The Type Ia Supernova 2021hem: A 2003fg-like Event in an Apparently Hostless Environment

Abstract

We report observations of Type Ia SN 2021hem, located in an apparently hostless environment. With a peak absolute B-band magnitude of -19.96 mag, and a lack of secondary maximum in near-infrared and i-band light curves make it resemble 2003fg-like events. The slowly evolving light curves, and the earliest spectrum showing CII absorption lines, further support this classification. Fireball model fit to early light curves yields a time of first light of -16.43 days relative to B-band maximum. The first detection occurs 1.51 days before the onset of the fireball-like flux rise. This early emission, and $(g-r{)}_0$ color, is inconsistent with circumstellar or companion interaction. Instead, shallow ${}^{56}$Ni mixing or an asymmetric ${}^{56}$Ni distribution offers a plausible explanation. SN2021hem is the fifth known 2003fg-like SN with early-time excess flux emission. The estimated mass of radioactive ${}^{56}$Ni in SN2021hem is $1.00\pm 0.09M_{\odot }$. Deep GTC imaging obtained 2.5 yr after the explosion (with $m_{lim,r}=24.4$ mag and ${\mu }_{lim,r}=26.3\mathrm{mag}\mathrm{arse}{\mathrm{c}}^{-2}$), reveals no coincident host, thereby ruling out most faint dwarf and UDGs. Alternatively, assuming the nearest plausible AGN host galaxy, at a distance of 104 kpc, implies a hyper-velocity progenitor ejected at $\sim$2200 km/s by AGN interaction. A faint diffuse feature ~6 kpc from the SN site has also been detected in the image, with its surface brightness of a UDGs. However, it is unclear whether it is a galaxy and is associated with SN2021hem. Considering its large normalized directional light distance ($d_{DLR}\sim 3-4$) from SN, and its unusual elongation, it is a candidate of low probability to be the host galaxy of SN2021hem. These results identify SN2021hem as one of the strongest candidates for a hostless SN Ia, underscoring the diversity of luminous, slowly evolving, 2003fg-like explosions.

22pages, 15 figures, accepted for publication in Astronomy & Astrophysics

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