HST observations of SN2016gkg and its pre-explosion counterpart

We present Hubble Space Telescope (HST) observations of the type IIb supernova (SN) 2016gkg at 652, 1698, and 1795 days with the Advanced Camera for Surveys (ACS) and Wide Field Camera 3 (WFC3). Comparing to pre-explosion imaging from 2001 obtained with the Wide Field Planetary Camera 2, we demonstrate that SN 2016gkg is now fainter than its candidate counterpart in the latest WFC3 imaging, implying that the counterpart has disappeared and confirming that it was the SN progenitor star. We show the latest light curve and Keck spectra of SN 2016gkg, which implies that SN 2016gkg is declining more slowly than the expected rate for 56 Co decay during its nebular phase. We find that this emission is too luminous to be powered by other radioisotopes, thus we infer that SN 2016gkg is entering a new phase in its evolution where it is powered primarily by interaction with circumstellar matter. Finally, we re-analyze the progenitor star spectral energy distribution and late-time limits in the context of binary evolution models and including emission from a potential companion star and find that all companion stars would be fainter than our limiting magnitudes.

我们利用哈勃空间望远镜（Hubble Space Telescope, HST）的高级巡天相机（Advanced Camera for Surveys, ACS）和第三代广域相机（Wide Field Camera 3, WFC3），在爆发后652天、1698天及1795天对IIb型超新星（supernova, SN）2016gkg开展了观测。通过与2001年由第二代广域行星相机（Wide Field Planetary Camera 2）获取的爆发前图像对比，我们发现，在最新的WFC3图像中，SN 2016gkg的亮度已低于其候选前身星对应体，这表明该对应体已消失，从而证实其为该超新星的前身星（progenitor star）。我们展示了SN 2016gkg最新的光变曲线（light curve）和凯克望远镜（Keck）光谱数据，结果显示，在星云阶段（nebular phase），其亮度衰减速率慢于⁵⁶Co衰变的预期速率。我们发现该辐射亮度过高，无法由其他放射性同位素（radioisotope）提供能量，因此推断SN 2016gkg正进入演化新阶段，其能量主要来源于与星周物质（circumstellar matter）的相互作用。最后，结合双星演化模型（binary evolution model），我们重新分析了前身星的光谱能量分布（spectral energy distribution）及晚期限制条件（包括潜在伴星的辐射），发现所有可能的伴星亮度均低于我们的探测极限。