DETECTION of BROAD Hα EMISSION LINES in the LATE-TIME SPECTRA of A HYDROGEN-POOR SUPERLUMINOUS SUPERNOVA

iPTF13ehe is a hydrogen-poor superluminous supernova (SLSN) at z = 0.3434, with a slow-evolving light curve and spectral features similar to SN2007bi. It rises in 83-148 days to reach a peak bolometric luminosity of ∼1.3 × 1044 erg s-1, then decays slowly at 0.015 mag day-1. The measured ejecta velocity is ∼ 13,000 km s-1. The inferred explosion characteristics, such as the ejecta mass (70-220 Mo), and the total radiative and kinetic energy (Erad ∼ 1051 erg, Ekin ∼ 2 × 1053 erg), are typical of slow-evolving H-poor SLSN events. However, the late-time spectrum taken at +251 days (rest, post-peak) reveals a Balmer Hα emission feature with broad and narrow components, which has never been detected before among other H-poor SLSNe. The broad component has a velocity width of ∼4500 km s-1 and a ∼300 km s-1 blueward shift relative to the narrow component. We interpret this broad Hα emission with a luminosity of ∼2 × 1041 erg s-1 as resulting from the interaction between the supernova ejecta and a discrete H-rich shell, located at a distance of ∼4 × 1016 cm from the explosion site. This interaction causes the rest-frame r-band LC to brighten at late times. The fact that the late-time spectra are not completely absorbed by the shock-ionized H-shell implies that its Thomson scattering optical depth is likely ≤1, thus setting upper limits on the shell mass ≤30 Mo. Of the existing models, a Pulsational Pair Instability supernova model can naturally explain the observed 30 Mo H-shell, ejected from a progenitor star with an initial mass of (95-150) Mo about 40 years ago. We estimate that at least ∼15% of all SLSNe-I may have late-time Balmer emission lines