Aravind P. Ravi, Sahana Kumar, Raphael Baer-Way, Stefano Valenti, M. Modjaz, Bart F. A. van Baal, Anders Jerkstrand, Yize 一泽 Dong 董, Lindsey A. Kwok, Jeniveve Pearson, David J. Sand, D. Hiramatsu, Alexei V. Filippenko, Jennifer Andrews, Moira Andrews, Prasiddha Arunachalam, K Azalee Bostroem, Thomas G. Brink, L Chen, Collin Christy, Kyle W. Davis, Ali Esamdin, Joseph Farah, Ryan J. Foley, Emily Hoang, G. Hosseinzadeh, D. Andrew Howell, Brian Hsu, Ruifeng Huang, Abdusamatjan Iskandar, Daryl Janzen, Saurabh W. Jha, Ravjit Kaur, M. Lundquist, Curtis McCully, Darshana Mehta, Nicolás Meza-Retamal, Yuan Qi Ni, Kishore C. Patra, Conor Ransome, Manisha Shrestha, Nathan Smith, Bhagya Subrayan, K. Taggart, Xiaofeng Wang, Kathryn Wynn, Shengyu Yan, Yi 轶 Yang 杨, Weikang Zheng, Dan Coe
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.