Abstract Supernova (SN) 2025coe at a distance of ∼25 Mpc is the second-closest calcium-strong transient. It was discovered at a large projected offset of ∼34 kpc from its potential host galaxy NGC 3277. Multiband photometry of SN 2025coe indicates the presence of two peaks at day ∼2 and day ∼11 after explosion. Modeling the bolometric light curve, we find that the first peak can be reproduced either by shock cooling of a compact envelope ( R env ≈6–40 R ⊙ ; M env ≈0.1–0.2 M ⊙ ) or by interaction with close-in circumstellar material (CSM; R CSM ≲ 6 × 10 14 cm), or a combination of both. The second peak is dominated by radioactive decay of 56 Ni ( M ej ≈ 0.4–0.5 M ⊙ ; M 56 Ni ≈ 1.4 × 1 0 − 2 M ⊙ ). SN 2025coe rapidly evolves from the photospheric phase dominated by He I P Cygni profiles to nebular phase spectra dominated by strong Ca ii λλ 7291, 7323 and weak O i λλ 6300, 6364 emission lines. Simultaneous line profile modeling of Ca ii and O i at nebular phases shows that an asymmetric core-collapse explosion of a low-mass (≲3.3 M ⊙ ) He-core progenitor can explain the observed line profiles. Alternatively, lack of local star formation at the site of the SN explosion combined with a low ejecta mass is also consistent with a thermonuclear explosion due to a low-mass hybrid He-C/O white dwarf +C/O white dwarf merger.
Ravi et al. (2026) studied this question.