The Eye of Sauron in SN 2025ngs: a Short-plateau Cousin of SN 1998S with Evidence for a Ring-like Circumstellar Medium

TL;DR

SN 2025ngs is a peculiar supernova exhibiting a short-plateau light curve, complex spectral features, and evidence of a ring-like circumstellar medium, bridging characteristics of type IIn and IIP supernovae.

Contribution

This study presents the first detailed analysis of SN 2025ngs, revealing a ring-like CSM and spectral similarities to SN 1998S, expanding understanding of supernova diversity.

Findings

SN 2025ngs shows a 70-day plateau with re-emerging interaction features.

High-resolution spectra reveal a double-horned Hα profile indicating a disk-like CSM.

SN 2025ngs is spectroscopically similar to SN 1998S despite photometric differences.

Abstract

Interacting supernovae probe the twilight years of massive stars, exhibiting signatures of interaction between the supernova ejecta and surrounding material expelled from the progenitor. We present the peculiar interacting supernova, SN\,2025ngs in NGC5961 (37.8 Mpc). This transient toes the line between strongly interacting supernovae (type IIn) and type IIP supernovae. SN 2025ngs presents photometrically as a short-plateau supernova, with a plateau duration, t$_{\mathrm{PT}}^{}\approx70$ days. Interaction features subside within a week post-explosion, consistent with the growing number of flash supernovae, giving way to a short period where a typical IIP spectrum is exhibited. Towards the drop off the plateau, interaction features re-emerge, exhibiting complex H$\alpha$ profiles throughout the rest of the transient evolution. We compare with models of early spectra, finding the abundances generally consistent with a supergiant progenitor with a high mass-loss rate (10$^{-3}$ M$_\odot$ yr$^{-1}$). Early, high-resolution spectra reveal a double-horned H$\alpha$ profile, providing strong evidence for shock interaction with a proximate disk-like circumstellar medium. Spectroscopically, SN 2025ngs closely resembles the luminous SN 1998S, despite photometric differences, with SN 2025ngs having a relatively modest peak magnitude of $M_\mathrm{V}=-17.9$ mag, adding another member to the surprisingly diverse 98S-like group.