The luminous and double-peaked type Ic supernova 2019stc: evidence for multiple energy sources

We present optical photometry and spectroscopy of SN 2019stc (=ZTF19acbonaa), an unusual Type Ic supernova (SN Ic) at a redshift of z = 0.117. SN 2019stc exhibits a broad double-peaked light curve, with the first peak having an absolute magnitude of M_r = -20.0 mag, and the second peak, about 80 rest-frame days later, M_r = -19.2 mag. The total radiated energy is large, E_rad ≍ 2.5 × 10⁵⁰ 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 ≍ 3.2 M_⊙). 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 ≍ 4 M_⊙, a magnetar spin period of P_spin ≍ 7.2 ms, and magnetic field of B ≍ 10¹⁴ G, and f_Ni ≲ 0.2 (consistent with SNe Ic). The prominent second peak cannot be naturally accommodated with radioactive heating or magnetar spin-down, but instead can be explained as circumstellar interaction with ≍0.7 M_⊙ of hydrogen-free material located ≍400 au from the progenitor. Accounting for the ejecta mass, circumstellar shell mass, and remnant neutron star mass, we infer a CO core mass prior to explosion of ≍6.5 M_⊙. The host galaxy has a metallicity of ≍0.26 Z_⊙, low for SNe Ic but consistent with SLSNe. Overall, we find that SN 2019stc is a transition object between normal SNe Ic and SLSNe....