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 similar to 1.3 x 10(44) erg s(-1), then decays slowly at 0.015 mag day(-1). The measured ejecta velocity is similar to 13,000 km s(-1). The inferred explosion characteristics, such as the ejecta mass (70-220M(circle dot)), and the total radiative and kinetic energy (E-rad similar to 10(51) erg, E-kin similar to 2 x 10(53) 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 Ha 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 similar to 4500 km s(-1) and a similar to 300 km s(-1) blueward shift relative to the narrow component. We interpret this broad Ha emission with a luminosity of similar to 2 x 10(41) erg s(-1) as resulting from the interaction between the supernova ejecta and a discrete H-rich shell, located at a distance of similar to 4 x 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 <= 30M(circle dot). Of the existing models, a Pulsational Pair Instability supernova model can naturally explain the observed 30M(circle dot) H-shell, ejected from a progenitor star with an initial mass of (95-150) M-circle dot about 40 years ago. We estimate that at least similar to 15% of all SLSNe-I may have late-time Balmer emission lines.