NEXUS: Spectral Variability of Little Red Dots and Blue Active Galactic Nuclei at 2 z 6

We present spectral measurements for 17 Little Red Dots (LRDs) and 14 blue broad-line active galactic nuclei (AGNs) at 2 z 6 using multi-epoch JWST NIRSpec MSA spectra from the NEXUS program, sampling rest-frame timescales of about 1-3 months. Overall, the LRD population shows significantly enhanced Balmer decrement compared with both blue JWST AGNs at similar redshifts and 56 low-redshift broad-line AGNs matched in H alpha luminosity. The rest-optical continua of LRDs show little ensemble variability (rms 3%), and the total H alpha emission also shows weaker ensemble variability compared with low-redshift AGNs matched in H alpha luminosity and rest-frame timescales. Based on the flux uncertainties, we constrain the intrinsic H alpha rms variability to be 4% for the LRD population over these timescales. Combining our results with recent broad-line variability measurements of LRDs over yearly to decade timescales reveals a low-level white-noise pattern across all timescales, in stark contrast to the variability amplitude (about 6% over monthly timescales) and red-noise pattern observed in normal AGNs. These results add to the growing observational studies that suggest population-wise, LRDs have weak variability both in optical continuum and broad-line emission. Furthermore, the distinct white-noise broad-line variability pattern suggests different production mechanisms of broad-line emission in LRDs as opposed to normal AGNs, and/or different properties of the driving ionizing flux from the central engine.