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

Bose, S (ORCID:0000000335293854); Stritzinger, M D (ORCID:0000000255711833); Malmgaard, A; Miller, C J; Elias-Rosa, N (ORCID:0000000213819125); Fynbo, J_P U (ORCID:0000000281498298); Ashall, C; Burns, C R (ORCID:0000000346256629); DerKacy, J M (ORCID:0000000275666080); Galbany, L (ORCID:0000000212966887); Gutiérrez, C P (ORCID:0000000323752064); Hoogendam, W B (ORCID:0000000339539532); Hsiao, E Y (ORCID:0000000310392928); Jensen, E_A M (ORCID:0000000331973430); Medler, K (ORCID:000000017186105X); Alburai, A (ORCID:0009000727315562); Anderson, J (ORCID:0000000302273451); Baron, E (ORCID:0000000153931608); Duarte, J (ORCID:0000000218233860); Gromadzki, M (ORCID:0000000216501518); Inserra, C (ORCID:0000000239684409); Mazzali, P A; Müller-Bravo, T E (ORCID:0000000339397167); Lundqvist, P (ORCID:0000000236648082); Reguitti, A (ORCID:0000000342542724); Salmaso, I (ORCID:0000000314500869); Sand, D J (ORCID:000000034102380X); Valerin, G (ORCID:0000000233344585)

doi:10.1051/0004-6361/202558053

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 absoluteB-band magnitude ofM_B, max = −19.96 ± 0.29 mag, SN 2021hem lies at the luminous end of the SNe Ia distribution. Its near-infrared andi-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 Δm₁₅(B) = 1.02 ± 0.02 mag;s_BV = 0.94 ± 0.05) and the earliest spectrum showing C IIλ6580 andλ7235 absorption lines further support this classification. Other spectroscopic features, including Si IIline 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 oft_first= −16.43^+0.45_−0.38days relative toB-band maximum. The first photometric detection occurs 1.51^+0.45_−0.38days before the onset of fireball-like flux rise. This early emission, together with the intrinsic (g − r)₀color, is inconsistent with circumstellar or companion interaction. Instead, shallow⁵⁶Ni mixing or an asymmetric⁵⁶Ni 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 radioactive⁵⁶Ni 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 ofm_lim, r = 24.4 mag and a surface-brightness limit ofμ_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 is assumed, the progenitor would need to be a hypervelocity star ejected at ≈2200 km s⁻¹from the host by AGN interaction. A faint diffuse feature ≈6 kpc from the SN site has also been detected in the GTC image, and its surface brightness is within the limits of UDGs. It is unclear whether it is a galaxy and is associated with SN 2021hem, however. Based on its large normalized directional light distance (d_DLR ≈ 3 − 4) from the SN and its unusual elongation, the probability that this is the candidate host galaxy of SN 2021hem is low. These results identify SN 2021hem as one of the strongest candidates for a hostless SN Ia and underscore the diversity of luminous slowly evolving 2003fg-like explosions and the wide range of environments in which they may occur.

More Like this