We report the discovery of a transient seen in a strongly lensed arc at
redshift zs​=1.2567 in \emph{Hubble Space Telescope} imaging of the
Abell 370 galaxy cluster. The transient is detected at 29.51±0.14 AB mag in
a WFC3/UVIS F200LP difference image made using observations from two different
epochs, obtained in the framework of the \emph{Flashlights} program, and is
also visible in the F350LP band (mF350LP​∼30.53 AB). The transient
is observed on the negative-parity side of the critical curve at a distance of
∼0.6′′ from it, greater than previous examples of lensed stars. The large
distance from the critical curve yields a significantly smaller
macro-magnification, but our simulations show that bright, O/B-type supergiants
can reach sufficiently high magnifications to be seen at the observed position
and magnitude. In addition, the observed transient image is a trailing image
with an observer-frame time delay of ∼+0.8 days from its expected
counterpart, so that any transient lasting for longer than that should have
also been seen on the minima side and is thus excluded. This, together with the
blue color we measure for the transient (mF200LP​−mF350LP​∼[−0.7,−1] AB mag), rules out most other transient candidates such as
(kilo)novae, for example, and makes a lensed star the prime candidate. Assuming
the transient is indeed a lensed star as suggested, many more such events
should be detected in the near future in cluster surveys with the \emph{Hubble
Space Telescope} and \emph{James Webb Space Telescope}.Comment: 11 pages. 5 figures. Comments are welcom