Investigating photometric and spectroscopic variability in the multiply-imaged Little Red Dot A2744-QSO1

Lukas J. Furtak, Amy R. Secunda, Jenny E. Greene, Adi Zitrin, Ivo Labbé, Miriam Golubchik, Rachel Bezanson, Vasily Kokorev, Hakim Atek, Gabriel B. Brammer, Iryna Chemerynska, Sam E. Cutler, Pratika Dayal, Robert Feldmann, Seiji Fujimoto, Joel Leja, Yilun Ma, Jorryt Matthee, Rohan P. Naidu, Erica J. Nelson, Richard Pan, Sedona H. Price, Katherine A. Suess, Bingjie Wang, John R. Weaver, Katherine E. Whitaker

First listed 2025-02-11 | Last updated 2025-05-07

Abstract

JWST observations have uncovered a new population of red, compact objects at high redshifts dubbed `Little Red Dots' (LRDs), which typically show broad emission lines and are thought to be dusty Active Galactic Nuclei (AGN). Some of their other features, however, challenge the AGN explanation, such as prominent Balmer breaks and extremely faint or even missing metal high-ionization lines, X-ray, or radio emission, including in deep stacks. Time variability is another, robust, test of AGN activity. Here, we exploit the $z=7.045$ multiply-imaged LRD A2744-QSO1, which offers a particularly unique test of variability due to lensing-induced time delays between the three images spanning 22 yr (2.7 yr in the rest-frame), to investigate its photometric and spectroscopic variability. We find the equivalent widths (EWs) of the broad H$α$ and H$β$ lines, which are independent of magnification and other systematics, to exhibit significant variations, up to $18\pm3$ % for H$α$ and up to $22\pm8$ % in H$β$, on a timescale of 875 d (2.4 yr) in the rest-frame. This suggests that A2744-QSO1 is indeed an AGN. We find no significant photometric variability beyond the limiting systematic uncertainties, so it currently cannot be determined whether the EW variations are due to line-flux or continuum variability. These results are consistent with a typical damped random walk (DRW) variability model for an AGN like A2744-QSO1 ($M_{\mathrm{BH}}=4\times10^7 \mathrm{M}_{\odot}$) given the sparse sampling of the light-curve with the available data. Our results therefore support the AGN interpretation of this LRD, and highlight the need for further photometric and spectroscopic monitoring in order to build a detailed and reliable light-curve.

Short digest

Exploiting the three lensed images of the z=7.045 LRD A2744-QSO1 with 22 yr (observer; 2.7 yr rest) delays, the authors test variability using two NIRSpec epochs and multi-epoch NIRCam. Magnification-insensitive EWs of broad Hα and Hβ vary by up to 18±3% and 22±8%, respectively, over 875 d (2.4 yr) in the rest frame, coherently across images. No significant broadband photometric variability is detected beyond ≈0.3 mag systematics, so the EW changes cannot yet be tied to line- versus continuum-driven variability. A DRW model for MBH≈4×10^7 M⊙ matches the sparse sampling, bolstering the AGN interpretation and motivating continued monitoring.

Key figures to inspect

• Figure 1: Compare the prism spectra of images A/B/C across the two NIRSpec epochs; note that the improved reductions now capture the full broad Hα profile, and the continuum-scaled zooms let you judge EW changes independent of magnification.
• Figure 2: Track the rest-frame EW evolution of Hα and Hβ between images (especially C→A), quantifying the ≈18% (Hα) and ≈22% (Hβ) drops over the 875 d rest-frame interval and the consistency between the two Balmer lines.
• Figure 3: Inspect the de-magnified, time-delay–corrected NIRCam light curves (with synthetic NIRSpec photometry) to see that scatter stays within ≈0.3 mag including systematics; also check the flagged lensing/contamination systematics, particularly for image B near a cluster galaxy.
• Figure 4: Examine DRW simulations for MBH≈4×10^7 M⊙ sampled at the actual cadence; the histograms show low probability of >0.3 mag events in F115W/F356W, explaining the nondetection of significant photometric variability.