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

Bose, S.; Stritzinger, Maximilian; Malmgaard, A.; Miller, C. J.; Elias-Rosa, Nancy; Fynbo, J.; Ashall, Chris; Burns, Christopher R.; DerKacy, J. M.; Galbany, Lluís; Gutiérrez, Claudia P.; Hoogendam, Willem B.; Hsiao, E. Y.; Jensen, Erik; Medler, Kyle; Alburai, Alaa; Anderson, Joseph P.; Baron, Edward; Duarte, J.; Gromadzki, Mariusz; Inserra, Cosimo; Mazzali, Paolo A.; Müller-Bravo, Tomás E.; Lundqvist, Peter; Reguitti, Andrea; Salmaso, Irene; Sand, David J.; Valerin, Giorgio

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

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 ea...

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Autores:	Bose, S., Stritzinger, Maximilian, Malmgaard, A., Miller, C. J., Elias-Rosa, Nancy, Fynbo, J., Ashall, Chris, Burns, Christopher R., DerKacy, J. M., Galbany, Lluís, Gutiérrez, Claudia P., Hoogendam, Willem B., Hsiao, E. Y., Jensen, Erik, Medler, Kyle, Alburai, Alaa, Anderson, Joseph P., Baron, Edward, Duarte, J., Gromadzki, Mariusz, Inserra, Cosimo, Mazzali, Paolo A., Müller-Bravo, Tomás E., Lundqvist, Peter, Reguitti, Andrea, Salmaso, Irene, Sand, David J., Valerin, Giorgio
Tipo de recurso:	artículo
Estado:	Versión publicada
Fecha de publicación:	2026
País:	España
Institución:	Consejo Superior de Investigaciones Científicas (CSIC)
Repositorio:	DIGITAL.CSIC. Repositorio Institucional del CSIC
OAI Identifier:	oai:dnet:digitalcsic_::573b2c66b9f14c42ef72f53082ab116f
Acceso en línea:	http://hdl.handle.net/10261/426195 http://arxiv.org/abs/2511.07529v2
Access Level:	acceso abierto
Palabra clave:	Supernovae: general Supernovae: individual: 2021hem

