The rise and fall of an extraordinary Ca-rich transient -- The discovery of ATLAS19dqr/SN 2019bkc

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. 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 ∼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 \ArII, 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. ; SJP, KM, MRM, and KS are supported by H2020 ERC grant no. 758638. MF is supported by a Royal Society – Science Foundation Ireland University Research Fellowship. L.G. was funded by the European Union's Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie grant agreement No. 839090. MG is supported by the Polish NCN MAESTRO grant 2014/14/A/ST9/00121. CPG acknowledges support from EU/FP7-ERC grant no. [615929]. GL was supported by a research grant (19054) from VILLUM FONDEN. AF acknowledges the support of an ESO Studentship. TWC acknowledgments the funding provided by the Alexander von Humboldt Foundation. PGJ acknowledges funding from the European Research Council under ERC Consolidator Grant agreement no 647208. The Liverpool Telescope is operated on the island of La Palma by Liverpool John Moores University in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias with financial support from the UK Science and Technology Facilities Council. This work has made use of data from the Asteroid Terrestrial-impact Last Alert System (ATLAS) project. ATLAS is primarily funded to search for near earth asteroids through NASA grants NN12AR55G, 80NSSC18K0284, and 80NSSC18K1575; byproducts of the NEO search include images and catalogues from the survey area.