2017ApJ...844..160A

We present the discovery of a rapidly evolving transient by the Korean Microlensing Telescope Network Supernova Program (KSP). KSP is a novel high-cadence supernova survey that offers deep (∼21.5 mag in BV I bands), nearly continuous wide-field monitoring for the discovery of early and/or fast optical transients. KSP-OT-201509a, reported here, was discovered on 2015 September 27 during the KSP commissioning run in the direction of the nearby galaxy NGC 300, and stayed above detection limit for ∼22 days. We use our BV I light curves to constrain the ascent rate, -3.7(7) mag day^–1 in V, decay timescale, t₂^V=1.7(6) days, and peak absolute magnitude, -9.65 M_V -9.25 mag. We also find evidence for a short-lived pre-maximum halt in all bands. The peak luminosity and light-curve evolution make KSP-OT-201509a consistent with a bright, rapidly decaying nova outburst. We discuss constraints on the nature of the progenitor and its environment using archival Hubble Space Telescope (HST)/ACS images and conclude with a broad discussion on the nature of the system.