Quasar Broad Absorption Line Variability: Understanding the Driving Mechanisms

Understanding the main driver of absorption line variability in quasars is crucial for using this variability to constrain outflow properties. However, the exact cause of absorption line variability in quasars remains unclear. Two mechanisms are most commonly favored: gas clouds moving across the line of sight (TM scenario) and changes in optical depth due to changes in the ionizing flux (IC scenario). Additionally, changes in the density of the absorber can also cause variability.

BAL variability may be caused by changes in coverage fraction due to gas transverse motion (e.g., Hamann et al. 2008; Shi et al. 2016) or changes in ionization states of the absorption gas (e.g., Crenshaw, Kraemer & George 2003, and references therein). Some studies have attempted to identify the leading mechanism of BAL variability by analyzing the relation between variation of BALs and that of the continuum/emission, and the relation between variation of different ions.

Both transverse motion and ionization change of the outflow cloud can be mechanisms that drive BAL variability. Observational evidence supports both scenarios. For the TM scenario, early works have not found an obvious relation between BALs and continuum variability (Gibson et al. 2008; Vivek et al. 2014; Wildy et al. 2014); in recent works, the strength of P V in BAL outflows inferred that the BALs are very saturated with large total column densities, which may offer an evidence for the TM model (Capellupo et al. 2017; Moravec et al. 2017; McGraw et al. 2018). For the IC scenario, coordinated multitrough variability (e.g., Capellupo et al. 2012, 2013; Filiz Ak et al. 2013; Wildy et al. 2014; Wang et al. 2015) and the coordinated variability between BALs and the ionizing continuum (Wang et al. 2015) have been found, these results tend to support the IC scenario. For instance, Filiz Ak et al. (2013) estimated that (56 ﾱ 7)% of trough variations are arising from a mechanism correlated between troughs, such as ICs. Besides, Wang et al. (2015) showed that BAL gas can, in principle, show large changes in response to only small continuum variations. More recently, based on the multiepoch observed BAL quasars from SDSS-I/II/III, He et al. (2017) have estimated statistically that at least 80% of the BAL variations are mainly driven by variations of the ionizing continuum. Based on the same data sample of He et al. (2017), Lu et al. (2018, hereafter LLQ2018) have revealed moderate anticorrelations with a high significance level between fractional equivalent width (EW) variations (DEW/EW) of BALs (for both Si IV and C IV BALs) and the fractional flux variations of the continuum (DF/Fcont).

Lu et al. 2018; Lu & Lin 2018c; Vivek 2019 for BAL). These correlations indicate that ionization change (IC) is the dominant mechanism for UV absorption line variations, although they cannot completely rule out other variation mechanisms such as transverse motion (TM) of absorbing gas across sight lines.

Previous BAL variability studies have presented evidence in favor of the ionization driven BAL variability (for eg., Barlow 1994; Filiz Ak et al. 2013; Wang et al. 2015; He et al. 2017; Lu), absorber motions across the line of sight (for eg., Gibson et al. 2008; Hamann et al. 2008; Hall et al. 2011; Filiz Ak et al. 2012; Vivek et al. 2012, 2016) or a combination of both effects (for eg., Capellupo et al. 2012). In these studies, coordinated variations of different velocity absorption components of the same ion are attributed to changes in ionization. BAL variabilities occurring over small portions of the troughs are considered either to be due to clouds crossing the line of sight or due to the difference in the density or the covering fraction of the absorbers at different velocities.