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.

我们展示了 SN 2019stc (=ZTF19acbonaa) 的光学光度和光谱学，这是一颗红移z = 0.117的不寻常的 Ic 型超新星 (SN Ic) 。SN 2019stc 呈现出宽阔的双峰光变曲线，第一个峰值的绝对星等为M _r = -20.0 mag，第二个峰值在大约 80 天后静帧后，M _r = -19.2 mag。总辐射能量大，E _rad ≈ 2.5 10 ⁵⁰ 尔格。尽管 SN 2019stc 的光度很大，接近 I 型超光速超新星 (SLSNe) 的光度，但 SN 2019stc 表现出典型的 SN Ic 光谱，弥合了 SLSNe 和 SNe Ic 之间的差距。光谱表明存在 Fe 峰元素，但仅使用放射性加热对第一个光曲线峰进行建模会导致镍质量分数异常高，f _Ni ≈ 0.31 ( M _Ni ≈ 3.2 M _⊙ )。相反，如果我们使用组合的磁星自旋下降和放射性加热模型对第一个峰值进行建模，我们会发现与M _ej ≈ 4 M _⊙、磁星自旋周期P _自旋≈ 7.2 ms 和磁场B的更好匹配≈ 10 ¹⁴ G，并且f _Ni ≲ 0.2（与 SNe Ic 一致）。突出的第二个峰不能自然地适应放射性加热或磁星自旋下降，而是可以解释为与 ≈0.7 M _⊙的无氢物质的星周相互作用，该物质位于距离前身 ≈400 au。考虑到喷射物质量、星周壳质量和残余中子星质量，我们推断爆炸前的 CO 核心质量约为 6.5 M _⊙。宿主星系的金属丰度为 ≈0.26 Z _⊙，对于 SNe Ic 较低，但与 SLSNe 一致。总的来说，我们发现 SN 2019stc 是正常 SNe Ic 和 SLSNe 之间的过渡对象。