The Luminous and Double-Peaked Type Ic Supernova 2019stc: Evidence for Multiple Energy Sources

Abstract

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, $E_{\mathrm{rad}}\approx 2.5\times 10^{50}$ 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 $f_{\mathrm{Ni}}\approx 31\%$ ($M_{\mathrm{Ni}}\approx 3.2$ 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 $M_{\mathrm{ej}}\approx 4$ M${}_{\odot }$, a magnetar spin period of $P_{\mathrm{spin}}\approx 7.2$ ms and magnetic field of $B\approx 10^{14}$ G, and $f_{\mathrm{Ni}}\lesssim 0.2$ (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.

14 pages, 13 figures, Accepted to ApJ

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