The rise and fall of an extraordinary Ca-rich transient

The discovery of ATLAS19dqr/SN 2019bkc^⋆

¹ School of Physics, Trinity College Dublin, The University of Dublin, Dublin 2, Ireland
e-mail: sipren.astro@gmail.com
² Astrophysics Research Centre, School of Mathematics and Physics, Main Physics Building, Queen’s University Belfast, Belfast, County Antrim BT7 1NN, UK
³ Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Straße 1, 85748 Garching bei München, Germany
⁴ European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching bei München, Germany
⁵ Physik-Department, Technische Universität München, James-Franck-Straße 1, 85748 Garching bei München, Germany
⁶ School of Physics & Astronomy, Cardiff University, Queens Buildings, The Parade, Cardiff CF24 3AA, UK
⁷ Institute of Cosmology and Gravitation, University of Portsmouth, Portsmouth, PO1 3FX UK
⁸ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstraße 1, 85748 Garching, Germany
⁹ European Southern Observatory, Alonso de Córdova 3107, Casilla 19, Santiago, Chile
¹⁰ Department of Physics, Florida State University, Tallahassee, FL 32306, USA
¹¹ School of Physics, O’Brien Centre for Science North, University College Dublin, Belfield, Dublin 4, Ireland
¹² Departamento de Física Teórica y del Cosmos, Universidad de Granada, 18071 Granada, Spain
¹³ Department of Particle Physics and Astrophysics, Weizmann Institute of Science, Rehovot 76100, Israel
¹⁴ Astronomical Observatory, University of Warsaw, Al. Ujazdowskie 4, 00-478 Warszawa, Poland
¹⁵ Department of Physics and Astronomy, University of Southampton, Southampton SO17 1BJ, UK
¹⁶ Astrophysics Research Institute, Liverpool John Moores University, IC2, Liverpool Science Park, 146 Brownlow Hill, Liverpool L3 5RF, UK
¹⁷ SRON, Netherlands Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
¹⁸ Department of Astrophysics/IMAPP, Radboud University, PO Box 9010, 6500 GL Nijmegen, The Netherlands
¹⁹ Tuorla Observatory, Department of Physics and Astronomy, University of Turku, 20014 Turku, Finland
²⁰ DTU Space, National Space Institute, Technical University of Denmark, Elektrovej 327, 2800 Kgs. Lyngby, Denmark
²¹ Institute for Astronomy, University of Edinburgh, Royal Observatory, Blackford Hill EH9 3HJ, UK
²² Birmingham Institute for Gravitational Wave Astronomy and School of Physics and Astronomy, University of Birmingham, Birmingham B15 2TT, UK

Received: 16 August 2019
Accepted: 18 February 2020

Abstract

This work presents the observations and analysis of ATLAS19dqr/SN 2019bkc, an extraordinary rapidly evolving transient event located in an isolated environment, tens of kiloparsecs from any likely host. Its light curves rise to maximum light in 5−6 d and then display a decline of Δm₁₅ ∼ 5 mag. With such a pronounced decay, it has one of the most rapidly evolving light curves known for a stellar explosion. The early spectra show similarities to normal and “ultra-stripped” type Ic SNe, but the early nebular phase spectra, which were reached just over two weeks after explosion, display prominent calcium lines, marking SN 2019bkc as a Ca-rich transient. The Ca emission lines at this phase show an unprecedented and unexplained blueshift of 10 000–12 000 km s⁻¹. Modelling of the light curve and the early spectra suggests that the transient had a low ejecta mass of 0.2−0.4 M_⊙ and a low kinetic energy of (2−4) × 10⁵⁰ erg, giving a specific kinetic energy E_k/M_ej ∼ 1 [10⁵¹ erg]/M_⊙. The origin of this event cannot be unambiguously defined. While the abundance distribution used to model the spectra marginally favours a progenitor of white dwarf origin through the tentative identification of Ar II, the specific kinetic energy, which is defined by the explosion mechanism, is found to be more similar to an ultra-stripped core-collapse events. SN 2019bkc adds to the diverse range of physical properties shown by Ca-rich events.