1100 days in the life of the supernova 2018ibb

The best pair-instability supernova candidate, to date^⋆

¹ The Oskar Klein Centre, Department of Physics, Stockholm University, Albanova University Center, 106 91 Stockholm, Sweden
e-mail: steve.schulze@fysik.su.se
² The Oskar Klein Centre, Department of Astronomy, Stockholm University, Albanova University Center, 106 91 Stockholm, Sweden
³ Heidelberger Institut für Theoretische Studien, Schloss-Wolfsbrunnenweg 35, 69118 Heidelberg, Germany
⁴ Technische Universität München, TUM School of Natural Sciences, Physik-Department, James-Franck-Straße 1, 85748 Garching, Germany
⁵ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild Straße 1, 85748 Garching, Germany
⁶ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstraße 1, 85748 Garching, Germany
⁷ Department of Particle Physics and Astrophysics, Weizmann Institute of Science, 234 Herzl St, 76100 Rehovot, Israel
⁸ The Caltech Optical Observatories, California Institute of Technology, Pasadena, CA 91125, USA
⁹ Finnish Centre for Astronomy with ESO (FINCA), 20014 University of Turku, Turku, Finland
¹⁰ Tuorla Observatory, Department of Physics and Astronomy, University of Turku, 20014 Turku, Finland
¹¹ DTU Space, National Space Institute, Technical University of Denmark, Elektrovej 327, 2800 Kgs. Lyngby, Denmark
¹² Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
¹³ Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, Belfast BT7 1NN, UK
¹⁴ Department of Astronomy, University of California, Berkeley, CA 94720-3411, USA
¹⁵ Birmingham Institute for Gravitational Wave Astronomy and School of Physics and Astronomy, University of Birmingham, Birmingham B15 2TT, UK
¹⁶ Nordita, Stockholm University and KTH Royal Institute of Technology Hannes Alfvéns väg 12, 106 91 Stockholm, Sweden
¹⁷ Division of Physics, Mathematics and Astronomy, California Institute of Technology, Pasadena, CA 91125, USA
¹⁸ INAF – Osservatorio di Astrofisica e Scienza dello Spazio, Via Piero Gobetti 93/3, 40129 Bologna, Italy
¹⁹ Physics Department and Tsinghua Center for Astrophysics (THCA), Tsinghua University, Beijing 100084, PR China
²⁰ Gemini Observatory/NSF’s NOIRLab, Casilla 603, La Serena, Chile
²¹ MIT-Kavli Institute for Astrophysics and Space Research, 77 Massachusetts Ave., Cambridge, MA 02139, USA
²² Department of Astrophysics, University of Vienna, Türkenschanzstraße 17, 1180 Vienna, Austria
²³ Cosmic Dawn Center (DAWN), Denmark; Niels Bohr Institute, Copenhagen University, Jagtvej 128, 2200 Copenhagen N, Denmark
²⁴ Department of Astronomy, Cornell University, Ithaca, NY 14853, USA
²⁵ Cardiff Hub for Astrophysics Research and Technology, School of Physics & Astronomy, Cardiff University, Queens Buildings, The Parade, Cardiff CF24 3AA, UK
²⁶ Tuorla Observatory, Department of Physics and Astronomy, University of Turku, 20014 Turku, Finland
²⁷ Finnish Centre for Astronomy with ESO (FINCA), University of Turku, 20014 Turku, Finland
²⁸ Astrophysics Research Institute, Liverpool John Moores University, Liverpool Science Park, 146 Brownlow Hill, Liverpool L3 5RF, UK
²⁹ Large Binocular Telescope Observatory, University of Arizona, 933 N. Cherry Ave., Tucson, AZ 85721, USA
³⁰ George Mason University, Department of Physics & Astronomy, MS 3F3, 4400 University Dr., Fairfax, VA 22030, USA
³¹ GSI Helmholtzzentrum für Schwerionenforschung, Planckstraße 1, 64291 Darmstadt, Germany
³² Department of Astronomy and Astrophysics, University of California, Santa Cruz, CA 95064, USA
³³ INAF – Osservatorio Astronomico di Padova, vicolo dell’Osservatorio 5, 35122 Padova, Italy
³⁴ INAF – Osservatorio Astronomico d’Abruzzo, Via M. Maggini snc, 64100 Teramo, Italy
³⁵ European Southern Observatory, Alonso de Córdova 3107, Casilla 19, Santiago, Chile
³⁶ Millennium Institute of Astrophysics MAS, Nuncio Monsenor Sotero Sanz 100, Off. 104, Providencia, Santiago, Chile
³⁷ INAF – Istituto di Astrofisica Spaziale e Fisica cosmica Milano (IASF), Via Alfonso Corti 12, 20133 Milano, Italy
³⁸ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans, s/n, 08193 Barcelona, Spain
³⁹ Institut d’Estudis Espacials de Catalunya (IEEC), 08034 Barcelona, Spain
⁴⁰ Astronomical Observatory, University of Warsaw, Al. Ujazdowskie 4, 00-478 Warszawa, Poland
⁴¹ IPAC, California Institute of Technology, 1200 E. California Blvd, Pasadena, CA 91125, USA
⁴² Center for Astrophysics, Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138-1516, USA
⁴³ Las Cumbres Observatory, 6740 Cortona Dr. Suite 102, Goleta, CA 93117, USA
⁴⁴ Department of Physics, University of California, Santa Barbara, CA 93106-9530, USA
⁴⁵ Department of Physics, University of California, 1 Shields Avenue, Davis, CA 95616-5270, USA
⁴⁶ Université de Lyon, Université Claude-Bernard Lyon 1, CNRS/IN2P3, IP2I Lyon, 69622 Villeurbanne, France
⁴⁷ Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA
⁴⁸ The NSF AI Institute for Artificial Intelligence and Fundamental Interactions, Cambridge, USA

