A Tale of Two Type Ia Supernovae: The fast-declining siblings SNe 2015bo and 1997cn

Abstract

We present optical and near-infrared photometric and spectroscopic observations of the fast-declining Type Ia Supernova (SN) 2015bo. SN 2015bo is under-luminous (M${}_B$ = -17.67 $\pm$ 0.15 mag) and has a fast-evolving light curve ($\hat{I}”{\mathrm{m}}_{15}(\mathrm{B})$ = 1.91 $\pm$ 0.01 mag and $s_{BV}$ = 0.48 $\pm$ 0.01). It has a unique morphology in the $V-r$ color curve, where it is bluer than all other SNe in the comparison sample. A ${}^{56}$Ni mass of 0.17 $\pm$ 0.03 $M_{\odot }$ was derived from the peak bolometric luminosity, which is consistent with its location on the luminosity-width relation. Spectroscopically, SN 2015bo is a Cool SN in the Branch classification scheme. The velocity evolution measured from spectral features is consistent with 1991bg-like SNe. SN 2015bo has a SN twin (similar spectra) and sibling (same host galaxy), SN 1997cn. Distance moduli of $\hat{I}¼$ = 34.33 $\pm$ 0.01 (stat) $\pm$0.11 (sys) mag and $\hat{I}¼$ = 34.34 $\pm$ 0.04 (stat) $\pm$ 0.12 (sys) mag were derived for SN 2015bo and SN 1997cn, respectively. These distances are consistent at the 0.06-$\ddot{I}ƒ$ level with each other, and are also consistent with distances derived using surface-brightness fluctuations and redshift-corrected cosmology. This suggests that fast-declining SNe could be accurate distance indicators which should not be excluded from future cosmological analyses.

Accepted to ApJ. 29 pages, 22 figures