Strongly lensed supernovae in focus: deblending marginally resolved lenses via joint modeling of ground and space-based imaging

Abstract

The Rubin Observatory Legacy Survey of Space and Time (LSST) is projected to discover hundreds of gravitationally lensed Type Ia supernovae (glSNe), offering an unprecedented opportunity for time-delay cosmography and independent measurements of the Hubble constant. However, the majority of Rubin-discovered glSNe will be marginally resolved in ground-based imaging, making accurate deblending of the multiply-imaged supernovae from the foreground lens and host galaxy a critical challenge. We present a new framework to extract light curves of the multiply-imaged SNe from blended glSNe systems via joint forward modeling of multi-epoch, multi-band high-resolution space-based imaging and ground-based imaging using \texttt{Scarlet2}, combined with Gaussian Process time-delay inference using \texttt{GausSN}. Using pixel-level simulations of Rubin and Roman imaging of typical glSNe systems from the Goldstein et al. 2019 catalog across a range of angular separations, we quantify pipeline performance as a function of source blending and space-based follow-up availability. We find that Rubin-only observations are sufficient to recover time-delays with sub-day mean errors, outperforming configurations that include supplemental Roman epochs. SN image separations are recovered to $\sim 10^{-2}$ arcsecond precision even when sources are initialized with significant spatial offsets. These results suggest transient light curves, time-delays, and image positions can be extracted from Rubin data alone, while more accurate lens mass modeling can be performed separately using archival high-resolution imaging, reducing the need for prompt space-based follow-up and substantially increasing the number of systems suitable for time-delay cosmography.