Rapidly Declining Hostless Type Ia Supernova KSP-OT-201509b from the KMTNet Supernova Program: Transitional Nature and Constraint on <SUP>56</SUP>Ni Distribution and Progenitor Type

We report the early discovery and multicolor (BVI) high-cadence light-curve analyses of the rapidly declining sub-Chandrasekhar Type Ia supernova KSP-OT-201509b (= AT 2015cx) from the KMTNet Supernova Program. The Phillips and color stretch parameters of KSP-OT-201509b are Delta M-B,M-15 1.62 mag and s(BV) 0.54, respectively, at an inferred redshift of 0.072. These, together with other measured parameters (such as the strength of the secondary I-band peak, colors, and luminosity), identify the source to be a rapidly declining Type Ia of a transitional nature that is closer to Branch-normal than 91bg-like. Its early light-curve evolution and bolometric luminosity are consistent with those of homologously expanding ejecta powered by radioactive decay and a Type Ia SN explosion with 0.32 0.01 M of synthesized Ni-56 mass, 0.84 0.12 M of ejecta mass, and (0.61 0.14) x 10(51) erg of ejecta kinetic energy. While its B - V and V - I colors evolve largely synchronously with the changes in the I-band light curve, as found in other supernovae, we also find the presence of an early redward evolution in V - I prior to -10 days since peak. The bolometric light curve of the source is compatible with a stratified Ni-56 distribution extended to shallow layers of the exploding progenitor. Comparisons between the observed light curves and those predicted from ejecta-companion interactions clearly disfavor Roche lobe-filling companion stars at large separation distances, thus supporting a double-degenerate scenario for its origin. The lack of any apparent host galaxy in our deep stack images reaching a sensitivity limit of similar to 28 mag arcsec(-2) makes KSP-OT-201509b a hostless Type Ia supernova and offers new insights into supernova host galaxy environments.