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

TL;DR

This study discovers gravitationally lensed 'little red dots' at high redshift showing long-term variability, indicating they are a new class of active galactic nuclei with stellar-like envelopes, revealed through Einstein cross observations.

Contribution

It provides the first evidence of long-term variability in LRDs, suggesting they are a distinct AGN class with massive, extended envelopes, observed via gravitational lensing and Einstein cross configurations.

Findings

LRDs are gravitationally lensed at z~4.3.

One LRD shows 0.7 mag brightness variations over decades.

Variability is consistent with temperature changes in a massive photosphere.

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.