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network_acronym_str	ES
network_name_str	España
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dc.title.none.fl_str_mv	The Type Ia Supernova 2021hem: A 2003fg-like Event in an Apparently Hostless Environment
title	The Type Ia Supernova 2021hem: A 2003fg-like Event in an Apparently Hostless Environment
spellingShingle	The Type Ia Supernova 2021hem: A 2003fg-like Event in an Apparently Hostless Environment Bose, S. Supernovae: general Supernovae: individual: 2021hem
title_short	The Type Ia Supernova 2021hem: A 2003fg-like Event in an Apparently Hostless Environment
title_full	The Type Ia Supernova 2021hem: A 2003fg-like Event in an Apparently Hostless Environment
title_fullStr	The Type Ia Supernova 2021hem: A 2003fg-like Event in an Apparently Hostless Environment
title_full_unstemmed	The Type Ia Supernova 2021hem: A 2003fg-like Event in an Apparently Hostless Environment
title_sort	The Type Ia Supernova 2021hem: A 2003fg-like Event in an Apparently Hostless Environment
dc.creator.none.fl_str_mv	Bose, S. Stritzinger, Maximilian Malmgaard, A. Miller, C. J. Elias-Rosa, Nancy Fynbo, J. Ashall, Chris Burns, Christopher R. DerKacy, J. M. Galbany, Lluís Gutiérrez, Claudia P. Hoogendam, Willem B. Hsiao, E. Y. Jensen, Erik Medler, Kyle Alburai, Alaa Anderson, Joseph P. Baron, Edward Duarte, J. Gromadzki, Mariusz Inserra, Cosimo Mazzali, Paolo A. Müller-Bravo, Tomás E. Lundqvist, Peter Reguitti, Andrea Salmaso, Irene Sand, David J. Valerin, Giorgio
author	Bose, S.
author_facet	Bose, S. Stritzinger, Maximilian Malmgaard, A. Miller, C. J. Elias-Rosa, Nancy Fynbo, J. Ashall, Chris Burns, Christopher R. DerKacy, J. M. Galbany, Lluís Gutiérrez, Claudia P. Hoogendam, Willem B. Hsiao, E. Y. Jensen, Erik Medler, Kyle Alburai, Alaa Anderson, Joseph P. Baron, Edward Duarte, J. Gromadzki, Mariusz Inserra, Cosimo Mazzali, Paolo A. Müller-Bravo, Tomás E. Lundqvist, Peter Reguitti, Andrea Salmaso, Irene Sand, David J. Valerin, Giorgio
author_role	author
author2	Stritzinger, Maximilian Malmgaard, A. Miller, C. J. Elias-Rosa, Nancy Fynbo, J. Ashall, Chris Burns, Christopher R. DerKacy, J. M. Galbany, Lluís Gutiérrez, Claudia P. Hoogendam, Willem B. Hsiao, E. Y. Jensen, Erik Medler, Kyle Alburai, Alaa Anderson, Joseph P. Baron, Edward Duarte, J. Gromadzki, Mariusz Inserra, Cosimo Mazzali, Paolo A. Müller-Bravo, Tomás E. Lundqvist, Peter Reguitti, Andrea Salmaso, Irene Sand, David J. Valerin, Giorgio
author2_role	author author author author author author author author author author author author author author author author author author author author author author author author author author author
dc.contributor.none.fl_str_mv	Independent Research Fund Denmark Aarhus University Research Foundation Ministerio de Ciencia e Innovación (España) Agencia Estatal de Investigación (España) Consejo Superior de Investigaciones Científicas (España) Generalitat de Catalunya European Commission European Research Council Fundação para a Ciência e a Tecnologia (Portugal) Bose, S. [0000-0003-3529-3854] Stritzinger, Maximilian [0000-0002-5571-1833] Elias-Rosa, Nancy [0000-0002-1381-9125] Fynbo, J. [0000-0002-8149-8298] Galbany, Lluís [0000-0002-1296-6887] Consejo Superior de Investigaciones Científicas [https://ror.org/02gfc7t72]
dc.subject.none.fl_str_mv	Supernovae: general Supernovae: individual: 2021hem
topic	Supernovae: general Supernovae: individual: 2021hem
description	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.
publishDate	2026
dc.date.none.fl_str_mv	2026 2026 2026
dc.type.none.fl_str_mv	info:eu-repo/semantics/article http://purl.org/coar/resource_type/c_6501 Publisher's version info:eu-repo/semantics/publishedVersion
format	article
status_str	publishedVersion
dc.identifier.none.fl_str_mv	http://hdl.handle.net/10261/426195 http://arxiv.org/abs/2511.07529v2
url	http://hdl.handle.net/10261/426195 http://arxiv.org/abs/2511.07529v2
dc.language.none.fl_str_mv	Inglés
language_invalid_str_mv	Inglés
dc.relation.none.fl_str_mv	#PLACEHOLDER_PARENT_METADATA_VALUE# #PLACEHOLDER_PARENT_METADATA_VALUE# #PLACEHOLDER_PARENT_METADATA_VALUE# info:eu-repo/grantAgreement/AEI/Plan Estatal de Investigación Científica y Técnica y de Innovación 2021-2023/PID2023-151307NB-I00 info:eu-repo/grantAgreement/AEI//CEX2020-001058-M info:eu-repo/grantAgreement/EC/HE/101125877 https://doi.org/10.1051/0004-6361/202558053 Sí
dc.rights.none.fl_str_mv	info:eu-repo/semantics/openAccess
eu_rights_str_mv	openAccess
dc.format.none.fl_str_mv	application/pdf
dc.publisher.none.fl_str_mv	EDP Sciences
publisher.none.fl_str_mv	EDP Sciences
dc.source.none.fl_str_mv	reponame:DIGITAL.CSIC. Repositorio Institucional del CSIC instname:Consejo Superior de Investigaciones Científicas (CSIC)
instname_str	Consejo Superior de Investigaciones Científicas (CSIC)
