Seeing Double: ASASSN-18bt Exhibits a Two-component Rise in the Early-time K2 Light

Shappee, B. J.; Holoien, T. W. -S.; Drout, M. R.; Auchettl, K.; Stritzinger, M. D.; Kochanek, C. S.; Stanek, K. Z.; Shaya, E.; Narayan, G.; Brown, J. S.; Bose, S.; Bersier, D.; Brimacombe, J.; Chen, Ping; Dong, Subo; Holmbo, S.; Katz, B.; Munoz, J. A.; Mutel, R. L.; Post, R. S.; Prieto, J. L.; Shields, J.; Tallon, D.; Thompson, T. A.; Vallely, P. J.; Villanueva, S., Jr.; Denneau, L.; Flewelling, H.; Heinze, A. N.; Smith, K. W.; Stalder, B.; Tonry, J. L.; Weiland, H.; Barclay, T.; Barentsen, G.; Cody, A. M.; Dotson, J.; Foerster, F.; Garnavich, P.; Gully-Santiago, M.; Hedges, C.; Howell, S.; Kasen, D.; Margheim, S.; Mushotzky, R.; Rest, A.; Tucker, B. E.; Villar, A.; Zenteno, A.; Beerman, G.; Bjella, R.; Castillo, G.; Coughlin, J.; Elsaesser, B.; Flynn, S.; Gangopadhyay, R.; Griest, K.; Hanley, M.; Kampmeier, J.; Kloetzel, R.; Kohnert, L.; Labonde, C.; Larsen, R.; Larson, K. A.; McCalmont-Everton, K. M.; McGinn, C.; Migliorini, L.; Moffatt, J.; Muszynski, M.; Nystrom, V.; Osborne, D.; Packard, M.; Peterson, C. A.; Redick, M.; Reedy, L. H.; Ross, S. E.; Spencer, B.; Steward, K.; Van Cleve, J. E.; Vinicius de Miranda Cardoso, J.; Weschler, T.; Wheaton, A.; Bulger, J.; Chambers, K. C.; Flewelling, H. A.; Huber, M. E.; Lowe, T. B.; Magnier, E. A.; Schultz, A. S. B.; Waters, C. Z.; Willman, M.; Baron, E.; Chen, Zhihao; Derkacy, James M.; Huang, Fang; Li, Linyi; Li, Wenxiong; Li, Xue; Mo, Jun; Rui, Liming; Sai, Hanna; Wang, Lifan; Wang, Lingzhi; Wang, Xiaofeng; Xiang, Danfeng; Zhang, Jicheng; Zhang, Jujia; Zhang, Kaicheng; Zhang, Tianmeng; Zhang, Xinghan; Zhao, Xulin; Brown, P. J.; Hermes, J. J.; Nordin, J.; Points, S.; Sodor, A.; Strampelli, G. M.; ASAS-SN; ATLAS; Kepler K2; Kepler Spacecraft Team; Pan-STARRS; PTSS TNTS
2019
ASTROPHYSICAL JOURNAL
DOI
10.3847/1538-4357/aaec79
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 similar to 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 Ni-56 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 rho < 4.5 x 10(5) cm(-3) at a radius of 4 x 10(15) cm from the progenitor star. Assuming a wind-like environment, we place mass loss limits of <(M)over dot> < 8 x 10(-6) M-circle dot yr(-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.