Panning for gold, but finding helium: Discovery of the ultra-stripped supernova SN 2019wxt from gravitational-wave follow-up observations

¹ Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
² INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Piero Gobetti 93/3, 40129 Bologna, Italy
³ Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai 200030, PR China
⁴ Peng Cheng Laboratory, Shenzhen 518066, PR China
⁵ Instituto de Astrofísica and Centro de Astroingeniería, Facultad de Física, Pontificia Uni versidad Católica de Chile, Casilla 306, Santiago 22, Chile
⁶ Millennium Institute of Astrophysics (MAS), Nuncio Monseñor Sótero Sanz 100, Providencia, Santiago, Chile
⁷ Space Science Institute, 4750 Walnut Street, Suite 205, Boulder, Colorado 80301, USA
⁸ INAF – Osservatorio Astronomico di Padova, 35122 Padova, Italy
⁹ INAF – Brera Astronomical Observatory, Via Bianchi 46, 23807 Merate (LC), Italy
¹⁰ Jodrell Bank Centre for Astrophysics, Department of Physics and Astronomy, The University of Manchester, M13 9PL Manchester, UK
¹¹ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
¹² Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
¹³ Gran Sasso Science Institute, Viale F. Crispi 7, 67100 L’Aquila (AQ), Italy
¹⁴ INFN – Laboratori Nazionali del Gran Sasso, 67100 L’Aquila (AQ), Italy
¹⁵ School of Physics, O’Brien Centre for Science North, University College Dublin, Belfield, Dublin 4, Ireland
¹⁶ INAF – Osservatorio Astronomico d’Abruzzo, Via M. Maggini s.n.c., 64100 Teramo, Italy
¹⁷ INAF – Osservatorio Astronomico di Roma, Via di Frascati 33, 00078 Monteporzio Catone (RM), Italy
¹⁸ Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
¹⁹ Departamento de Astrofísica, Universidad de La Laguna, 38206 La Laguna, Tenerife, Spain
²⁰ GRANTECAN, Cuesta de San Josè s/n, 38712 Breña Baja, La Palma, Spain
²¹ Université Paris Cité, CNRS, Astroparticule et Cosmologie, 75013 Paris, France
²² The Oskar Klein Centre, Department of Astronomy, Stockholm University, AlbaNova, 10691 Stockholm, Sweden
²³ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstraße 1, 85748 Garching, Germany
²⁴ INAF – Istituto di Radioastronomia, Bologna, Via Gobetti 101, 40127 Bologna, Italy
²⁵ ASI Science Data Centre, Via del Politecnico snc, 00133 Rome, Italy
²⁶ European Centre for Theoretical Studies in Nuclear Physics and Related Areas (ECT⋆), Fondazione Bruno Kessler, Trento, Italy
²⁷ INFN-TIFPA, Trento Institute for Fundamental Physics and Applications, Via Sommarive 14, 38123 Trento, Italy
²⁸ GSI Helmholtzzentrum für Schwerionenforschung, Planckstraße 1, 64291 Darmstadt, Germany
²⁹ Konkoly observatory, ELKH Research Centre for Astronomy and Earth Sciences, Konkoly Thege Miklós út 15-17, 1121 Budapest, Hungary
³⁰ CSFK, MTA Centre of Excellence, Konkoly Thege Miklós út 15-17, 1121 Budapest, Hungary
³¹ Institute of Physics, ELTE Eötvös Loránd University, Pázmány Péter sétány 1/A, 1117 Budapest, Hungary
³² School of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ, UK
³³ Astrophysics Research Centre, School of Mathematics and Physics, Queen’s University Belfast, BT7 1NN Belfast, UK
³⁴ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans, s/n, 08193 Barcelona, Spain
³⁵ Institut d’Estudis Espacials de Catalunya (IEEC), 08034 Barcelona, Spain
³⁶ DARK, Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200 Copenhagen N, Denmark
³⁷ Universitá di Bologna, Via Zamboni 33, 40126 Bologna (BO), Italy
³⁸ Institute for Gravitational Wave Astronomy and School of Physics and Astronomy, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
³⁹ Astronomical Observatory, University of Warsaw, Al. Ujazdowskie 4, 00-478 Warszawa, Poland
⁴⁰ Cosmic Dawn Center (DAWN), Jagtvej 128, Tårn I, 2200 Copenhagen, Denmark
⁴¹ Niels Bohr Institute, University of Copenhagen, Lyngbyvej 2, 2100 Copenhagen N, Denmark
⁴² Centre for Astrophysics and Cosmology, Science Institute, University of Iceland, Dunhagi 5, 107 Reykjavík, Iceland
⁴³ Departamento de Física Teórica y del Cosmos, Universidad de Granada, 18071 Granada, Spain
⁴⁴ Department of Astrophysics/IMAPP, Radboud University, PO Box 9010 6500 GL Nijmegen, The Netherlands
⁴⁵ SRON, Netherlands Institute for Space Research, Niels Bohrweg 4, 2333 CA Leiden, The Netherlands
⁴⁶ Key Laboratory of Dark Matter and Space Astronomy, Purple Mountain Observatory, Chinese Academy of Sciences, Nanjing 210008, PR China
⁴⁷ Department of Physics and Astronomy, University of Turku, Vesilinnantie 5, Turku 20014, Finland
⁴⁸ DTU Space, National Space Institute, Technical University of Denmark, Elektrovej 327, 2800 Kongens Lyngby, Denmark
⁴⁹ Department of Physics, University of Warwick, Coventry CV4 7AL, UK
⁵⁰ School of Physics, Trinity College Dublin, University of Dublin, College Green, Dublin 2, Ireland
⁵¹ School of Physics and Astronomy, Monash University, Clayton, Victoria 3800, Australia
⁵² Joint Institute for VLBI ERIC, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands
⁵³ School of Sciences, European University Cyprus, Diogenes street, Engomi, 1516 Nicosia, Cyprus
⁵⁴ Astronomical Observatory Institute, Faculty of Physics, Adam Mickiewicz University, ul. Słoneczna 36, 60-286 Poznań, Poland
⁵⁵ Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany
⁵⁶ Departamento de Ciencias Fisicas, Universidad Andres Bello, Avda. Republica 252, Santiago, Chile
⁵⁷ Universitá degli Studi di Milano-Bicocca, Piazza della Scienza 3, 20126 Milano (MI), Italy
⁵⁸ INFN – Sezione di Milano-Bicocca, Piazza della Scienza 3, 20126 Milano (MI), Italy
⁵⁹ The Oskar Klein Centre, Department of Physics, Stockholm University, AlbaNova, 10691 Stockholm, Sweden
⁶⁰ School of Physics and Astronomy, University of Leicester, University Road, LE1 7RH Leicester, UK
⁶¹ The Department of Physics, The University of Auckland, Private Bag 92019, Auckland, New Zealand
⁶² Special Astrophysical Observatory, Russian Academy of Sciences, Nizhnii Arkhyz 369167, Russia
⁶³ GEPI, Observatoire de Paris, PSL University, CNRS, 5 Place Jules Janssen, 92190 Meudon, France
⁶⁴ Goethe University Frankfurt, Max-von-Laue-Strasse 1, Frankfurt am Main 60438, Germany
⁶⁵ Physics Department, Lancaster University, Lancaster LA1 4YB, UK
⁶⁶ Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
⁶⁷ Observatorio Astronómico de Quito, Escuela Politécnica Nacional, 170136 Quito, Ecuador

