Do Little Red Dots Vary?

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 casts 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 two to four times in a given wave band, 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 nexus campaign will detect changes in magnitude Δ m > 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 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.