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 \sim 1.3 \times 10 ^ { 44 } erg s ^ { -1 } , then decays slowly at 0.015 magnitude per day .
The measured ejecta velocity is \sim 13000 km s ^ { -1 } .
The inferred explosion characteristics , such as the ejecta mass ( 70 - 220 M _ { \odot } ) , the total radiative and kinetic energy ( E _ { rad } \sim 10 ^ { 51 } erg , E _ { kin } \sim 2 \times 10 ^ { 53 } erg ) , is typical of slow-evolving H-poor SLSN events .
However , the late-time spectrum taken at +251 days ( rest , post-peak ) reveals a Balmer H \alpha 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 \sim 4500 km s ^ { -1 } and a \sim 300 km s ^ { -1 } blue-ward shift relative to the narrow component .
We interpret this broad H \alpha emission with luminosity of \sim 2 \times 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 \sim 4 \times 10 ^ { 16 } 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 \leq 1 , thus setting upper limits on the shell mass \leq 30 M _ { \odot } .
Of the existing models , a Pulsational Pair Instability Supernova ( PPISN ) 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 % Â of all SLSNe-I may have late-time Balmer emission lines .