K2 Observations of SN 2018oh Reveal a Two-component Rising Light Curve for a Type Ia Supernova

Dimitriadis, G. and Foley, R. J. and Rest, A. and Kasen, D. and Piro, A. L. and Vinkó, József and Sárneczky, Krisztián and Pál, András and Bódi, Attila and Sódorné Bognár, Zsófia and Csák, Balázs and Cseh, Borbála and Kalup, Csilla and Könyves-Tóth, Réka and Kriskovics, Levente and Sódor, Ádám and Szabó, Róbert and Szakáts, Róbert and Zsidi, Gabriella (2019) K2 Observations of SN 2018oh Reveal a Two-component Rising Light Curve for a Type Ia Supernova. ASTROPHYSICAL JOURNAL LETTERS, 870 (1). ISSN 2041-8205

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Abstract

We present an exquisite 30 minute cadence Kepler (K2) light curve of the Type Ia supernova (SN Ia) 2018oh (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 4 m 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 SN Ia behavior is confirmed in our i-band light curve, and furthermore, SN 2018oh is especially blue during the early epochs. The flux excess peaks 2.14 ± 0.04 days after explosion, has a FWHM of 3.12 ± 0.04 days, a blackbody temperature of T=17,{500}-9,000+11,500 K, a peak luminosity of 4.3+/- 0.2× {10}37 {erg} {{{s}}}-1, and a total integrated energy of 1.27+/- 0.01× {10}43 {erg}. We compare SN 2018oh 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 ∼2× {10}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 SN 2018oh would provide strong support for a single-degenerate progenitor system.

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