Optical Variability Properties of Mini-BAL and NAL Quasars

TL;DR

This study investigates the variability of mini-BAL and NAL quasars over three years, finding limited UV flux variability and weak synchronization, which challenges the ionization state variability scenario as the main cause.

Contribution

It provides observational evidence that UV flux variability in mini-BAL and NAL quasars is insufficient to support the VIS scenario, suggesting additional mechanisms are involved.

Findings

UV flux variability over three years is too small for VIS scenario

No significant difference in variability between mini-BAL and NAL quasars

Weak synchronization between mini-BAL and quasar light curves

Abstract

While narrow absorption lines (NALs) are relatively stable, broad absorption lines (BALs) and mini-BAL systems usually show violent time variability within a few years via a mechanism that is not yet understood. In this study, we examine variable ionization state (VIS) scenario as a plausible mechanism, as previously suspected. Over three years, we performed photometric monitoring observations of four mini-BAL and five NAL quasars at $z_{\rm em}$ $\sim$ 2.0 - 3.1 using the 105 cm Kiso Schmidt Telescope in $u$, $g$, and $i$-bands. We also performed spectroscopic monitoring observation of one of our mini-BAL quasar (HS1603+3820) using the 188-cm Okayama Telescope over the same period as the photometric observations. Our main results are as follows: (1) Structure function (SF) analysis revealed that the quasar UV flux variability over three years was not large enough to support the VIS scenario, unless the ionization condition of outflow gas is very low. (2) There was no crucial difference between the SFs of mini-BAL and NAL quasars. (3) The variability of the mini-BAL and quasar light curves was weakly synchronized with a small time delay for HS1603+3820. These results suggest that the VIS scenario may need additional mechanisms such as a variable shielding by X-ray warm absorbers.