A Multiwavelength View of the Nearby Calcium-strong Transient SN 2025coe in the X-Ray, Near-infrared, and Radio Wavebands

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 SNe 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 multiwavelength observations obtained days to weeks postexplosion reveal new insights into these enigmatic transients. With the most robust near-IR (NIR) spectroscopic time series of a CaST collected to date, SN 2025coe shows spectral signatures characteristic of Type Ib SNe (SNe Ib; i.e., He-rich stripped-envelope SNe (SESNe)). SN 2025coe is the third X-ray-detected CaST and our analysis of Neil Gehrels Swift Observatory X-ray data suggests interaction with 0.12 ± 0.11 M ⊙ of circumstellar material (CSM) extending to at least 2 × 10 15 cm (∼30,000 R ⊙ ), while our analysis of the 1–240 GHz radio nondetections gives an outer radius of that CSM of at most ∼5 × 10 15 cm. This inferred nearby high-density CSM extending out to (3.5 ± 1.5) × 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-SN mass ejection may be a common feature of this subclass. Our work also expands upon recent studies of the optical properties of SN 2025coe and explores our current understanding of different progenitor systems that could possibly produce CaSTs.