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, $E_{rm rad}approx 2.5times 10^{50}$ 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 $f_{rm Ni}approx 31%$ ($M_{rm Ni}approx 3.2$ M$_odot$). Instead, if we model the first peak with a combined magnetar spin-down and radioactive heating model we find a better match with $M_{rm ej}approx 4$ M$_odot$, a magnetar spin period of $P_{rm spin}approx 7.2$ ms and magnetic field of $Bapprox 10^{14}$ G, and $f_{rm Ni}lesssim 0.2$ (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 $approx 0.7$ $M_odot$ of hydrogen-free material located $approx 400$ AU from the progenitor. Including the remnant mass leads to a CO core mass prior to explosion of $approx 6.5$ M$_odot$. The host galaxy has a metallicity of $approx 0.26$ Z$_odot$, low for SNe Ic but consistent with SLSNe. Overall, we find that SN,2019stc is a transition object between normal SNe Ic and SLSNe.