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SN 2018bsz: a Type I superluminous supernova with aspherical circumstellar material

SN 2018bsz: a Type I superluminous supernova with aspherical circumstellar material
SN 2018bsz: a Type I superluminous supernova with aspherical circumstellar material

We present a spectroscopic analysis of the most nearby Type I superluminous supernova (SLSN-I), SN 2018bsz. The photometric evolution of SN 2018bsz has several surprising features, including an unusual pre-peak plateau and evidence for rapid formation of dust ≥200 d post-peak. We show here that the spectroscopic and polarimetric properties of SN 2018bsz are also unique. While its spectroscopic evolution closely resembles SLSNe-I, with early O II absorption and C II P Cygni profiles followed by Ca, Mg, Fe, and other O features, a multi-component Hα profile appearing at ∼30 d post-maximum is the most atypical. The Hα is at first characterised by two emission components, one at ∼+3000 km s-1 and a second at ∼-7500 km s-1, with a third, near-zero-velocity component appearing after a delay. The blue and central components can be described by Gaussian profiles of intermediate width (FWHM ∼20006000 km s-1), but the red component is significantly broader (FWHM ≥ 10000 km s-1) and Lorentzian. The blue Hα component evolves towards a lower-velocity offset before abruptly fading at ∼+100 d post-maximum brightness, concurrently with a light curve break. Multi-component profiles are observed in other hydrogen lines, including Paβ, and in lines of Ca II and He I. Spectropolarimetry obtained before (10.2 d) and after (38.4 d) the appearance of the H lines shows a large shift on the Stokes Q-U plane consistent with SN 2018bsz undergoing radical changes in its projected geometry. Assuming the supernova is almost unpolarised at 10.2 d, the continuum polarisation at 38.4 d reaches P ∼1.8%, implying an aspherical configuration. We propose that the observed evolution of SN 2018bsz can be explained by highly aspherical, possibly disk-like, circumstellar material (CSM) with several emitting regions. After the supernova explosion, the CSM is quickly overtaken by the ejecta, but as the photosphere starts to recede, the different CSM regions re-emerge, producing the peculiar line profiles. Based on the first appearance of Hα, we can constrain the distance of the CSM to be less than ∼6.5× 1015 cm (430 AU), or even lower (≤87 AU) if the pre-peak plateau is related to an eruption that created the CSM. The presence of CSM has been inferred previously for other SLSNe-I, both directly and indirectly. However, it is not clear whether the rare properties of SN 2018bsz can be generalised for SLSNe-I, for example in the context of pulsational pair instability, or whether they are the result of an uncommon evolutionary path, possibly involving a binary companion.

Astrophysics - High Energy Astrophysical Phenomena, Supernovae: Individual: SN 2018bsz, Circumstellar matter
0004-6361
Pursiainen, M.
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Leloudas, G.
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Paraskeva, E.
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Cikota, A.
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Anderson, J. P.
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Angus, C. R.
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Brennan, S.
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Bulla, M.
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Camacho-Iñiguez, E.
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Charalampopoulos, P.
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Chen, T.-W.
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Delgado Mancheño, M.
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Fraser, M.
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Frohmaier, C.
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Galbany, L.
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Gutiérrez, C. P.
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Gromadzki, M.
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Inserra, C.
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Maund, J.
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Müller-Bravo, T. E.
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Muñoz Torres, S.
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Nicholl, M.
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Onori, F.
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Patat, F.
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Pessi, P. J.
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Roy, R.
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Spyromilio, J.
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Wiseman, P.
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Young, D. R.
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Pursiainen, M.
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Leloudas, G.
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Paraskeva, E.
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Cikota, A.
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Anderson, J. P.
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Angus, C. R.
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Brennan, S.
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Bulla, M.
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Camacho-Iñiguez, E.
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Charalampopoulos, P.
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Chen, T.-W.
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Delgado Mancheño, M.
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Fraser, M.
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Frohmaier, C.
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Galbany, L.
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Gutiérrez, C. P.
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Gromadzki, M.
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Inserra, C.
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Maund, J.
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Müller-Bravo, T. E.
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Muñoz Torres, S.
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Nicholl, M.
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Onori, F.
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Patat, F.
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Pessi, P. J.
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Roy, R.
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Spyromilio, J.
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Wiseman, P.
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Young, D. R.
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Pursiainen, M., Leloudas, G., Paraskeva, E., Cikota, A., Anderson, J. P., Angus, C. R., Brennan, S., Bulla, M., Camacho-Iñiguez, E., Charalampopoulos, P., Chen, T.-W., Delgado Mancheño, M., Fraser, M., Frohmaier, C., Galbany, L., Gutiérrez, C. P., Gromadzki, M., Inserra, C., Maund, J., Müller-Bravo, T. E., Muñoz Torres, S., Nicholl, M., Onori, F., Patat, F., Pessi, P. J., Roy, R., Spyromilio, J., Wiseman, P. and Young, D. R. (2022) SN 2018bsz: a Type I superluminous supernova with aspherical circumstellar material. Astronomy & Astrophysics, 666, [A30]. (doi:10.1051/0004-6361/202243256).

