SN 2018 bsz：具有非球面拱星材料的I型超亮超新星
SN 2018bsz: a Type I superluminous supernova with aspherical circumstellar material
摘要：
本文对SN 2018 bsz的I型超亮超新星（SLSN-I）进行了光谱分析。虽然它与SLSNe-I非常相似，但出现在$\sim30$ d最大值后的多组分H $α$线是最不典型的。H $α$的特征是有两个发射分量，一个在$+3000$ km/s，另一个在$-7500$ km/s，第三个在延迟后出现，速度接近零。蓝色和中心分量可以用中等宽度的高斯分布来描述，但红色分量明显更宽，是洛伦兹分布。蓝色分量在峰后$100$ d处衰减之前朝向较低速度演变，同时具有光变曲线中断。在包括Pa $β$在内的其他氢谱线以及Ca II和He I谱线中观察到多组分分布。在H线出现之前（10.2天）和之后（38.4天）获得的分光偏振显示斯托克斯$Q$ -$U$平面上的大位移，与SN 2018 bsz的几何结构发生根本变化一致。假设SN在10.2 d时几乎未极化，则在38.4 d时的连续极化达到$P \sim1.8\%$，这意味着高度不对称的配置。我们认为SN 2018 bsz的演化可以用高度非球面的CSM来解释。在SN爆炸之后，CSM很快被喷出物所取代，但是当光球开始后退时，不同的CSM区域重新出现，产生奇特的线轮廓。根据H $α$的首次出现，我们可以将CSM的距离限制为小于$430$ AU，或者如果峰前平台与产生CSM的喷发有关，则甚至更低（$<87$ AU）。CSM的存在已被推断为其他SLSNe-I。然而，目前尚不清楚SN 2018 bsz的罕见性质是否可以推广到SLSNe-I，或者它们是否是不寻常的进化路径的结果，可能涉及一个双星伴星。
英文摘要：
We present a spectroscopic analysis of Type I superluminous supernova (SLSN-I), SN 2018bsz. While it closely resembles SLSNe-I, the multi-component H$α$ line appearing at $\sim30$ d post-maximum is the most atypical. The H$α$ is characterised by two emission components, one at $+3000$ km/s and a second at $-7500$ km/s, with a third, near-zero velocity component appearing after a delay. The blue and central components can be described by Gaussian profiles of intermediate width, but the red component is significantly broader and Lorentzian. The blue component evolves towards lower velocity before fading at $100$ d post-peak, concurrently with a light curve break. Multi-component profiles are observed in other hydrogen lines including Pa$β$, and in lines of Ca II and He I. Spectropolarimetry obtained before (10.2 d) and after (38.4 d) the appearance of the H lines show a large shift on the Stokes $Q$ -- $U$ plane consistent with SN 2018bsz undergoing radical changes in its geometry. Assuming the SN is almost unpolarised at 10.2 d, the continuum polarisation at 38.4 d reaches $P \sim1.8\%$ implying a highly asymmetric configuration. We propose that the observed evolution of SN 2018bsz can be explained by highly aspherical CSM. After the SN explosion, the CSM is quickly overtaken by the ejecta, but as the photosphere starts to recede, the different CSM regions re-emerge producing the peculiar line profiles. Based on the first appearance of H$α$, we can constrain the distance of the CSM to be less than $430$ AU, or even lower ($<87$ AU) if the pre-peak plateau is related to an eruption that created the CSM. The presence of CSM has been inferred for other SLSNe-I. However, it is not clear whether the rare properties of SN 2018bsz can be generalised for SLSNe-I or whether they are the result of an uncommon evolutionary path, possibly involving a binary companion.
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