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