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We report the early discovery and multi-color ($BVI$) high-cadence lightcurve analyses of a rapidly-declining sub-Chandrasekhar Type Ia supernovaKSP-OT-201509b (= AT2015cx) from the KMTNet Supernova Program. The Phillipsparameter and color stretch parameter of KSP-OT-201509b (= AT2015cx) are$\Delta$$M_{B,15}$ $\simeq$ 1.62 mag and $s_{BV}$ $\simeq$ 0.54, respectively,at an inferred redshift of 0.072. These, together with other measuredparameters (such as the strength of the secondary $I$-band peak, colors andluminosity), identify the source to be a rapidly-declining Type Ia oftransitional nature that is closer to Branch Normal than 91bg-like. Its earlylight curve evolution and bolometric luminosity are consistent with those ofhomologously expanding ejecta powered by radioactive decay and a Type Ia SNexplosion with 0.32 $\pm$ 0.01 $M_{\odot}$ of synthesized $^{56}$Ni mass, 0.84$\pm$ 0.12 $M_{\odot}$ of ejecta mass and (0.61 $\pm$ 0.14) $\times$ 10$^{51}$erg of ejecta kinetic energy. While its $B-V$ and $V-I$ colors evolve largelysynchronously with the changes in the $I$-band light curve as found in othersupernovae, 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 iscompatible with a stratified $^{56}$Ni distribution extended to shallow layersof the exploding progenitor. Comparisons between the observed light curves andthose predicted from ejecta-companion interactions clearly disfavor RocheLobe-filling companion stars at large separation distances, thus supporting adouble-degenerate scenario for its origin. The lack of any apparent host galaxyin our deep stack images reaching a sensitivity limit of $\sim$ 28 $\rmmag\;arcsec^{-2}$ makes KSP-OT-201509b a hostless Type Ia supernova and offersnew insights into supernova host galaxy environments.

astrophysical journal/the astrophysical journal

Rapidly-Declining Hostless Type Ia Supernova KSP-OT-201509b from the  KMTNet Supernova Program: Transitional Nature and Constraint on $^{56}$Ni  Distribution and Progenitor Type