Determine the external-convergence correction from the lens environment

Determine the external convergence contributed by the surrounding galaxy-overdensity environment and line-of-sight matter distribution of SN 2025wny, in order to correct the mass-sheet degeneracy and obtain a robust cosmographic determination of H_0.

Background

The lens system appears associated with a galaxy-overdensity or possible cluster environment. Such surrounding matter can contribute an external convergence, represented through the mass-sheet transformation parameter xi, and can shift the inferred Hubble constant even when the observed image positions and shapes remain unchanged.

The paper adopts xi = 1 for its principal results and finds that current comparisons with external H_0 measurements are consistent with values near unity. Nevertheless, the authors state that the size of the environmental correction cannot be established from the overdensity alone. A dedicated characterization of the surrounding mass using spectroscopic and photometric observations is therefore needed.

References

However, the magnitude of this correction depends on the detailed distribution of matter along the line of sight and cannot be inferred reliably from the presence of a galaxy overdensity alone.

— Follow-up of SN 2025wny V: Lens Modelling and Cosmography of a Strongly Lensed Superluminous Supernova at $z = 2.015$ using Space Data  (2608.28430 - Mörtsell et al., 28 Aug 2026) in Section 4.3, “Impact of the mass-sheet degeneracy”; Section 5.3, “SN 2025wny as a cosmographic probe”