A UV Resonance Line Echo from a Shell Around a Hydrogen-Poor Superluminous Supernova
astro-ph.HE
/ Authors
R. Lunnan, C. Fransson, P. M. Vreeswijk, S. E. Woosley, G. Leloudas, D. A. Perley, R. M. Quimby, Lin Yan, N. Blagorodnova, B. D. Bue
and 14 more authors
S. B. Cenko, A. De Cia, D. O. Cook, C. U. Fremling, P. Gatkine, A. Gal-Yam, M. M. Kasliwal, S. R. Kulkarni, F. J. Masci, P. E. Nugent, A. Nyholm, A. Rubin, N. Suzuki, P. Wozniak
/ Abstract
Hydrogen-poor superluminous supernovae (SLSN-I) are a class of rare and energetic explosions discovered in untargeted transient surveys in the past decade. The progenitor stars and the physical mechanism behind their large radiated energies ($\sim10^{51}$ erg) are both debated, with one class of models primarily requiring a large rotational energy, while the other requires very massive progenitors to either convert kinetic energy into radiation via interaction with circumstellar material (CSM), or engender a pair-instability explosion. Observing the structure of the CSM around SLSN-I offers a powerful test of some scenarios, though direct observations are scarce. 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 $\sim 50-55~{\rm M}_{\odot}$, corresponding to an initial mass of $\sim 115~{\rm M}_{\odot}$.