A multiwavelength view of the nearby Calcium-Strong Transient SN 2025coe in the X-Ray, Near-Infrared, and Radio Wavebands

Abstract

Calcium-strong transients (CaSTs) are a subclass of faint and rapidly evolving supernovae (SNe) that exhibit strong calcium features and notably weak oxygen features. The small but growing population of CaSTs exhibits some aspects similar to thermonuclear supernovae but others similar to massive star core-collapse events, leading to intriguing questions on their physical origins. SN~2025coe is one of the nearest CaSTs discovered to date, and our coordinated multi-wavelength observations obtained days to weeks post-explosion reveal new insights on these enigmatic transients. With the most robust NIR spectroscopic time-series of a CaST collected to date, SN 2025coe shows spectral signatures characteristic of Type Ib SNe (i.e. He-rich stripped-envelope SNe). SN~2025coe is the third X-ray detected CaST and our analysis of \textit{Swift} X-ray data suggest interaction with 0.12 $\pm 0.11M_{\odot }$ of circumstellar material (CSM) extending to at least $2\times 10^{15}$cm ($\sim 30,000R_{\odot }$, while our analysis of the 1--240 GHz radio non-detections gives an outer radius of that CSM of at most $\sim 5\times 10^{15}$ cm. This inferred nearby high-density CSM extending out to $(3.5\pm 1.5)\times 10^{15}$ cm is similar to that seen in the other two X-ray detected CaSTs, and its presence suggests that either intensive mass-loss from a massive star or some exotic pre-supernova mass ejection may be a common feature of this subclass. Our work also expands upon recent studies on the optical properties of SN 2025coe and explores our current understanding of different progenitor systems that could possibly produce CaSTs.

submitted to ApJ on Jan 26, 2026 accepted for publication May 14, 2026