Received: 15 August 2022
Accepted: 9 May 2023

Abstract

We present the results from multi-wavelength observations of a transient discovered during an intensive follow-up campaign of S191213g, a gravitational wave (GW) event reported by the LIGO-Virgo Collaboration as a possible binary neutron star merger in a low latency search. This search yielded SN 2019wxt, a young transient in a galaxy whose sky position (in the 80% GW contour) and distance (∼150 Mpc) were plausibly compatible with the localisation uncertainty of the GW event. Initially, the transient’s tightly constrained age, its relatively faint peak magnitude (M_i ∼ −16.7 mag), and the r-band decline rate of ∼1 mag per 5 days appeared suggestive of a compact binary merger. However, SN 2019wxt spectroscopically resembled a type Ib supernova, and analysis of the optical-near-infrared evolution rapidly led to the conclusion that while it could not be associated with S191213g, it nevertheless represented an extreme outcome of stellar evolution. By modelling the light curve, we estimated an ejecta mass of only ∼0.1 M_⊙, with ⁵⁶Ni comprising ∼20% of this. We were broadly able to reproduce its spectral evolution with a composition dominated by helium and oxygen, with trace amounts of calcium. We considered various progenitor channels that could give rise to the observed properties of SN 2019wxt and concluded that an ultra-stripped origin in a binary system is the most likely explanation. Disentangling genuine electromagnetic counterparts to GW events from transients such as SN 2019wxt soon after discovery is challenging: in a bid to characterise this level of contamination, we estimated the rate of events with a volumetric rate density comparable to that of SN 2019wxt and found that around one such event per week can occur within the typical GW localisation area of O4 alerts out to a luminosity distance of 500 Mpc, beyond which it would become fainter than the typical depth of current electromagnetic follow-up campaigns.