Do Little Red Dots Vary?

Amy Secunda, Rachel S. Somerville, Yan-Fei Jiang, Jenny E. Greene, Lukas J. Furtak, Adi Zitrin

First listed 2025-09-03 | Last updated 2025-09-03

Abstract

Little red dots (LRDs), high-redshift, compact, red objects with V-shaped spectra, are one of the most exciting and perplexing discoveries made by the James Webb Space Telescope (JWST). While the simplest explanation for LRDs is that they are high redshift active galactic nuclei (AGN), due to their compactness and frequent association with broad line emission, the lack of corresponding X-ray emission and observed variability cast doubt on this picture. Here, we simulate LRD light curves using both traditional models for sub-Eddington AGN variability derived empirically from lower-redshift AGN observations and moderately super-Eddington AGN disk models from radiation magnetohydrodynamic simulations to examine the reason for the lack of variability. We find that even though most LRDs have only been observed 2--4 times in a given waveband, we should still be detecting significantly more variability if traditional sub-Eddington AGN variability models can be applied to LRDs. Instead, our super-Eddington model light curves are consistent with the lack of observed LRD variability. In addition, the ongoing high-cadence {\sc nexus} campaign will detect changes in magnitude, $Δm>1$, for traditional sub-Eddington models, but will only observe significant continuum variability for the lowest mass LRDs for our super-Eddington AGN models. Even if LRDs lack continuum variability, we find that the ongoing spectroscopic JWST campaign {\sc twinkle} should observe broad emission line variability as long as soft X-ray irradiation manages to reach the broad line region from the inner disk. Our models show that super-Eddington accretion can easily explain the lack of continuum variability in LRDs.

Short digest

Simulated light curves for little red dots are generated under two variability prescriptions: an empirical sub‑Eddington DRW tuned to low‑z AGN and a radiation‑MHD–motivated super‑Eddington disk model. With today’s 2–4‑epoch JWST sampling, the DRW case would have produced far more rest‑UV/optical variability than is seen, while the super‑Eddington model naturally matches the near‑steady continua. The high‑cadence nexus imaging should yield Δm>1 mag if DRW holds, but only the lowest‑mass LRDs vary appreciably in the super‑Eddington scenario; meanwhile, twinkle spectroscopy is still expected to detect broad‑line variability if soft X‑rays can reach the BLR. Net result: super‑Eddington accretion neatly explains the missing continuum variability while allowing line changes.

Key figures to inspect

• Figure 1: Compare PSDs from the super‑Eddington simulations to a sub‑Eddington DRW—note how increasing ˙M suppresses UV/soft‑X variability and how the extrapolated optical PSD sits well below the DRW expectation.
• Figure 2: Read off the simulated Δm distributions in F115W and F356W for past JWST/HST cadences vs nexus; see that DRW predicts detectable variability already, while the super‑Eddington tracks largely sit under the current photometric limits.
• Figure 3: Inspect the percent broad‑line flux change histograms—only the soft‑X driven super‑Eddington case clears the twinkle detection threshold, highlighting the need for BLR‑reaching soft X‑ray irradiation.
• Figure 4: Trend of Δm with black‑hole mass in F356W for nexus cadence—DRW rises above the detectability line broadly, but super‑Eddington variability is only detectable toward the lowest masses, quantifying where continuum changes might appear first.