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

TL;DR

This study uses gravitational lensing time delays to detect spectroscopic variability in a high-redshift object, providing strong evidence that the Little Red Dot A2744-QSO1 is an active galactic nucleus (AGN).

Contribution

It demonstrates the first detection of spectroscopic variability in a high-redshift LRD, confirming its AGN nature through lensing-induced time delay analysis.

Findings

Significant EW variations in broad Hα and Hβ lines over 2.4 years rest-frame.

No significant photometric variability detected beyond systematic uncertainties.

Results align with a damped random walk model typical of AGN variability.

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$\alpha$ and H$\beta$ lines, which are independent of magnification and other systematics, to exhibit significant variations, up to $18\pm3$ % for H$\alpha$ and up to $22\pm8$ % in H$\beta$, 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.