reponame_str	DIGITAL.CSIC. Repositorio Institucional del CSIC
collection	DIGITAL.CSIC. Repositorio Institucional del CSIC
repository.name.fl_str_mv
repository.mail.fl_str_mv
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spelling	The Type Ia Supernova 2021hem: A 2003fg-like Event in an Apparently Hostless EnvironmentBose, S.Stritzinger, MaximilianMalmgaard, A.Miller, C. J.Elias-Rosa, NancyFynbo, J.Ashall, ChrisBurns, Christopher R.DerKacy, J. M.Galbany, LluísGutiérrez, Claudia P.Hoogendam, Willem B.Hsiao, E. Y.Jensen, ErikMedler, KyleAlburai, AlaaAnderson, Joseph P.Baron, EdwardDuarte, J.Gromadzki, MariuszInserra, CosimoMazzali, Paolo A.Müller-Bravo, Tomás E.Lundqvist, PeterReguitti, AndreaSalmaso, IreneSand, David J.Valerin, GiorgioSupernovae: generalSupernovae: individual: 2021hemWe 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.We are grateful to Subo Dong for suggesting our team to follow-up SN 2021hem, Mark Phillips for discussions, and Morgran Fraser for observational support. Funding of the Aarhus supernova group during the creation of this research comes from grants via the Independent Research Fund Denmark (IRFD; grant numbers 8021-00170B and 10.46540/2032-00022B), and the Aarhus University Research Foundation (Nova project AUFF-E-2023-9-28). L.G. and C.P.G. recognize the support from the Spanish Ministerio de Ciencia e Innovación (MCIN) and the Agencia Estatal de Investigación (AEI) 10.13039/501100011033 under the PID2023-151307NB-I00 SNNEXT project, from Centro Superior de Investigaciones Científicas (CSIC) under the PIE project 20215AT016 and the program Unidad de Excelencia María de Maeztu CEX2020-001058-M, and from the Departament de Recerca i Universitats de la Generalitat de Catalunya through the 2021-SGR-01270 grant. C.P.G. acknowledges financial support from the Secretary of Universities and Research (Government of Catalonia) and by the Horizon 2020 Research and Innovation Programme of the European Union under the Marie Skłodowska-Curie and the Beatriu de Pinós 2021 BP 00168 programme. L.G. acknowledges financial support from AGAUR, CSIC, MCIN and AEI 10.13039/501100011033 under projects PID2023-151307NB-I00, PIE 20215AT016, ILINK23001, COOPB2304. Based on observations made with the Nordic Optical Telescope, owned in collaboration by the University of Turku and Aarhus University, and operated jointly by Aarhus University, the University of Turku and the University of Oslo, representing Denmark, Finland and Norway, the University of Iceland and Stockholm University at the Observatorio del Roque de los Muchachos, La Palma, Spain, of the Instituto de Astrofisica de Canarias. The NOT data were obtained under programme ID P62-505 and P66-506. Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere, Chile, as part of ePESSTO+ (the advanced Public ESO Spectroscopic Survey for Transient Objects Survey – PI: Inserra). ePESSTO+ observations were obtained under ESO program ID 1103.D-0328. T.E.M.B. is funded by Horizon Europe ERC grant no. 101125877. J. D. acknowledges support by FCT for CENTRA through the Project No. UIDB/00099/2025. J. D. also acknowledges support by FCT under the PhD grant 2023.01333.BD.With funding from the Spanish government through the "María de Maeztu Unit of Excellence" accreditation (CEX2020-001058-M)Peer reviewedEDP SciencesIndependent Research Fund DenmarkAarhus University Research FoundationMinisterio de Ciencia e Innovación (España)Agencia Estatal de Investigación (España)Consejo Superior de Investigaciones Científicas (España)Generalitat de CatalunyaEuropean CommissionEuropean Research CouncilFundação para a Ciência e a Tecnologia (Portugal)Bose, S. [0000-0003-3529-3854]Stritzinger, Maximilian [0000-0002-5571-1833]Elias-Rosa, Nancy [0000-0002-1381-9125]Fynbo, J. [0000-0002-8149-8298]Galbany, Lluís [0000-0002-1296-6887]Consejo Superior de Investigaciones Científicas [https://ror.org/02gfc7t72]202620262026info:eu-repo/semantics/articlehttp://purl.org/coar/resource_type/c_6501Publisher's versioninfo:eu-repo/semantics/publishedVersionapplication/pdfhttp://hdl.handle.net/10261/426195http://arxiv.org/abs/2511.07529v2reponame:DIGITAL.CSIC. Repositorio Institucional del CSICinstname:Consejo Superior de Investigaciones Científicas (CSIC)Inglés#PLACEHOLDER_PARENT_METADATA_VALUE##PLACEHOLDER_PARENT_METADATA_VALUE##PLACEHOLDER_PARENT_METADATA_VALUE#info:eu-repo/grantAgreement/AEI/Plan Estatal de Investigación Científica y Técnica y de Innovación 2021-2023/PID2023-151307NB-I00info:eu-repo/grantAgreement/AEI//CEX2020-001058-Minfo:eu-repo/grantAgreement/EC/HE/101125877https://doi.org/10.1051/0004-6361/202558053Síinfo:eu-repo/semantics/openAccessoai:dnet:digitalcsic_::573b2c66b9f14c42ef72f53082ab116f2026-05-22T06:33:51Z
score	15,812429

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

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