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On 2018 Feb. 4.41, the All-Sky Automated Survey for SuperNovae (ASAS-SN)discovered ASASSN-18bt in the K2 Campaign 16 field. With a redshift ofz=0.01098 and a peak apparent magnitude of B_{max}=14.31, ASASSN-18bt is thenearest and brightest SNe Ia yet observed by the Kepler spacecraft. Here wepresent the discovery of ASASSN-18bt, the K2 light curve, and pre-discoverydata from ASAS-SN and the Asteroid Terrestrial-impact Last Alert System(ATLAS). The K2 early-time light curve has an unprecedented 30-minute cadenceand photometric precision for an SN~Ia light curve, and it unambiguously showsa ~4 day nearly linear phase followed by a steeper rise. Thus, ASASSN-18btjoins a growing list of SNe Ia whose early light curves are not well describedby a single power law. We show that a double-power-law model fits the datareasonably well, hinting that two physical processes must be responsible forthe observed rise. However, we find that current models of the interaction witha non-degenerate companion predict an abrupt rise and cannot adequately explainthe initial, slower linear phase. Instead, we find that existing, publishedmodels with shallow 56Ni are able to span the observed behavior and, withtuning, may be able to reproduce the ASASSN-18bt light curve. Regardless, moretheoretical work is needed to satisfactorily model this and other early-timeSNe~Ia light curves. Finally, we use Swift X-ray non-detections to constrainthe presence of circumstellar material (CSM) at much larger distances and lowerdensities than possible with the optical light curve. For a constant densityCSM these non-detections constrain rho<4.5 * 10^5 cm^-3 at a radius of 4 *10^15cm from the progenitor star. Assuming a wind-like environment, we placemass-loss limits of Mdot< 8 * 10^-6 M_sun yr^-1 for v_w=100 km s^-1, ruling outsome symbiotic progenitor systems.

Seeing Double: ASASSN-18bt Exhibits a Two-Component Rise in the Early-Time K2 Light Curve