Saturation Effect on Photoionization-driven Broad Absorption Line Variability

TL;DR

This study investigates how saturation effects influence broad absorption line variability in quasars, demonstrating that ionization changes driven by continuum variations primarily cause observed Si IV BAL changes, while C IV BALs are less variable due to saturation.

Contribution

The paper provides a detailed analysis of the saturation effect on BAL variability, combining case studies and statistical analysis to clarify the role of ionization changes and continuum variability.

Findings

Si IV BALs show significant variability linked to continuum changes.

C IV BALs exhibit less variability due to saturation effects.

Continuum variability is a key driver of BAL changes.

Abstract

We study the saturation effect on broad absorption line (BAL) variability through a variation phenomenon, which shows significant variation in Si IV BAL but no, or only small, change in C IV BAL (hereafter Phenomenon I). First, we explore a typical case showing Phenomenon I, quasar SDSS J153715.74+582933.9 (hereafter J1537+5829). We identify four narrow absorption line (NAL) systems within its Si IV BAL and two additional NAL systems within its C IV BAL, and confirm their coordinated weakening. Combining with the obvious strengthening of the ionizing continuum, we attribute the BAL variability in J1537+5829 to the ionization changes caused by the continuum variations. Secondly, a statistical study based on multiobserved quasars from SDSS-I/II/III is presented. We confirm that (1) the moderate anticorrelation between the fractional variations of Si IV BALs and the continuum in 74 quasars that show Phenomenon I and (2) the sample showing BAL variations tends to have larger ionizing continuum variations. These results reveal the ubiquitous effect of the continuum variability on Phenomenon I and BAL variation. We attribute the relative lack of variation of C IV BALs in Phenomenon I to the saturation effects. Nonetheless, these absorbers are not very optically thick in Si IV and the ionization changes in response to continuum variations could be the main driver of their variations. Finally, we find that the saturation effect on BAL variability can well explain many phenomena of BAL variations that have been reported before.