Optical light curves of the type Ic SN 2019stc

SN 2019stc exhibits a broad double-peaked light curve, with the first peak having an absolute magnitude of M_r_=-20.0mag, and the second peak, about 80 rest-frame days later, M_r_=-19.2mag. The total radiated energy is large, E_rad_~2.5x10^50^erg.

Despite its large luminosity, approaching those of Type I superluminous supernovae (SLSNe), SN 2019stc exhibits a typical SN Ic spectrum, bridging the gap between SLSNe and SNe Ic. The spectra indicate the presence of Fe-peak elements, but modeling of the first light-curve peak with radioactive heating alone leads to an unusually high nickel mass fraction of f_Ni_~0.31 (M_Ni_~3.2M_{sun}_). Instead, if we model the first peak with a combined magnetar spin-down and radioactive heating model we find a better match with M_ej_~4M_{sun}_, a magnetar spin period of P_spin_~7.2ms, and magnetic field of B~10^14^G, and f_Ni_<~0.2 (consistent with SNe Ic).