Investigating photometric and spectroscopic variability in the multiply imaged little red dot A2744-QSO1

¹ Department of Physics, Ben-Gurion University of the Negev, P.O. Box 653, Be’er-Sheva 84105, Israel
² Center for Computational Astrophysics, Flatiron Institute, New York, NY 10010, USA
³ Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544, USA
⁴ Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Melbourne, VIC 3122, Australia
⁵ Department of Physics & Astronomy and PITT PACC, University of Pittsburgh, Pittsburgh, PA 15260, USA
⁶ Department of Astronomy, The University of Texas at Austin, Austin, TX 78712, USA
⁷ Institut d’Astrophysique de Paris, CNRS, Sorbonne Université, 98bis Boulevard Arago, 75014 Paris, France
⁸ Cosmic Dawn Center (DAWN), Copenhagen, Denmark
⁹ Niels Bohr Institute, University of Copenhagen, Jagtvej 128, Copenhagen, Denmark
¹⁰ Department of Astronomy, University of Massachusetts, Amherst, MA 01003, USA
¹¹ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands
¹² Department of Astrophysics, Universität Zürich, Winterthurerstrasse 190, CH-8044 Zurich, Switzerland
¹³ Department of Astronomy & Astrophysics, The Pennsylvania State University, University Park, PA 16802, USA
¹⁴ Institute for Computational & Data Sciences, The Pennsylvania State University, University Park, PA 16802, USA
¹⁵ Institute for Gravitation and the Cosmos, The Pennsylvania State University, University Park PA 16802, USA
¹⁶ Institute of Science and Technology Austria (ISTA), Am Campus 1, 3400 Klosterneuburg, Austria
¹⁷ MIT Kavli Institute for Astrophysics and Space Research, 70 Vassar Street, Cambridge, 02139 Massachusetts, USA
¹⁸ Department for Astrophysical and Planetary Science, University of Colorado, Boulder, CO 80309, USA
¹⁹ Département d’Astronomie, Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland
²⁰ Department of Physics and Astronomy, Tufts University, 574 Boston Ave., Medford MA 02155, USA

^⋆ Corresponding author: furtak@post.bgu.ac.il

Received: 12 February 2025
Accepted: 2 May 2025

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 (AGNs). 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 Hα and Hβ lines, which are independent of magnification and other systematics, to exhibit significant variations, of up to 18 ± 3% for Hα and up to 22 ± 8% in Hβ, 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 variability model for an AGN such as A2744-QSO1 (M_BH = 4 × 10⁷ M_⊙) 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.