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

TL;DR

This study investigates the variability of high-redshift Little Red Dots (LRDs) using the CHAR model, finding that their observed lack of variability could imply they are either faint or intrinsically luminous AGNs, with implications for early universe black hole activity.

Contribution

It applies the CHAR model to LRD variability analysis, providing observational criteria to distinguish their nature and suggesting their variability properties differ from low-redshift quasars.

Findings

Observed variability in LRDs is dominated by measurement uncertainties.

Lack of variability can be explained if AGNs contribute less than 30% of luminosity or are intrinsically luminous.

Future observations require about 200 LRDs with specific temporal and photometric precision.

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.