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

¹ Department of Physics and Astronomy, Aarhus University Ny Munkegade 120 DK-8000 Aarhus C, Denmark
² Department of Astronomy, University of Michigan Ann Arbor MI 48109, USA
³ INAF – Osservatorio Astronomico di Padova Vicolo dell’Osservatorio 5 I-35122 Padova, Italy
⁴ Cosmic DAWN Center, Niels Bohr Institute, University of Copenhagen Lyngbyvej 2 2100 Copenhagen Ø, Denmark
⁵ Institute for Astronomy, University of Hawai’i 2680 Woodlawn Drive Honolulu HI 96822, USA
⁶ Observatories of the Carnegie Institution for Science 813 Santa Barbara St Pasadena CA 91101, USA
⁷ Space Telescope Science Institute 3700 San Martin Drive Baltimore MD 21218-2410, USA
⁸ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans s/n E-08193 Barcelona, Spain
⁹ Institut d’Estudis Espacials de Catalunya (IEEC) E-08034 Barcelona, Spain
¹⁰ Department of Physics, Florida State University 77 Chieftan Way Tallahassee FL 32306, USA
¹¹ Department of Physics, Chalmers University of Technology SE-412 96 Göteborg, Sweden
¹² European Southern Observatory Alonso de Córdova 3107 Casilla 19 Santiago, Chile
¹³ Planetary Science Institute, 1700 East Fort Lowell Road Suite 106 Tucson AZ 85719-2395, USA
¹⁴ CENTRA, Instituto Superior Técnico, Universidade de Lisboa Av. Rovisco Pais 1 1049-001 Lisboa, Portugal
¹⁵ Astronomical Observatory, University of Warsaw Al. Ujazdowskie 4 00-478 Warszawa, Poland
¹⁶ Cardiff Hub for Astrophysics Research and Technology, School of Physics & Astronomy, Cardiff University Queens Buildings The Parade Cardiff CF24 3AA, UK
¹⁷ Astrophysics Research Institute, Liverpool John Moores University, IC2, Liverpool Science Park 146 Brownlow Hill Liverpool L3 5RF, UK
¹⁸ School of Physics, Trinity College Dublin, The University of Dublin Dublin 2, Ireland
¹⁹ Instituto de Ciencias Exactas y Naturales (ICEN), Universidad Arturo Prat Iquique, Chile
²⁰ The Oskar Klein Centre, Department of Physics, Stockholm University AlbaNova 10691 Stockholm, Sweden
²¹ INAF – Osservatorio Astronomico di Capodimonte Salita Moiariello 16 80131 Napoli, Italy
²² Steward Observatory, University of Arizona 933 North Cherry Avenue Tucson AZ 85721-0065, USA

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Received: 10 November 2025
Accepted: 24 November 2025

Abstract

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 Δm₁₅(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 ⁵⁶Ni mixing or an asymmetric ⁵⁶Ni 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 ⁵⁶Ni 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 is assumed, the progenitor would need to be a hypervelocity star ejected at ≈2200 km s⁻¹ from 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.