The Most Rapidly Declining Type I Supernova 2019bkc/ATLAS19dqr

Understand

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.