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arXiv · 2511.07529

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

Subhash Bose·M. D. Stritzinger·A. Malmgaard·C. J. Miller·N. Elias-Rosa·J. P. U. Fynbo·C. Ashall·C. R. Burns·J. M. DerKacy·L. Galbany·C. P. Gutiérrez·W. B. Hoogendam·E. Y. Hsiao·E. A. M. Jensen·K. Medler·Alaa Alburai·J. Anderson·E. Baron·J. Duarte·M. Gromadzki·C. Inserra·P. A. Mazzali·T. E. Müller-Bravo·P. Lundqvist·A. Reguitti·I. Salmaso·D. J. Sand·G. Valerin
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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 $1.00\pm0.09 M_\odot$. Deep GTC imaging obtained 2.5 yr after the explosion (with $m_{lim,r}=24.4$ mag and $μ_{lim,r} = 26.3\rm~mag~arsec^{-2}$), 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}\sim3-4$) 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.

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Subhash Bose, M. D. Stritzinger, A. Malmgaard, C. J. Miller, N. Elias-Rosa, J. P. U. Fynbo, C. Ashall, C. R. Burns, J. M. DerKacy, L. Galbany, C. P. Gutiérrez, W. B. Hoogendam, E. Y. Hsiao, E. A. M. Jensen, K. Medler, Alaa Alburai, J. Anderson, E. Baron, J. Duarte, M. Gromadzki, C. Inserra, P. A. Mazzali, T. E. Müller-Bravo, P. Lundqvist, A. Reguitti, I. Salmaso, D. J. Sand, G. Valerin. 2025-11-24. The Type Ia Supernova 2021hem: A 2003fg-like Event in an Apparently Hostless Environment. https://doi.org/10.1051/0004-6361%2F202558053

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