Record type: Article

Abstract

We present a spectroscopic analysis of the most nearby Type I superluminous supernova (SLSN-I), SN 2018bsz. The photometric evolution of SN 2018bsz has several surprising features, including an unusual pre-peak plateau and evidence for rapid formation of dust ≥200 d post-peak. We show here that the spectroscopic and polarimetric properties of SN 2018bsz are also unique. While its spectroscopic evolution closely resembles SLSNe-I, with early O II absorption and C II P Cygni profiles followed by Ca, Mg, Fe, and other O features, a multi-component Hα profile appearing at ∼30 d post-maximum is the most atypical. The Hα is at first characterised by two emission components, one at ∼+3000 km s-1 and a second at ∼-7500 km s-1, with a third, near-zero-velocity component appearing after a delay. The blue and central components can be described by Gaussian profiles of intermediate width (FWHM ∼20006000 km s-1), but the red component is significantly broader (FWHM ≥ 10000 km s-1) and Lorentzian. The blue Hα component evolves towards a lower-velocity offset before abruptly fading at ∼+100 d post-maximum brightness, concurrently with a light curve break. Multi-component profiles are observed in other hydrogen lines, including Paβ, and in lines of Ca II and He I. Spectropolarimetry obtained before (10.2 d) and after (38.4 d) the appearance of the H lines shows a large shift on the Stokes Q-U plane consistent with SN 2018bsz undergoing radical changes in its projected geometry. Assuming the supernova is almost unpolarised at 10.2 d, the continuum polarisation at 38.4 d reaches P ∼1.8%, implying an aspherical configuration. We propose that the observed evolution of SN 2018bsz can be explained by highly aspherical, possibly disk-like, circumstellar material (CSM) with several emitting regions. After the supernova explosion, the CSM is quickly overtaken by the ejecta, but as the photosphere starts to recede, the different CSM regions re-emerge, producing the peculiar line profiles. Based on the first appearance of Hα, we can constrain the distance of the CSM to be less than ∼6.5× 1015 cm (430 AU), or even lower (≤87 AU) if the pre-peak plateau is related to an eruption that created the CSM. The presence of CSM has been inferred previously for other SLSNe-I, both directly and indirectly. However, it is not clear whether the rare properties of SN 2018bsz can be generalised for SLSNe-I, for example in the context of pulsational pair instability, or whether they are the result of an uncommon evolutionary path, possibly involving a binary companion.

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Accepted/In Press date: 22 June 2022
e-pub ahead of print date: 30 September 2022
Published date: 30 September 2022
Additional Information: Funding Information: We thank the anonymous referee for comments that helped improve this paper. Based on (in part) 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). ePESSTO observations were obtained under ESO program ID 1103.D-0328 (PI: Smartt). Based on (in part) observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO programme 2101.D-5023, M.P., G.L. and P.C. are supported by a research grant (19054) from VILLUM FONDEN. S.B. would like to thank their support from Science Foundation Ireland and the Royal Society (RS-EA/3471) M.B. acknowledges support from the Swedish Research Council (Reg. no. 2020-03330). E.C. acknowledges additional support from the National Agency for Research and Development (ANID)/Scholarship Program/Doctorado Nacional grant 2021 - 21211203. T.-W.C. acknowledges the EU Funding under Marie Skłodowska-Curie grant H2020-MSCA-IF-2018-842471. M.F. is supported by a Royal Society - Science Foundation Ireland University Research Fellowship L.G. acknowledges financial support from the Spanish Ministerio de Ciencia e Innovación (MCIN), the Agencia Estatal de Investigación (AEI) 10.13039/501100011033, and the European Social Fund (ESF) “Investing in your future” under the 2019 Ramón y Cajal program RYC2019-027683-I and the PID2020-115253GA-I00 HOSTFLOWS 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. M.G. is supported by the EU Horizon 2020 research and innovation programme under grant agreement No 101004719. T.E.M.-B. acknowledges financial support from the Spanish Ministerio de Ciencia e Innovación (MCIN), the Agencia Estatal de Investigación (AEI) 10.13039/501100011033 under the PID2020-115253GA-I00 HOSTFLOWS project, and from Centro Superior de Investigaciones Científicas (CSIC) under the PIE project 20215AT016. M.N. is supported by the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No. 948381) and by a Fellowship from the Alan Turing Institute. F.O. acknowledges the support of the GRAWITA/PRIN-MIUR project: “The new frontier of the Multi-Messenger Astrophysics: follow-up of electromagnetic transient counterparts of gravitational wave sources” and the support of HORIZON2020: AHEAD2020 grant agreement n.871158. Publisher Copyright: © M. Pursiainen et al. 2022.
Keywords: Astrophysics - High Energy Astrophysical Phenomena, Supernovae: Individual: SN 2018bsz, Circumstellar matter

Identifiers

Local EPrints ID: 471901
URI: http://eprints.soton.ac.uk/id/eprint/471901
ISSN: 0004-6361
PURE UUID: f19e9282-9677-4f00-af94-d6bd2a905d9d
ORCID for C. Inserra: ORCID iD orcid.org/0000-0002-3968-4409
ORCID for P. Wiseman: ORCID iD orcid.org/0000-0002-3073-1512

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Date deposited: 22 Nov 2022 17:41
Last modified: 17 Mar 2024 03:48

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Contributors

Author: M. Pursiainen
Author: G. Leloudas
Author: E. Paraskeva
Author: A. Cikota
Author: J. P. Anderson
Author: C. R. Angus
Author: S. Brennan
Author: M. Bulla
Author: E. Camacho-Iñiguez
Author: P. Charalampopoulos
Author: T.-W. Chen
Author: M. Delgado Mancheño
Author: M. Fraser
Author: C. Frohmaier
Author: L. Galbany
Author: C. P. Gutiérrez
Author: M. Gromadzki
Author: C. Inserra ORCID iD
Author: J. Maund
Author: T. E. Müller-Bravo
Author: S. Muñoz Torres
Author: M. Nicholl
Author: F. Onori
Author: F. Patat
Author: P. J. Pessi
Author: R. Roy
Author: J. Spyromilio
Author: P. Wiseman ORCID iD
Author: D. R. Young

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