The rise and fall of an extraordinary Ca-rich transient

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 Δ m 15 ∼ 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) × 10 50 erg, giving a specific kinetic energy E k / M ej ∼ 1 [10 51 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.