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

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 MR, 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 Mej=0.29−0.15+0.14M⊙ , a 56Ni mass of M56Ni=2.4−0.05+0.06×10−2M⊙ , and a progenitor envelope with mass Me=1.4−1.2+6.9×10−3M⊙ and radius Re=13.5−11.1+64.1R⊙ . 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 56Ni, and the presence of an extended, low-mass envelope.