Received: 9 May 2023
Accepted: 11 October 2023

Abstract

Stars with zero-age main sequence masses between 140 and 260 M_⊙ are thought to explode as pair-instability supernovae (PISNe). During their thermonuclear runaway, PISNe can produce up to several tens of solar masses of radioactive nickel, resulting in luminous transients similar to some superluminous supernovae (SLSNe). Yet, no unambiguous PISN has been discovered so far. SN 2018ibb is a hydrogen-poor SLSN at z = 0.166 that evolves extremely slowly compared to the hundreds of known SLSNe. Between mid 2018 and early 2022, we monitored its photometric and spectroscopic evolution from the UV to the near-infrared (NIR) with 2–10 m class telescopes. SN 2018ibb radiated > 3 × 10⁵¹ erg during its evolution, and its bolometric light curve reached > 2 × 10⁴⁴ erg s⁻¹ at its peak. The long-lasting rise of > 93 rest-frame days implies a long diffusion time, which requires a very high total ejected mass. The PISN mechanism naturally provides both the energy source (⁵⁶Ni) and the long diffusion time. Theoretical models of PISNe make clear predictions as to their photometric and spectroscopic properties. SN 2018ibb complies with most tests on the light curves, nebular spectra and host galaxy, and potentially all tests with the interpretation we propose. Both the light curve and the spectra require 25–44 M_⊙ of freshly nucleosynthesised ⁵⁶Ni, pointing to the explosion of a metal-poor star with a helium core mass of 120–130 M_⊙ at the time of death. This interpretation is also supported by the tentative detection of [Co II] λ 1.025 μm, which has never been observed in any other PISN candidate or SLSN before. We observe a significant excess in the blue part of the optical spectrum during the nebular phase, which is in tension with predictions of existing PISN models. However, we have compelling observational evidence for an eruptive mass-loss episode of the progenitor of SN 2018ibb shortly before the explosion, and our dataset reveals that the interaction of the SN ejecta with this oxygen-rich circumstellar material contributed to the observed emission. That may explain this specific discrepancy with PISN models. Powering by a central engine, such as a magnetar or a black hole, can be excluded with high confidence. This makes SN 2018ibb by far the best candidate for being a PISN, to date.