SN 2025coe: A Multiple-Peaked Calcium-Strong Transient from A White-Dwarf Progenitor

TL;DR

This study addresses calcium-strong transients (CaSTs), focusing on SN 2025coe, a nearby multi-peaked CaST with a large offset from its host. It combines a ~100-day, multi-band photometric and spectroscopic campaign, including Swift/UVOT ToO data, with bolometric light-curve modeling to extract physical ejecta parameters. The analysis reveals three peaks in the light curve, a rapid transition to the nebular phase with a high [Ca II]/[O I] ratio, and a best-fit ejecta mass of $M_{ m ej} = 0.29^{+0.14}_{-0.15}\,M_\odot$, a $^{56}$Ni mass of $M_{^{56}{\rm Ni}} = 2.4^{+0.06}_{-0.05}\times 10^{-2}\,M_\odot$, and an extended, low-mass envelope with $M_e = 1.4^{+6.9}_{-1.2}\times 10^{-3}\,M_\odot$ and $R_e = 13.5^{+64.1}_{-11.1}\,R_\odot$. The authors propose a tidal-disruption scenario in which a hybrid HeCO WD is disrupted by a low-mass CO WD, naturally accounting for the low ejecta mass, the small $^{56}$Ni fraction, and the presence of the extended envelope, with implications for CaST progenitor channels and their environmental dependence.

Abstract

SN 2025coe is a calcium-strong transient located at an extremely large projected offset $\sim$39.3 kpc from the center of its host, the nearby early-type galaxy NGC 3277 at a distance of $\sim$25.5 Mpc. In this paper, we present multi-band photometric and spectroscopic observations spanning $\sim$100 days post-discovery. Its multi-band light curves display {multiple} distinct peaks: (1) an initial peak at $t \approx 1.6$ days attributed to shock cooling emission, (2) a secondary peak of $M_{R, \, peak} \approx$ $-$15.8 mag at $t \approx 10.2$ days powered by radioactive decay, and (3) a {possible} late-time bump at $t \approx 42.8$ 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 $M_{\rm ej} = 0.29^{+0.14}_{-0.15} \, M_{\odot}$, a $^{56}$Ni mass of $M_{\rm ^{56}Ni} = 2.4^{+0.06}_{-0.05} \times 10^{-2} M_{\odot}$, and a progenitor envelope with mass $M_e = 1.4^{+6.9}_{-1.2} \times 10^{-3} \, M_{\odot}$ and radius $R_e = 13.5^{+64.1}_{-11.1} \, R_{\odot}$. 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.

Figures (2)

• Figure 1: SN 2025coe and its host galaxy NGC 3277. The image is a composite of BV$R$-band observations obtained by TRT at $t = 1.7$ days after the discovery. Red cross marks the location of SN 2025coe.
• Figure 2: The multi-band light curves of SN 2025coe are shown with phases relative to the first clear-band detection (MJD 60730.6). Gray dashed lines mark the epochs corresponding to the multiple photometric peaks. No extinction corrections are applied.