We present photometric and spectroscopic observations of SN 2013aa and SN 2017cbv, two nearly identical type Ia supernovae (SNe Ia) in the host galaxy NGC 5643. The optical photometry has been obtained using the same telescope and instruments used by the Carnegie Supernova Project. This eliminates most instrumental systematics and provides light curves in a stable and well-understood photometric system. Having the same host galaxy also eliminates systematics due to distance and peculiar velocity, providing an opportunity to directly test the relative precision of SNe Ia as standard candles. The two SNe have nearly identical decline rates, negligible reddening, and remarkably similar spectra and, at a distance of $\sim 20$ Mpc, are ideal as potential calibrators for the absolute distance using primary indicators such as Cepheid variables. We discuss to what extent these two SNe can be considered twins and compare them with other supernova "siblings" in the literature and their likely progenitor scenarios. Using 12 galaxies that hosted 2 or more SNe~Ia, we find that when using SNe~Ia, and after accounting for all sources of observational error, one gets consistency in distance to 3 percent.

中文翻译：

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SN 2013aa 和 SN 2017cbv：螺旋星系 NGC 5643 中的两个同胞 Ia 型超新星

我们展示了 SN 2013aa 和 SN 2017cbv 这两个在宿主星系 NGC 5643 中几乎完全相同的 Ia 型超新星 (SNe Ia) 的光度和光谱观测。使用与卡内基超新星项目相同的望远镜和仪器获得了光学测光。这消除了大多数仪器系统，并在稳定且易于理解的光度系统中提供光曲线。由于距离和特殊速度，拥有相同的宿主星系也消除了系统性，提供了直接测试 SNe Ia 作为标准烛光的相对精度的机会。这两个 SNe 具有几乎相同的下降率、可忽略不计的红化和非常相似的光谱，并且在 $\sim 20 $ Mpc 的距离处，非常适合作为使用主要指标（如造父变星变量）的绝对距离的潜在校准器。我们讨论这两个超新星在多大程度上可以被视为双胞胎，并将它们与文献中的其他超新星“兄弟姐妹”及其可能的祖先场景进行比较。使用拥有 2 个或更多 SNe~Ia 的 12 个星系，我们发现当使用 SNe~Ia 时，在考虑到所有观测误差来源后，距离的一致性达到 3%。