# The Most Rapidly Declining Type I Supernova 2019bkc/ATLAS19dqr

## TL;DR

SN 2019bkc is the fastest declining Type I supernova observed, with unique light curve and spectral features that challenge existing models of fast-evolving supernovae.

## Contribution

This paper reports the discovery and detailed analysis of SN 2019bkc, the most rapidly declining Type I supernova, highlighting its unprecedented characteristics and the inadequacy of current models.

## Key findings

- SN 2019bkc has the fastest decline rates among Type I SNe.
- Its light curves and spectra are unique compared to similar fast-evolving SNe.
- Existing models cannot fully explain SN 2019bkc's observations.

## Abstract

We report observations of the hydrogen-deficient supernova (SN) 2019bkc/ATLAS19dqr. With B- and r-band decline between peak and 10 days post peak of Delta m_10(B)=5.24+/-0.07 mag and Delta m_10(r)=3.85+/-0.10$ mag, respectively, SN 2019bkc is the most rapidly declining SN I discovered so far. While its closest matches are the rapidly declining SN 2005ek and SN 2010X, the light curves and spectra of SN 2019bkc show some unprecedented characteristics. SN 2019bkc appears "hostless," with no identifiable host galaxy near its location, although it may be associated with the galaxy cluster MKW1 at z = 0.02. We evaluate a number of existing models of fast-evolving SNe, and we find that none of them can satisfactorily explain all aspects of SN 2019bkc observations.