Detection of Broad H$\alpha$ 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
within (83-148)days (rest-frame) to reach a peak bolometric luminosity of
1.3x$10^{44}$erg/s, then decays very slowly at 0.015mag. per day. The measured
ejecta velocity is 13000km/s. The inferred explosion characteristics, such as
the ejecta mass (67-220$M_\odot$), the total radiative and kinetic energy
($10^{51}$ & 2x$10^{53}$erg respectively), is typical of a slow-evolving H-poor
SLSN event. However, the late-time spectrum taken at +251days reveals a Balmer
Halpha 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 ~4500km/s and has a ~300km/s blue-ward shift relative to the narrow
component. We interpret this broad H$\alpha$ emission with luminosity of
$\sim$2$\times10^{41}$\,erg\,s$^{-1}$ as resulting from the interaction between
the supernova ejecta and a discrete H-rich shell, located at a distance of
$\sim4\times10^{16}$\,cm from the explosion site. This ejecta-CSM 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 CSM shell
implies that its Thomson scattering optical depth is likely <1, thus setting
upper limits on the CSM mass <30$M_\odot$ and the volume number density
<4x$10^8cm^{-3}$. Of the existing models, a Pulsational Pair Instability
Supernova model can naturally explain the observed 30$M_\odot$ H-shell, ejected
from a progenitor star with an initial mass of (95-150)$M_\odot$ about 40 years
ago. We estimate that at least $\sim$15\%\ of all SLSNe-I may have late-time
Balmer emission lines.
