Investigating the Correlation Between Ionizing Continuum and C IV BAL Variability at Different Ionization Levels

Broad absorption line (BAL) variability is a subject of great importance in the study of quasar outflows. Understanding the variation mechanism of BALs is a central issue in this field. One effective way to determine the variation mechanism is to investigate the correlation between changes in absorption lines and the continuum. Early studies based on relatively small quasar samples reported no detectable correlation between changes in the BAL equivalency width (EW) and the continuum flux. However, recent studies have found anti-correlations between the variation of the continuum and both C IV and Si IV BAL troughs in several BAL quasar samples. These anti-correlations reveal the widespread effects of ionizing continuum variability on BALs, providing evidence for photoionization-driven BAL variability.

The question of whether these correlations extend to lower and higher ionization levels is still under debate. Some studies have found no clear correlation between absorption line and continuum flux variabilities in low-ionization quasars. However, it has been suggested that this lack of correlation may be due to the large variability in light curves observed in targets with significant absorption line variability. These studies propose that the observed variability can be explained by a combination of continuum variations and clouds transiting across our line of sight.

In typical disk-wind models, BALs with different ionization levels can be explained by the viewing angle effect. Lower degrees of ionization are observed along lines of sight closer to the accretion disc plane. This view is supported by both observations and simulation studies. For example, previous research on C IV BAL troughs, and their potential association with Si IV and/or Al III BAL troughs, found correlated changes between ionization level, kinematics, and column density, consistent with the predictions of accretion-disk wind models. Recent numerical simulations have also shown that low ionization absorption features result in wider and deeper BALs, while high ionization species exhibit higher blue-edge velocities compared to the low ionization lines.

Building upon the work of Filiz Ak et al. (2014), this paper aims to explore the correlation between the fractional variation of the ionizing continuum and C IV BALs with different ionization levels. The paper is organized as follows: Section 2 presents the data preparation and measurements of BALs and spectra. Section 3 contains the results and discussions. Finally, Section 4 provides the conclusion and outlines potential future research directions.