SN 2025coe: A Multiple-peaked Calcium-strong Transient from a White-dwarf Progenitor

Abstract SN 2025coe is a calcium-strong transient located at an extremely large projected offset ∼39.3 kpc from the center of its host, the nearby early-type galaxy NGC 3277 at a distance of ∼25.5 Mpc. In this paper, we present multiband photometric and spectroscopic observations spanning ∼100 days postdiscovery. Its multiband light curves display multiple distinct peaks: (1) an initial peak at t ≈ 1.6 day attributed to shock cooling emission, (2) a secondary peak of M R , peak ≈ −15.8 mag at t ≈ 10.2 days powered by radioactive decay, and (3) a possible late-time bump at t ≈ 42.8 days likely caused by ejecta–circumstellar material/clump interaction. Spectral evolution of SN 2025coe reveals a fast transition to the nebular phase within 2 months, where it exhibits an exceptionally high [Ca II ]/[O I ] ratio larger than 6. Modeling of the bolometric light curve suggests an ejecta mass of M ej = 0.2 9 − 0.15 + 0.14 M ⊙ , a 56 Ni mass of M 56 Ni = 2 . 4 − 0.05 + 0.06 × 1 0 − 2 M ⊙ , and a progenitor envelope with mass M e = 1 . 4 − 1.2 + 6.9 × 1 0 − 3 M ⊙ and radius R e = 13 . 5 − 11.1 + 64.1 R ⊙ . The tidal disruption of a hybrid HeCO white dwarf (WD) by a low-mass CO WD provides a natural explanation for the low ejecta mass, the small fraction of 56 Ni, and the presence of an extended, low-mass envelope.