Late-time Hubble Space Telescope Observations of a Hydrogen-poor Superluminous Supernova Reveal the Power-law Decline of a Magnetar Central Engine

Abstract

The light-curve diversity of hydrogen-poor superluminous supernovae (SLSNe) has kept open the possibility that multiple power sources account for the population. Specifically, pair-instability explosions (PISNe), which produce large masses of 56 Ni, have been argued as the origin of some slowly-evolving SLSNe. Here we present detailed observations of SN 2016inl (=PS16fgt), a slowly-evolving SLSN at z = 0.3057, whose unusually red spectrum matches PS1-14bj, an SLSN with an exceptionally long rise time consistent with a PISN. Ground-based and Hubble Space Telescope data, spanning about 800 rest-frame days, reveal a significant light-curve flattening, similar to that seen in SN 2015bn and much slower than the decline rate expected from radioactive decay of 56 Co. We therefore conclude that despite its slow evolution, SN 2016inl is inconsistent with a PISN. Instead, the light-curve evolution matches the expected power-law spindown of a magnetar central engine, but with a shallower power law ( L ∝ t −2.8 ) compared to that in SN 2015bn, indicating a possible difference in the γ -ray opacity between the two events. Analytical modeling indicates typical magnetar engine parameters, but one of the highest ejecta masses (≈20 M ⊙ ) inferred for an SLSN. Our results indicate that monitoring the late-time light-curve evolution of SLSNe provides a powerful diagnostic of their energy source.