Twins for life? A comparative analysis of the Type Ia supernovae 2011fe and 2011by

The nearby Type Ia supernovae (SNe Ia) 2011fe and 2011by had nearly identical\nphotospheric phase optical spectra, light-curve widths, and photometric\ncolours, but at peak brightness SN 2011by reached a fainter absolute magnitude\nin all optical bands and exhibited lower flux in the near-ultraviolet (NUV).\nBased on those data, Foley & Kirshner (2013) argue that the progenitors of SNe\n2011by and 2011fe were supersolar and subsolar, respectively, and that SN\n2011fe generated 1.7 times the amount of 56Ni as SN 2011by. With this work, we\nextend the comparison of these SNe Ia to 10 days before and 300 days after\nmaximum brightness with new spectra and photometry. We show that the nebular\nphase spectra of SNe 2011fe and 2011by are almost identical, and do not support\na factor of 1.7 difference in 56Ni mass. Instead, we find it plausible that the\nTully-Fisher distance for SN 2011by is an underestimate, in which case these\nSNe Ia may have reached similar peak luminosity, formed similar amounts of\n56Ni, and had lower metallicity progenitors than previously estimated.\nRegardless of the true distance to SN 2011by, we find that the relative\nprogenitor metallicity difference remains well supported by their disparity in\nNUV flux, which we show to be even stronger at pre-maximum epochs --- although\ncontributions from differences in total ejecta mass, viewing angle, or\nprogenitor density cannot be ruled out. We also demonstrate that, independent\nof distance modulus, SN 2011by exhibits a late-time luminosity excess that\ncannot be explained by a light echo, but is more likely to be the result of\ngreater energy trapping by the nucleosynthetic products of SN 2011by.\n