Continuum Variability in AGN: Evidence for Systematically Suppressed Fluctuations in the BAL Population

Abstract

We aim to compare the flux variability of Broad Absorption Line quasars (BAL) to non-BAL quasars, controlling for black hole parameters to inform models concerning the generation of the BAL phenomenon. Using SDSS DR16 and ZTF $g$- and $r$-band light curves, we select quasars with $1.57 \leq z \leq 2.00$ and $18.5 \leq rmag \leq 19.8$. This redshift range ensures that the $g$-band covers the C IV emission line (and trough in BALs) while the $r$-band probes continuum variability, and allows black hole mass (MBH) estimates via Mg II. We quantify variability using excess variance and damped random walk (DRW) parameters ($σ_{\mathrm{DRW}}$, $τ_{\mathrm{DRW}}$). We also compared the DRW metrics in bins of MBH and Eddington ratio (REdd) to isolate the influence of the BAL phenomenon in objects with the same physical properties. Excess variance and $σ_{\mathrm{DRW}}$ are consistently smaller for BALs, confirming lower long-term variability, while the $g$-band exhibits higher variability than the $r$-band across both populations. These differences persist in fixed bins of MBH and REdd, indicating that the suppressed variability in BALs is not simply driven by differences in these properties between BAL and non-BAL samples. These results show that BAL quasars are systematically less variable than non-BAL quasars in both bands, confirming that this suppression is an intrinsic feature of the continuum rather than an effect of emission or absorption line contamination. The samples could be made to agree if BAL MBH values were systematically overestimated by a factor $\gtrsim 4$, implying a significantly higher REdd. Alternatively, the lower variability in BALs can be related to their lower X-ray luminosities, or to significant nuclear obscuration, if the inner part of the accretion disc were more obscured in BALs and more variable than the rest of the disc.