On 2018 February 4.41, the All-Sky Automated Survey for SuperNovae (ASAS-SN) discovered ASASSN-18bt in the K2 Campaign 16 field. With a redshift of z=0.01098 and a peak apparent magnitude of B max =14.31, ASASSN-18bt is the nearest and brightest SNe Ia yet observed by the Kepler spacecraft. Here we present the discovery of ASASSN-18bt, the K2 light curve, and prediscovery data from ASAS-SN and the Asteroid Terrestrial-impact Last Alert System. The K2 early-time light curve has an unprecedented 30-minute cadence and photometric precision for an SNIa light curve, and it unambiguously shows a ∼4 day nearly linear phase followed by a steeper rise. Thus, ASASSN-18bt joins a growing list of SNe Ia whose early light curves are not well described by a single power law. We show that a double-power-law model fits the data reasonably well, hinting that two physical processes must be responsible for the observed rise. However, we find that current models of the interaction with a nondegenerate companion predict an abrupt rise and cannot adequately explain the initial, slower linear phase. Instead, we find that existing published models with shallow 56 Ni are able to span the observed behavior and, with tuning, may be able to reproduce the ASASSN-18bt light curve. Regardless, more theoretical work is needed to satisfactorily model this and other early-time SNeIa light curves. Finally, we use Swift X-ray nondetections to constrain the presence of circumstellar material (CSM) at much larger distances and lower densities than possible with the optical light curve. For a constant-density CSM, these nondetections constrain ρ<4.5×10 5 cm −3 at a radius of 4×10 15 cm from the progenitor star. Assuming a wind-like environment, we place mass loss limits of M M 8 10 yr 6 1 <´-☉ for v w =100 km s −1 , ruling out some symbiotic progenitor systems. This work highlights the power of well-sampled early-time data and the need for immediate multiband, high-cadence follow-up for progress in understanding SNeIa.

We present the discovery of PS18kh, a tidal disruption event (TDE) discovered at the center of SDSS J075654.53+341543.6 (d 337 Mpc) by the Pan-STARRS Survey for Transients. Our dataset includes pre-discovery survey data from Pan-STARRS, the All-Sky Automated Survey for Supernovae (ASAS-SN), and the Asteroid Terrestrial-impact Last Alert System (ATLAS) as well as high-cadence, multi-wavelength follow-up data from ground-based telescopes and Swift, spanning from 56 days before peak light until 75 days after. The optical/UV emission from PS18kh is well-fit as a blackbody with temperatures ranging from T 14000 K to T 22000 K and it peaked at a luminosity of L 9.8 × 10 43 ergs s −1 . PS18kh radiated E = (3.82 ± 0.25) × 10 50 ergs over the period of observation, with (1.58 ± 0.22) × 10 50 ergs being released during the rise to peak. Spectra of PS18kh show a changing, boxy/double-peaked Hα emission feature, which becomes more prominent over time. Using a wind+elliptical disk+spiral arm model, we model the physical properties of the accretion disk and the stellar debris following the disruption of the star, finding that the stellar Corresponding author: T. W.-S. Holoien tholoien@carnegiescience.edu arXiv:1808.02890v2 [astro-ph.HE] 14 Aug 2018 2 HOLOIEN, ET AL.debris initially absorbs the emission from the disk but becomes optically thin over time. The disk has an inner radius of r in ∼ 500r g and an outer radius of r out ∼ 15000r g .