SN 2015bp: adding to the growing population of transitional Type Ia supernovae

Abstract

Photometric and spectroscopic observations of Type Ia supernova 2015bp are presented, spanning ∼−6 to ∼+141 d since B-band maximum. Also presented are unpublished HCT spectra of type Ia iPTF13ebh between −11 and +34 d since B-band maximum. SN 2015bp shows rapidly declining light curves with Δm15(B) = 1.72 ± 0.04. The I-band light curve shows a clear secondary maximum and peaks before the B-band maximum, placing SN 2015bp in the transitional category of SNe Ia. The spectral evolution of SN 2015bp resembles other transitional SNe Ia rather than 1991bg-like events. The C ii λ6580 feature is detected in both SN 2015bp and iPTF13ebh, though it is present till the epoch of B-band maximum in the case of SN 2015bp. The velocity gradients of Si ii λ6355 place SN 2015bp and iPTF13ebh in the FAINT subclass, whereas pseudo-equivalent widths of Si ii features place them in the Cool (CL) subclass of SNe Ia. The bolometric light curve of SN 2015bp indicates that ∼0.2 M⊙ of 56Ni was synthesized in the explosion, with a total ejected mass of ∼0.9 M⊙, suggesting a sub-Chandrasekhar mass white dwarf progenitor.