The underluminous Type la supernova 2005bl and the class of objects similar to SN 1991bg

Optical observations of the Type Ia supernova (SN Ia) 2005b1 in NGC 4070, obtained from -6 to +66d with respect to the B-band maximum, are presented. The photometric evolution is characterized by rapidly declining light curves [Delta m(15)(B)(true) = 1.93] and red colours at peak and soon thereafter. With M-B,M-max = -17.24 the SN is an underluminous SN Ia, similar to the peculiar SNe 1991bg and 1999by. This similarity also holds for the spectroscopic appearance, the only remarkable difference being the likely presence of carbon in pre-maximum spectra of SN 2005b1. A comparison study among underluminous SNe la is performed, based on a number of spectrophotometric parameters. Previously reported correlation of the light-curve decline rate with peak luminosity and R(Si) are confirmed, and a large range of post-maximum Si II lambda 6355 velocity gradients is encountered. 1D synthetic spectra for SN 2005b1 are presented, which confirm the presence of carbon and suggest an overall low burning efficiency with a significant amount of leftover unburned material. Also, the Fe content in pre-maximum spectra is very low, which may point to a low metallicity of the precursor. Implications for possible progenitor scenarios of underluminous SNe la are briefly discussed.