We present photometric and spectroscopic observations of SN2013aa and SN2017cbv, two nearly identical type Ia supernovae (SNeIa) in the host galaxy NGC5643. 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 SNeIa as standard candles. The two SNe have nearly identical decline rates, negligible reddenings, and remarkably similar spectra, and, at a distance of ~20Mpc, they are ideal 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 two or more SNe Ia, we find that when using SNeIa, and after accounting for all sources of observational error, one gets consistency in distance to 3%.

Cone search capability for table J/ApJ/895/118/tablea2 (Optical local sequence photometry of SN2013aa and SN2017cbv)

Cone search capability for table J/ApJ/895/118/tablea3 (NIR local sequence photometry for SN2013aa)