The enigmatic double-peaked stripped-envelope SN 2023aew

¹ Finnish Centre for Astronomy with ESO (FINCA), University of Turku, 20014 Turku, Finland
² Department of Physics and Astronomy, University of Turku, 20014 Turku, Finland
³ Aalto University Metsähovi Radio Observatory, Metsähovintie 114, 02540 Kylmälä, Finland
⁴ Aalto University Department of Electronics and Nanoengineering, PO Box 15500 00076 Aalto, Finland
⁵ School of Physics, University College Dublin, L.M.I. Main Building, Beech Hill Road, Dublin 4 D04 P7W1, Ireland
⁶ Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
⁷ School of Sciences, European University Cyprus, Diogenes Street, Engomi, 1516 Nicosia, Cyprus
⁸ Department of Astronomy, Kyoto University, Kitashirakawa-Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan
⁹ Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark
¹⁰ Oskar Klein Centre, Department of Astronomy, Stockholm University, Albanova University Centre, 106 91 Stockholm, Sweden
¹¹ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
¹² Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans s/n, 08193 Barcelona, Spain
¹³ Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Paseo del Bosque s/n, B1900FWA La Plata, Argentina
¹⁴ Instituto de Astrofísica de La Plata (IALP), CCT-CONICET-UNLP, Paseo del Bosque S/N, B1900FWA La Plata, Argentina
¹⁵ Kavli Institute for the Physics and Mathematics of the Universe (WPI), The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8583, Japan
¹⁶ School of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ, UK
¹⁷ Institut d’Estudis Espacials de Catalunya (IEEC), Gran Capitá, 2-4, Edifici Nexus, Desp. 201, 08034 Barcelona, Spain
¹⁸ Nishi-Harima Astronomical Observatory, Center for Astronomy, University of Hyogo, 407-2 Nishigaichi, Sayo-cho, Sayo, Hyogo 679-5313, Japan
¹⁹ Department of Physics, University of Crete, Vasilika Bouton, 70013 Heraklion, Greece
²⁰ Department of Physics, University of Warwick, Gibbet Hill Road, Coventry CV4 7AL, UK
²¹ Hiroshima Astrophysical Science Center, Hiroshima University, Higashi-Hiroshima, Hiroshima 739-8526, Japan
²² Indian Institute of Astrophysics, II Block, Koramangala, Bengaluru, 560034 Karnataka, India
²³ Pondicherry University, Chinna Kalapet, Kalapet, Puducherry 605014, India
²⁴ Yunnan Observatories, Chinese Academy of Sciences, Kunming 650216, PR China
²⁵ International Centre of Supernovae, Yunnan Key Laboratory, Kunming 650216, PR China
²⁶ Key Laboratory for the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences, Kunming 650216, PR China
²⁷ European Southern Observatory, Alonso de Córdova 3107, Casilla 19, Santiago, Chile
²⁸ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate (LC), Italy

Received: 30 January 2024
Accepted: 7 June 2024

Abstract

We present optical and near-infrared photometry and spectroscopy of SN 2023aew and our findings on its remarkable properties. This event, initially resembling a Type IIb supernova (SN), rebrightens dramatically ∼90 d after the first peak, at which time its spectrum transforms into that of a SN Ic. The slowly evolving spectrum specifically resembles a post-peak SN Ic with relatively low line velocities even during the second rise. The second peak, reached 119 d after the first peak, is both more luminous (M_r = −18.75 ± 0.04 mag) and much broader than those of typical SNe Ic. Blackbody fits to SN 2023aew indicate that the photosphere shrinks almost throughout its observed evolution, and the second peak is caused by an increasing temperature. Bumps in the light curve after the second peak suggest interaction with circumstellar matter (CSM) or possibly accretion. We consider several scenarios for producing the unprecedented behavior of SN 2023aew. Two separate SNe, either unrelated or from the same binary system, require either an incredible coincidence or extreme fine-tuning. A pre-SN eruption followed by a SN requires an extremely powerful, SN-like eruption (consistent with ∼10⁵¹ erg) and is also disfavored. We therefore consider only the first peak a true stellar explosion. The observed evolution is difficult to reproduce if the second peak is dominated by interaction with a distant CSM shell. A delayed internal heating mechanism is more likely, but emerging embedded interaction with a CSM disk should be accompanied by CSM lines in the spectrum, which are not observed, and is difficult to hide long enough. A magnetar central engine requires a delayed onset to explain the long time between the peaks. Delayed fallback accretion onto a black hole may present the most promising scenario, but we cannot definitively establish the power source.