We present a strong lensing analysis of COOL J1241+2219, the brightest known
gravitationally lensed galaxy at z≥5, based on new multi-band Hubble
Space Telescope (HST) imaging data. The lensed galaxy has a redshift of
z=5.043, placing it shortly after the end of the Epoch of Reionization, and an
AB magnitude z_AB=20.47 mag (Khullar et al. 2021). As such, it serves as a
touchstone for future research of that epoch. The high spatial resolution of
HST reveals internal structure in the giant arc, from which we identify 15
constraints and construct a robust lens model. We use the lens model to extract
cluster mass and lensing magnification. We find that the mass enclosed within
the Einstein radius of the z=1.001 cluster lens is
M(<5.77'')=1.079−0.007+0.023, significantly lower than other known
strong lensing clusters at its redshift. The average magnification of the giant
arc is =76−20+40, a factor of 2.4−0.7+1.4 greater
than previously estimated from ground-based data; the flux-weighted average
magnification is =92−31+37 We update the current
measurements of the stellar mass and star formation rate (SFR) of the source
for the revised magnification, log(M⋆/M⊙)=9.7±0.3 and SFR=10.3−4.4+7.0M⊙yr−1. The powerful lensing
magnification acting upon COOL J1241+2219 resolves the source and enables
future studies of the properties of its star formation on a clump-by-clump
basis. The lensing analysis presented here will support upcoming
multiwavelength characterization with HST and JWST data of the stellar mass
assembly and physical properties of this high-redshift lensed galaxy.Comment: Submitted to Ap