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

Shuying Zhou, Mouyuan Sun, Zijian Zhang, Jie Chen, Luis C. Ho

First listed 2025-08-22 | Last updated 2025-08-22

Abstract

The high-redshift ($z>4$) compact sources with ``V-shaped" spectral energy distributions (SEDs), known as Little Red Dots (LRDs), are 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 applicable to reproduce AGN conventional variability, to study the variability of $22$ LRDs in \citet{Tee2025}. 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 $\lesssim30\%$ of the observed luminosities, or they are intrinsically luminous AGNs. We use simulations to demonstrate the observational requirements to effectively investigate LRDs via variability: first, a sample of about $200$ LRDs; second, each LRD has two observations separated by at least two years in the observed frame; third, the photometric uncertainty is $\leq 0.07$ 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.

Short digest

Applies the CHAR (corona-heated disk reprocessing) model to the Tee et al. (2025) 22-object LRD sample to interpret their multi-epoch photometry. Finds that the measured |Δm| is dominated by photometric uncertainties; within CHAR the non-detections are consistent with either AGN light fractions ≲30% or intrinsically luminous AGN whose fractional variability is small. Simulations specify what it would take to break this degeneracy: ≈200 LRDs, two epochs separated by ≥2 years (observed frame), and σ_phot ≤0.07 mag. If variability still fails to appear, it would point to accretion modes unlike those of low‑z quasars, offering independent constraints on what powers LRDs.

Key figures to inspect

• Figure 1: Compare LRD bolometric luminosities (from F115W/F150W) to SDSS S82 quasars to see LRDs sit 1–2 dex lower, underscoring why extrapolating S82 variability scalings is risky.
• Figure 2: Inspect the |Δm| distributions across assumed AGN fractions with/without injected measurement noise to see how host dilution plus current uncertainties wash out intrinsic variability signals.
• Figure 3: Use the A–D test “acceptance” maps versus luminosity correction factor (τ; with A_V on the right axis) in F115W/F150W to read off f_AGN upper limits and how they loosen as τ increases.
• Figure 4: Contrast Δχ² grids for σ_phot≈0.14 vs 0.07 mag (N≈200, Δt=2 yr) to see that current precision yields only f_AGN upper limits, while improved photometry enables joint constraints on f_AGN and τ.
• Method emphasis: Note how the CHAR-timescale (thermal) framing ties the expected variability amplitude to luminosity, motivating the survey design in Figs. 3–4.