Ionizing Continuum Variability Impacts BAL Variations in HiBAL Quasars: Evidence from Anti-Correlations and Saturation Effects

As shown in Table 1, the Spearman rank correlation analysis confirms the anti-correlation between the fractional flux variations of the continuum and the fractional EW variations of the BALs, including CIV00, CIVs0, CIVS0, and SiIVS0. It is worth noting that the CIV BAL with lower ionization exhibits a smaller correlation coefficient. However, when the CIV BAL is accompanied by AlIII BAL/mini-BAL, no significant correlation is detected. These statistical results are consistent with the expectations of the disk-wind model and provide important insights into the variation mechanism of BALs with different ionization levels.
\nOn one hand, we confirm the anti-correlations between the fractional variations of the continuum and BALs in three groups (CIV00, CIVs0, and CIVS0) of HiBALs. Previous studies have observed anti-correlations between the fractional variations of the continuum and absorption lines in several BAL samples (Lu et al. 2018; Lu & Lin 2018; Vivek 2019) as evidence for ionization-driven BAL variation. However, these studies did not compare the EW variations of BALs at different ionization states with their fractional variations of the continuum. Our study expands on this by finding anti-correlations between the fractional flux variations of the continuum and the fractional EW variations for three BAL groups without AlIII BAL/mini-BAL, covering a wider ionization-potential range. This finding provides further evidence of the widespread impact of ionizing continuum variability on the variation of HiBALs (Weymann et al. 1991). Additionally, if different groups of BALs represent different viewing inclinations, the anti-correlations observed in our study provide evidence for the ubiquitous effects of ionizing continuum variability across a wide range of viewing inclinations, in accordance with the typical disk-wind model (Murray et al. 1995; Proga et al. 2000; Higginbottom et al. 2013).
\nOn the other hand, we found no significant correlations between the fractional flux variations of the continuum and the fractional EW variations for the BAL groups with AlIII BAL/mini-BAL (CIVsa, CIVSA, CIVSa, SiIVSA, and AlIIISA), with P-values of the Spearman coefficient greater than 1E-2. We speculate that the lack of correlations in these groups may be due to the effects of variable shielding gas, BAL saturation, ionization state of the outflow, or other BAL variation mechanisms, especially the saturation effect. This observation is supported by the study conducted by Filiz Ak et al. (2014), which showed that when BAL troughs from lower ionization transitions are present, CIV troughs tend to be stronger and wider (see their Figure 5), but exhibit less fractional EW variation (see their Figure 11) and more saturation. This suggests that CIV troughs might experience more saturation when BAL troughs from lower ionization transitions are present. Hamann2019 also obtained empirical results showing that compared to the LoBALs, the CIV BAL in the composite spectrum of the HiBALs are deeper and wider (see Figure 4 in Hamann2019). This can also be inferred from our Figure 2, which shows that the EW distribution of the CIVSA group has a smaller standard deviation (σ) for the best-fitting Gaussian component (σ=0.213) compared to the CIV00 (σ=0.390) and CIVS0 (σ=0.357) groups. This is because the fractional EW variation measurements can only reflect the lower limits of the true optical depth and column density variations for saturated troughs. However, the Fcont distribution of the CIVSA sample (with a standard deviation σ=0.271 for the best-fitting Gaussian component) still exhibits a wide range comparable to the CIV00 (σ=0.363) and CIVS0 (σ=0.292) samples. Considering the significant impact of line-saturation in the CIVSA group, we speculate that the effects of ionizing continuum variability may also exist in the BALs that accompany AlIII BAL/mini-BAL. In other words, the ubiquitous effects of ionizing continuum variability may extend to even lower viewing inclinations.
\nIn summary, the statistical analysis based on the fractional variation of BALs and the continuum reveals the widespread effects of ionizing continuum variability on the variation of outflow absorption across a wide range of ionization levels and viewing inclinations.