Little red dot variability over a century reveals black hole envelope via a giant Einstein cross

Zijian Zhang, Mingyu Li, Masamune Oguri, Xiaojing Lin, Kohei Inayoshi, Catherine Cerny, Dan Coe, Jose M. Diego, Seiji Fujimoto, Linhua Jiang, Guillaume Mahler, Jorryt Matthee, Rohan P. Naidu, Keren Sharon, Yue Shen, Adi Zitrin, Abdurro'uf, Hollis Akins, Joseph F. V. Allingham, Ricardo Amorín, Yoshihisa Asada, Hakim Atek, Franz E. Bauer, Maruša Bradač, Larry D. Bradley, Zheng Cai, Sebastiano Cantalupo, Christopher Conselice, Liang Dai, Pratika Dayal, Eiichi Egami, Daniel J. Eisenstein, Andreas L. Faisst, Xiaohui Fan, Qinyue Fei, Brenda L. Frye, Yoshinobu Fudamoto, Lukas J. Furtak, Miriam Golubchik, Mauro González-Otero, Yuichi Harikane, Tiger Yu-Yang Hsiao, Yolanda Jiménez-Teja, Jeyhan S. Kartaltepe, Tomokazu Kiyota, Anton M. Koekemoer, Kotaro Kohno, Vasily Kokorev, Nimisha Kumari, Ivo Labbe, Claudia D. P. Lagos, Conor Larison, Yongming Liang, Ray A. Lucas, Jianwei Lyu, Nicholas S. Martis, Georgios E. Magdis, Matteo Messa, Minami Nakane, Gaël Noirot, Rafael Ortiz, Masami Ouchi, Justin D. R. Pierel, Marc Postman, Naveen Reddy, Massimo Ricotti, Daniel Schaerer, Raffaella Schneider, Charles C. Steidel, Wei Leong Tee, Roberta Tripodi, James A. A. Trussler, Hiroya Umeda, Francesco Valentino, Eros Vanzella, Feige Wang, Rogier Windhorst, Yunjing Wu, Zihao Wu, Hiroto Yanagisawa, Jinyi Yang, Fengwu Sun

First listed 2025-12-04 | Last updated 2025-12-04

Abstract

"Little red dots" (LRDs) represent a new population of astronomical objects uncovered by JWST whose nature remains debated. Although many LRDs are suspected as active galactic nuclei (AGN), they show little variability on days-years timescales. We report the discovery of two gravitationally lensed LRDs at redshift $\sim$4.3 behind the cluster RXCJ2211-0350, one of which (RX1) is quadruply imaged with time delays spanning $\sim$130 years. RX1 exhibits intrinsic color and brightness variations of up to 0.7 magnitude among its images. These changes are consistent with blackbody-temperature variations of a photosphere, indicating long-term variability analogous to Cepheid-like pulsations but in a far more extended ($R \sim 2000$ AU) and massive ($M \gtrsim 10^6 \, M_{\odot}$) systems. These results suggest LRDs as a distinct class of AGN with stellar-like envelopes.

Short digest

Two lensed little red dots are found at z ~ 4.3 behind RXCJ2211–0350, with R2211‑RX1 forming a near‑perfect Einstein cross whose images have ~130‑year time delays. Inter‑image color and brightness offsets up to 0.7 mag encode intrinsic, century‑scale variability best explained by blackbody temperature changes in a photospheric envelope. Modeling implies a vast envelope (R ~ 2000 AU, M ≳ 10^6 Msun) around a central black hole, producing Cepheid‑like, long‑timescale variations. This positions LRDs as a distinct AGN class with stellar‑like envelopes and uses strong lensing to time‑slice their evolution.

Key figures to inspect

• Figure 1: Einstein cross of R2211‑RX1 — inspect the four image parities, placement relative to critical curves, and the predicted ~130‑yr arrival‑time spread that yields a century‑baseline variability test.
• Figures S1–S2: 10‑band NIRCam imaging/SEDs from VENUS (PID 6882; 2025‑10‑16) — verify the V‑shaped continuum and compact morphology defining the LRDs; compare filter‑by‑filter fluxes across images.
• RX1 color–magnitude/SED‑fit panel — check the up‑to‑0.7 mag inter‑image differences and the blackbody‑fit temperature shifts driving the color changes; note how alternatives (e.g., extinction/microlensing) are addressed if shown.
• Lens model maps and source‑plane reconstruction (incl. Fig. S4) — read off caustics, magnifications, and time‑delay contours that produce the giant cross and century‑scale delays.
• Envelope constraint plot — locate the inferred R ~ 2000 AU and M ≳ 10^6 Msun and the implied dynamical timescale supporting a Cepheid‑like envelope around the black hole.