Emission-line Variable Active Galactic Nuclei at Cosmic Noon from HETDEX

Abstract We present the first statistical census of emission-line variable active galactic nuclei (EVA) at cosmic noon by combining untargeted and deep Hobby–Eberly Telescope Dark Energy Experiment (HETDEX) spectroscopy with multiepoch spectra from the Sloan Digital Sky Survey, DESI, and LAMOST. Anchoring all candidates to a HETDEX spectroscopic epoch and requiring an active galactic nucleus (AGN) classification in either the HETDEX or external epoch(s), we identify a homogeneous sample of 100 EVA at z ∼ 1.5, including 98 that are newly identified. Emission-line variability is selected primarily through statistically significant line-flux changes, supplemented by extensive visual inspections using contemporaneous photometric light curves. The resulting incidence fraction is f EVA ∼ 0.9%. The rest-frame intervals between spectroscopic epochs span ∼1–10 yr, with brightening and dimming events exhibiting statistically indistinguishable characteristic timescales (Δ T ∼ 2.2 and ∼2.6 yr, respectively). A key result is the characterization of the Baldwin effect in the time domain: while many EVA follow the ensemble Baldwin effect (eBeff) between two epochs, a substantial fraction exhibit apparent anti-eBeff responses. Time-resolved spectroscopy of an individual source reveals that the intrinsic equivalent width–luminosity relation is nonstationary, with the line-to-continuum responsivity systematically evolving from stronger to weaker across successive variability cycles; sparse two-epoch sampling of this evolving intrinsic Baldwin evolution naturally produces both eBeff-like and anti-eBeff behaviors. Finally, EVA show no strong preference for extreme Eddington ratios but exhibit a mild tendency toward lower λ Edd values relative to matched control samples, driven primarily by sources observed in their dim states. Together, these results establish a coherent framework for interpreting emission-line variability in AGN at the peak epoch of cosmic black hole growth.