RELICS: Strong-lensing analysis of the massive clusters MACS
  J0308.9+2645 and PLCK G171.9-40.7
  Strong gravitational lensing by galaxy clusters has become a powerful tool
for probing the high-redshift Universe, magnifying distant and faint background
galaxies. Reliable strong lensing (SL) models are crucial for determining the
intrinsic properties of distant, magnified sources and for constructing their
luminosity function. We present here the first SL analysis of MACS J0308.9+2645
and PLCK G171.9-40.7, two massive galaxy clusters imaged with the Hubble Space
Telescope in the framework of the Reionization Lensing Cluster Survey (RELICS).
We use the Light-Traces-Mass modeling technique to uncover sets of multiply
imaged galaxies and constrain the mass distribution of the clusters. Our SL
analysis reveals that both clusters have particularly large Einstein radii
($\theta_E>30"$ for a source redshift of $z_s=2$), providing fairly large areas
with high magnifications, useful for high-redshift galaxy searches ($\sim2$
arcmin$^{2}$ with $\mu>5$ to $\sim1$ arcmin$^{2}$ with $\mu>10$, similar to a
typical \textit{Hubble Frontier Fields} cluster). We also find that MACS
J0308.9+2645 hosts a promising, apparently bright (J$\sim23.2-24.6$ AB),
multiply imaged high-redshift candidate at $z\sim6.4$. These images are amongst
the brightest high-redshift candidates found in RELICS. Our mass models,
including magnification maps, are made publicly available for the community
through the Mikulski Archive for Space Telescopes.
