# Highly Magnified Stars in Lensing Clusters: New Evidence in a Galaxy Lensed by MACS J0416.1-2403

## TL;DR

This paper presents evidence of a highly magnified star undergoing microlensing in a galaxy cluster, demonstrating the potential to study dark matter structure through caustic-transiting stars.

## Contribution

It provides the first detailed analysis of microlensing effects on a caustic-straddling arc, revealing a candidate star with significant magnification and variability, and discusses implications for dark matter studies.

## Key findings

- Detection of asymmetric surface brightness anomalies consistent with microlensing
- Identification of a candidate blue supergiant star with ~200 magnification
- Tentative evidence of increased mismatch near the critical line

## Abstract

We examine a caustic-straddling arc at $z=0.9397$ in the field of the galaxy cluster MACS J0416.1-2403 ($z=0.397$) using archival multiband HST images and show that its surface brightness exhibits anomalies that can be explained by a single highly magnified star undergoing microlensing. First, we show that the surface brightness pattern is not perfectly symmetric across the cluster critical curve, which is inconsistent with a locally smooth lens model; the location of the candidate star exhibits the most significant asymmetry. Second, our analysis indicates that the asymmetric feature has $\sim 30\%$ higher flux in the 2012 visits compared to the Frontier Fields program visits in 2014. Moreover, the variable asymmetric feature shows an anomalous color between the F814W and F105W filters in 2014. These anomalies are naturally explained by microlensing induced variability of a caustic-transiting blue supergiant in a star-forming region, with a mean magnification factor around $\mu \sim 200$. We extend this study to a statistical analysis of the whole arc image and find tentative evidence of the increased mismatch of the two images in the proximity of the critical line. Robust detection of one or multiple caustic-transiting stars in this arc will enable detailed follow-up studies that can shed light on the small-scale structure of the dark matter inside the cluster halo.