The brightness of SN 1991T and the uniformity of decline-rate and colour corrected absolute magnitudes of supernovae Ia

Abstract

We present a distance to NGC 4527, the host galaxy of the type Ia SN 1991T, measured by surface brightness fluctuations. This supernova has been labelled ``peculiar'' both on the grounds of its spectroscopic behaviour and its apparent overluminosity with respect to other supernovae. The distance modulus to NGC 4527 and thus to SN 1991T is 30.26+/-0.09. This relatively short distance largely removes the discrepancy with other Ia supernovae having similar light-curve characteristics and also removes the motivation for interpreting SN 1991T as a super-Chandrasekhar explosion. However, the reddening uncertainty results in significant uncertainty of the absolute magnitudes. We show further that, although the intrinsic absolute magnitudes among Ia supernovae are quite different, their brightness, corrected for decline-rate and colour, shows a high degree of uniformity, including underluminous events like SN 1991bg and SN 1997cn. In particular, the present data do not support a significant difference between corrected SNe Ia luminosities in late-type and early-type host galaxies. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS 5-26555.