On the Variability Features of Active Galactic Nuclei in Little Red Dots

Abstract The high-redshift ( z > 4) compact sources with “V-shaped” spectral energy distributions, known as Little Red Dots (LRDs), were discovered by the James Webb Space Telescope and provide valuable clues to the physics of active galactic nuclei (AGNs) in the early Universe. The nature of LRDs is controversial. Recently, several studies have investigated LRDs through variability, a characteristic feature of AGNs. These studies explore LRD variability by extrapolating empirical relationships from local quasars. Here, we adopt the Corona-heated Accretion-disk Reprocessing (CHAR) model, which is motivated by accretion physics and is applicable to reproducing AGN conventional variability, to study the variability of 22 LRDs in W. L. Tee et al. Our results indicate that the observed variability in LRDs is dominated by measurement uncertainties. Within the CHAR model, the lack of variability in LRDs can be explained by two scenarios: either AGNs contribute ≲30% of the observed luminosities, or they are intrinsically luminous AGNs. We use simulations to demonstrate the observational requirements to effectively investigate LRDs via variability with the following criteria: first, the sample has about 200 LRDs; second, each LRD has two observations separated by at least 2 yr in the observed frame; third, the photometric uncertainty is ≤0.07 mag. If the LRDs still lack variability under these conditions, the time–domain study would provide independent evidence that the accretion mode of LRDs differs significantly from low-redshift quasars.