Continuum variability in active galactic nuclei: Evidence for systematically suppressed fluctuations in the broad absorption line population

Vollständiger Abstract

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We aim to compare the flux variability of broad absorption line quasars (BAL) to non-BAL quasars using excess variance and damped random walk (DRW) metrics, controlling for intrinsic differences in the black hole parameters of their populations to inform models concerning the generation of the BAL phenomenon. We constructed a sample of light curves of BAL and non-BAL quasars by cross-matching spectroscopically confirmed quasars from the SDSS DR16 catalogue to the Zwicky Transient Facility database, in the g and r bands, selecting only objects in the ranges of 1.57 łeq z łeq 2.00 and 18.5 łeq rmag łeq19.8. The redshift range ensures that the g band covers the C, emission line (and trough in BALs) while the r band does not, mainly probing the continuum variability, and that the black hole masses ( ) and characteristic timescale (τ_ iv can be estimated from the width of the Mg, line for all objects. For each object, we quantified variability in both bands using the excess variance alongside the amplitude (σ_ ii DRW DRW ) of a DRW model. We also compared the DRW metrics in bins of and Eddington ratio (̊edd) to isolate the influence of the BAL phenomenon in objects with the same physical properties. The normalized excess variance and the σ_ DRW are consistently smaller for BALs, confirming that BAL quasars display lower long-term variability; furthermore, the g band exhibits higher variability than the r band across both populations. These differences persist when the samples are compared within fixed bins of and ̊edd, indicating that the suppressed variability in BAL quasars 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 the in the BAL sample were significantly overestimated, on average by a factor of ≳ 4. In this case their lower variability would be explained by their significantly higher ̊edd compared to the non-BAL sample. 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.