2025ApJ...980...48M

We report the results from a study of two massive (M_500c > 6.0 × 10¹⁴M_⊙) strong-lensing clusters selected from the South Pole Telescope cluster survey for their large Einstein radius (R_E > 40″), SPT-CL J2325-4111 and SPT-CL J0049-2440. Ground-based and shallow Hubble Space Telescope (HST) imaging indicated extensive strong-lensing evidence in these fields, with giant arcs spanning 18″ and 31″, respectively, motivating further space-based imaging follow-up. Here, we present multiband HST imaging and ground-based Magellan spectroscopy of the fields, from which we compile detailed strong-lensing models. The lens models of SPT-CL J2325-4111 and SPT-CL J0049-2440 were optimized using nine and eight secure multiply imaged systems with a final image-plane rms of 0$\mathop{.}\limits^{^{\prime\prime} }$63 and 0$\mathop{.}\limits^{^{\prime\prime} }$73, respectively. From the lensing analysis, we measure a projected mass density within 500 kpc of M(<500 kpc) = (7.30 ± 0.07) × 10¹⁴M_⊙ and $M(\lt 500\,\mathrm{kpc})=7.1{2}_{-0.19}^{+0.16}\times 1{0}^{14}$M_⊙ for these two clusters, and subhalo mass ratios of 0.12 ± 0.01 and $0.2{1}_{-0.05}^{+0.07}$, respectively. Both clusters produce a large area with high magnification (μ ≥ 3) for a source at z = 9, ${{ \mathcal A }}_{|\mu |\geqslant 3}^{{\rm{lens}}}=4.9{3}_{-0.04}^{+0.03}$arcmin² and ${{ \mathcal A }}_{|\mu |\geqslant 3}^{{\rm{lens}}}=3.6{4}_{-0.10}^{+0.14}$arcmin², respectively, placing them in the top tier of strong-lensing clusters. We conclude that these clusters are spectacular sightlines for further observations that will reduce the systematic uncertainties due to cosmic variance. This paper provides the community with two additional well-calibrated cosmic telescopes, as strong as the Frontier Fields and suitable for studies of the highly magnified background Universe.