AGN Variability with Rubin Observatory in the 2030s

Abstract

AGN variability offers a direct probe of accretion physics, disk structure, and black hole growth, but progress has been limited by sample size, cadence heterogeneity, and photometric systematics. The Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST) will deliver multi-band light curves for millions of AGN, enabling variability studies at a true population scale. We synthesize recent results from the Zwicky Transient Facility (ZTF), which demonstrate that optical variability amplitudes and timescales are primarily regulated by accretion state, with secondary dependence on black hole mass and redshift, and establish the feasibility of survey-driven continuum reverberation mapping. ZTF measurements reveal optical continuum-emitting region sizes that often exceed standard thin disk predictions, implicating diffuse continuum emission from the broad line region as a significant contributor to observed inter-band lags. We evaluate the implications of LSST cadence and survey strategy, particularly the deep drilling fields, for continuum and emission line reverberation mapping, changing-look AGN, extreme variability quasars, and periodic variability searches. Key limitations of broadband photometric variability are identified, including variable emission line contamination, diffuse BLR continuum emission, and cadence-dependent lag recoverability. We argue that realizing LSST's full scientific potential requires community-scale, standardized variability metric pipelines, probabilistic classification integrated with alert brokers for follow-up triggering, and complementary medium-band photometric observations to isolate the accretion disk continuum. Together, these elements will enable LSST to convert photometric variability into quantitative constraints on accretion disks, BLR structure, and supermassive black hole growth across cosmic time.