The Type Ia supernova 2021hem:A 2003fg-like event in an apparently hostless environment

We report observations of a Type Ia supernova (SN Ia) 2021hem that was discovered within 48 hours of last nondetection and is located in an apparently hostless environment. With a peak absolute B-band magnitude of M B, max = - 19.96 ± 0.29 mag, SN 2021hem lies at the luminous end of the SNe Ia distribution. Its near-infrared and i-band light curves lack the secondary maximum, which is otherwise ubiquitous to normal and 1991T-like SNe Ia. Instead, these properties cause SN 2021hem to closely resemble 2003fg-like events. The slowly evolving light curves (characterized by Δm 15 (B) = 1.02 ± 0.02 mag; s BV = 0.94 ± 0.05) and the earliest spectrum showing C IIλ6580 and λ7235 absorption lines further support this classification. Other spectroscopic features, including Si II line diagnostics, resemble those of normal SNe Ia. A fit of a fireball model to the early-time light curves yields a time of first light of t first = - 16.43 +0.45 - 0.38 days relative to B-band maximum. The first photometric detection occurs 1.51 +0.45 - 0.38 days before the onset of fireball-like flux rise. This early emission, together with the intrinsic (g - r) 0 color, is inconsistent with circumstellar or companion interaction. Instead, shallow 56 Ni mixing or an asymmetric 56 Ni distribution offers a plausible explanation for the delayed onset of the fireball flux rise, while a double-detonation scenario with a thin helium shell remains a less likely alternative. Notably, SN 2021hem represents the fifth known 2003fg-like SN that has early-time activity or excess flux emission. The estimated mass of radioactive 56 Ni synthesized in SN 2021hem is 1.00 ± 0.09 M ⊙ . Deep GTC imaging obtained 2.5 years after the explosion, with an estimated limiting magnitude of m lim, r = 24.4 mag and a surface-brightness limit of μ lim, r = 26.3 mag arcsec - 2 , revealed no coincident host. Most faint dwarf and ultradiffuse galaxies (UDGs) are therefore ruled out. Alternatively, if the nearest plausible AGN host galaxy located at a projected distance of 104 kpc ...