Hydrogen-poor superluminous supernovae ( SLSN-I ) are a class of rare and energetic explosions discovered in untargeted transient surveys in the past decade [ 1 , 2 ] .
The progenitor stars and the physical mechanism behind their large radiated energies ( \sim 10 ^ { 51 } erg ) are both debated , with one class of models primarily requiring a large rotational energy [ 3 , 4 ] , while the other requires very massive progenitors to either convert kinetic energy into radiation via interaction with circumstellar material ( CSM ) [ 5 , 6 , 7 , 8 ] , or engender a pair-instability explosion [ 9 , 10 ] .
Observing the structure of the CSM around SLSN-I offers a powerful test of some scenarios , though direct observations are scarce [ 11 , 12 ] .
Here , we present a series of spectroscopic observations of the SLSN-I iPTF16eh , which reveal both absorption and time- and frequency-variable emission in the Mg II resonance doublet .
We show that these observations are naturally explained as a resonance scattering light echo from a circumstellar shell .
Modeling the evolution of the emission , we find a shell radius of 0.1 pc and velocity of 3300 km s ^ { -1 } , implying the shell was ejected three decades prior to the supernova explosion .
These properties match theoretical predictions of pulsational pair-instability shell ejections , and imply the progenitor had a He core mass of \sim 50 - 55 ~ { } ~ { } M _ { \odot } , corresponding to an initial mass of \sim 115 ~ { } ~ { } M _ { \odot } .