# Variation of ionizing continuum: the main driver of Broad Absorption Line Variability

## TL;DR

This study analyzes quasar broad absorption line variability using SDSS data, showing that ionizing continuum changes primarily drive BAL variability, with some outflows responding on very short timescales of days.

## Contribution

It provides statistical evidence that ionizing continuum variations are the main driver of BAL variability, supported by a large multi-epoch quasar dataset and photoionization model predictions.

## Key findings

- Over 80% of BAL variability is driven by ionizing continuum changes.
- BAL outflows can respond within days, indicating short recombination times.
- The ratio of Si iv to C iv BALs correlates with continuum variations as predicted by models.

## Abstract

We present a statistical analysis of the variability of broad absorption lines (BALs) in quasars using the large multi-epoch spectroscopic dataset of the Sloan Digital Sky Survey Data Release 12 (SDSS DR12). We divide the sample into two groups according to the pattern of the variation of C iv BAL with respect to that of continuum: the equivalent widths (EW) of the BAL decreases (increases) when the continuum brightens (dims) as group T1; and the variation of EW and continuum in the opposite relation as group T2. We find that T2 has significantly (P_T<10-6 , Students T Test) higher EW ratios (R) of Si iv to C iv BAL than T1. Our result agrees with the prediction of photoionization models that C +3 column density increases (decreases) if there is a (or no) C +3 ionization front while R decreases with the incident continuum. We show that BAL variabilities in at least 80% quasars are driven by the variation of ionizing continuum while other models that predict uncorrelated BAL and continuum variability contribute less than 20%. Considering large uncertainty in the continuum flux calibration, the latter fraction may be much smaller. When the sample is binned into different time interval between the two observations, we find significant difference in the distribution of R between T1 and T2 in all time-bins down to a deltaT < 6 days, suggesting that BAL outflow in a fraction of quasars has a recombination time scale of only a few days.