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

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 twocomponent 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 erg s 37 1 ´-, and a total integrated energy of 1.27 0.01 10 erg 43 ´. 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 cm 12 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.

Explaining the Early Excess Emission of the Type Ia Supernova 2018oh by the Interaction of the Ejecta with Disk-originated Matter

“…The ejecta-DOM model has κ = 0.03 cm 2 /g, ∆t exp = 5 × 10 3 s, M DOM = 0.01 M , f DOM = 0.15 and the rest of the parameters as in section 2. The ejecta-companion model has a binary separation of a = 2 × 10 12 cm as in Dimitriadis et al (2019a). We show this fit mainly to illustrate that the two interaction models have similar fitting power.…”

“…While this cannot be strictly correct, we find the explosion time cannot be much earlier than the first detection time if we assume an interaction model. Shappee et al (2018), Li et al (2018) and Dimitriadis et al (2019a) separately compute different first light times from fitting models of the form L ∝ (t − t 0 ) α to the light curve. These first light times are t exp,fit = t FD − (0.19 − 0.45) days, i.e., 0.19 − 0.45 days before first K2 detection.…”

“…3 shows the excess luminosity after subtracting a L ∝ t 2 power-law fit from it. We use the same range as Dimitriadis et al (2019a) to fit a power-law model and subtract it from the K2 flux. The ejecta-DOM model fit to the rising part of the excess light curve has κ = 0.2 cm 2 /g, ∆t exp = 5 × 10 3 s, M DOM = 0.1 M , f DOM = 0.1 and the rest of the parameters as in section 2.…”

“…Several SNe Ia show early ( 5 days) excess emission in their light curve. Notable examples are normal SNe Ia SN 2012cg (Marion et al 2016), SN 2017cbv (Hosseinzadeh et al 2017) and SN 2018oh Dimitriadis et al 2019a), and the peculiar events iPTF14atg (Cao et al 2015) and MUSSES1604D (Jiang et al 2017). A comprehensive list of SNe Ia with early excess emission appears in Jiang et al (2018).…”

“…Their preferred explanation is the presence of 56 Ni in the outskirts of the ejecta, although existing models need tuning. Dimitriadis et al (2019a) studied the presence of 0.03 M of 56 Ni on the surface of the ejecta for SN 2018oh, but argue that although this model can explain the early light curve, the expected colors are redder than the observed color curve. They therefore favored an ejecta-companion collision model.…”