Rest-frame Ultraviolet Variability of Quasars in the Cosmic Himalayas Overdensity

Abstract

Some high-redshift overdensities show an enhanced incidence of AGNs, suggesting that large-scale environment may influence supermassive black hole fuelling and triggering. Whether environment also affects AGN variability remains unclear. Using the Cosmic Himalayas (CH), an extreme concentration of 11 type 1 quasars at z=2.16-2.20, we investigate possible offsets in rest-frame ultraviolet variability relative to quasars with comparable redshifts, black hole masses, and Eddington ratios. We construct a parent sample of 1,138 Stripe 82 quasars at 2.11<=z<=2.25 and combine photometry from the SDSS, the PS1, and the ZTF to build observer-frame g- and r-band light curves spanning up to 27~yr. We compare DRW parameters and ensemble structure functions (SFs) with control pseudo-samples matched in black hole mass and Eddington ratio, and use injection--recovery simulations to forecast the detectability of a CH-like offset. The CH quasars show a tendency toward shorter DRW damping timescales (median offsets of -0.32 and -0.13 dex in g and r) and slightly higher asymptotic amplitudes. None of the four DRW comparisons is statistically significant, including the largest offset in the g-band damping timescale (p=0.16). Ensemble SFs generally exceed matched-control medians, although 18 of 19 lag bins per band remain within the 95% pointwise envelopes. Assuming a true uniform -0.32 dex g-band timescale offset, injection--recovery simulations predict an 80% probability of nominal 3sigma detection with approximately 60 quasars of comparable variability scatter and data quality. If confirmed, the tentative variability offsets may suggest that large-scale environment influences accretion-flow variability beyond its potential role in quasar triggering. Larger samples with higher-precision light curves, such as those expected from the LSST, are required to test this possibility.