We present the discovery and high-cadence follow-up observations of SN 2018ivc, an unusual Type II supernova that exploded in NGC 1068 (D=10.1 Mpc). The light curve of SN 2018ivc declines piecewise-linearly, changing slope frequently, with four clear slope changes in the first 30 days of evolution. This rapidly changing light curve indicates that interaction between the circumstellar material and ejecta plays a significant role in the evolution. Circumstellar interaction is further supported by a strong X-ray detection. The spectra are rapidly evolving and dominated by hydrogen, helium, and calcium emission lines. We identify a rare high-velocity emission-line feature blueshifted at ~7800 km/s (in Ha, Hb, Pb, Pg, HeI, CaII), which is visible from day 18 until at least day 78 and could be evidence of an asymmetric progenitor or explosion. From the overall similarity between SN 2018ivc and SN 1996al, the \Ha{} equivalent width of its parent HII region, and constraints from pre-explosion archival Hubble Space Telescope images, we find that the progenitor of SN 2018ivc could be as massive as 52 Msun but is more likely <12 Msun. SN 2018ivc demonstrates the importance of the early discovery and rapid follow-up observations of nearby supernovae to study the physics and progenitors of these cosmic explosions.

中文翻译：

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NGC 1068 中不寻常的 II 型 SN 2018ivc 的发现和快速跟踪观测

我们展示了 SN 2018ivc 的发现和高频率后续观测，这是一颗在 NGC 1068 (D=10.1 Mpc) 中爆炸的不寻常 II 型超新星。SN 2018ivc的光变曲线呈分段线性下降，斜率变化频繁，在进化的前30天有4次明显的斜率变化。这种快速变化的光变曲线表明，星周物质和喷射物之间的相互作用在演化中起着重要作用。强大的 X 射线探测进一步支持了环绕恒星的相互作用。光谱正在迅速演变，并以氢、氦和钙发射线为主。我们发现了一个罕见的高速发射线特征，以~7800 公里/秒的速度蓝移（在 Ha、Hb、Pb、Pg、HeI、CaII），从第 18 天到至少第 78 天都可以看到，并且可能是不对称的祖先或爆炸。从 SN 2018ivc 和 SN 1996al 之间的整体相似性、其父 HII 区域的 \Ha{} 等效宽度以及来自爆炸前档案哈勃太空望远镜图像的限制，我们发现 SN 2018ivc 的祖先质量可能高达 52 Msun 但更有可能 <12 Msun。SN 2018ivc 证明了对附近超新星的早期发现和快速后续观测对研究这些宇宙爆炸的物理和起源的重要性。