak-emission-line quasars (WLQs) show significantly less optical variability over months and years compared to normal quasars, suggesting differences in their central engine or surrounding material.

Contribution

It provides the first systematic comparison of optical variability between WLQs and normal QSOs using large samples and multiple surveys, highlighting a new observational distinction.

Findings

WLQs exhibit 1.76 times milder variability than normal QSOs.

The variability difference is consistent across different datasets and bands.

Clumpiness of the torus may influence the variability and emission-line weakness.

Abstract

Weak-emission-line QSOs (WLQs) are an enigmatic subclass of the QSO population, as their optical/UV spectra are marked by abnormally weak (or absent) emission lines. To obtain much-needed additional clues to the origin of this and other known peculiarities of WLQs, we have determined the 'ensemble' optical variability characteristics for a large, well-defined sample of 76 radio-quiet WLQs and also for a matched control sample comprising 603 normal radio-quiet QSOs. This analysis was done using their light-curves recorded in the $g$ and $r$ bands, under the ZTF survey during 2018-2024, with a typical cadence of 3 days. We find that, compared to normal QSOs, WLQs exhibit systematically milder optical variability on month/year-like time scales (by a factor of $\sim$ 1.76$\pm$0.05 in amplitude). We have independently verified this by carrying out an equivalent analysis of the V-band light-curves acquired under the CRTS during 2007- 2014, with a typical cadence of 10 days. This new observational differentiator between WLQs and normal QSOs may provide clues to understanding the intriguing nature of WLQs. It is proposed that the clumpiness of the torus material flowing into the central engine may play a key role in explaining the observed differences between the WLQs and normal QSOs.