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Description
We present an exquisite 30minute cadence Kepler (K2) light curve of the Type Ia supernova (SNIa) 2018oh All-Sky Automated Survey for Supernovae (ASASSN-18bt), starting weeks before explosion, covering the moment of explosion and the subsequent rise, and continuing past peak brightness. These data are supplemented by multi-color Panoramic Survey Telescope (Pan-STARRS1) and Rapid Response System 1 and Cerro Tololo Inter-American Observatory 4m Dark Energy Camera (CTIO 4-m DECam) observations obtained within hours of explosion. The K2 light curve has an unusual two-component shape, where the flux rises with a steep linear gradient for the first few days, followed by a quadratic rise as seen for typical supernovae (SNe) Ia. This "flux excess" relative to canonical SNIa behavior is confirmed in our i-band light curve, and furthermore, SN2018oh is especially blue during the early epochs. The flux excess peaks 2.14{+/-}0.04 days after explosion, has a full width at half maximum (FWHM) of 3.12{+/-}0.04 days, a blackbody temperature of T=17500_-9000_^+11500^K, a peak luminosity of 4.3{+/-}0.2x10^37^erg/s, and a total integrated energy of 1.27{+/-}0.01x10^43^erg. We compare SN2018oh to several models that may provide additional heating at early times, including collision with a companion and a shallow concentration of radioactive nickel. While all of these models generally reproduce the early K2 light curve shape, we slightly favor a companion interaction, at a distance of ~2x10^12^cm based on our early color measurements, although the exact distance depends on the uncertain viewing angle. Additional confirmation of a companion interaction in future modeling and observations of SN2018oh would provide strong support for a single-degenerate progenitor system.
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