SN 2021wuf: A transitional type Ia supernova with a low-velocity gradient

Abstract

In this paper, we present an extensive analysis of SN 2021 wuf, a transition between Ia-norm and SN 1991T-like supernovae, which exploded at the periphery of the tidal bridge between the pair galaxy NGC 6500 and NGC 6501, at a redshift of z = 0.01. Our observations, ranging from −21 to +276 days relative to the B-band maximum light, reveal that SN 2021wuf exhibits properties akin to normal SNe Ia, with a peak absolute magnitude of Mmax(B) ∼ − 19.49 ± 0.10 mag and a post-peak decline rate of Δm15(B) ∼ 1.11 ± 0.06 mag. The peak bolometric luminosity of this SN is estimated as 1.58 × 1043 erg s−1, corresponding to a 56Ni mass of MNi ∼ 0.64 ± 0.05 M⊙. The spectral features, including high-velocity Si IIλ6355 lines, a plateau in the Si IIλ6355 velocity evolution and the nickel-to-iron ratio in the nebular phase, suggest a potential pulsating delayed detonation mechanism. The absence of intermediate-mass elements in the early phase and the high photospheric temperature, as inferred from the line-strength ratio of Si IIλ5972 to Si IIλ6355 (named as R(Si II)), further support this classification.