2018ApJ...865....9B

We present UV/optical observations of PS16aqv (SN 2016ard), a fast-evolving Type I superluminous supernova (SLSN-I) that reached a peak absolute magnitude of M_r ≃ -22.1. The lightcurves exhibit a significant undulation at 30 rest-frame days after peak, with a behavior similar to undulations seen in the slowly fading SLSN-I SN 2015bn. This similarity strengthens the case that fast and slow SLSNe-I form a continuum with a common origin. At ≃80 days after peak, the lightcurves exhibit a transition to a slow decline, followed by significant steepening, indicative of a plateau phase or a second significant undulation. Deep limits at ≃280 days after peak imply a tight constraint on the nickel mass, M_Ni <= 0.35 M_☉ (lower than for previous SLSNe-I), and indicate that some SLSNe-I do not produce significantly more nickel than normal Type Ic SNe. Using MOSFiT, we model the lightcurve with a magnetar central engine model and find P_spin ≃ 0.9 ms, B ≃ 1.5 x 10¹⁴ G, and M_ej ≃ 16 M_☉. The implied rapid spin-down time and large reservoir of available energy coupled with the high ejecta mass may account for the fast lightcurve and slow spectroscopic evolution. We also study PS16aqv's location within its host galaxy and find that it occurred at an offset of 2.46 ± 0.21 kpc from the central star-forming region. Aside from high extinction, the host properties are similar to most other SLSN-I host galaxies. The complexity in the lightcurves of PS16aqv and other events highlights the importance of obtaining well-sampled lightcurves for exploring deviations from a uniform decline.