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

We present optical and near-infrared photometric and spectroscopic observations of the fast-declining Type Ia supernova (SN) 2015bo. SN 2015bo is underluminous (M B = −17.50 ± 0.15 mag) and has a fast-evolving light curve (Δm15(B) = 1.91 ± 0.01 mag and s BV = 0.48 ± 0.01). It has a unique morphology in the observed V − r color curve, where it is bluer than all other supernovae (SNe) in the comparison sample. A 56Ni mass of 0.17 ± 0.03 M ⊙ 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 μ = 34.33 ± 0.01 (stat) ±0.11 (sys) mag and μ = 34.34 ± 0.04 (stat) ± 0.12 (sys) mag are derived for SN 2015bo and SN 1997cn, respectively. These distances are consistent at the 0.06σ level with each other, and they 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.