Optical observations of the fast declining Type Ib supernova iPTF13bvn

We present optical UBVRI photometry and medium resolution spectroscopy of the Type Ib supernova iPTF13bvn, spanning a phase of ∼-13 d to +71 d with respect to B-band maximum. The post-maximum decline rates indicate a fast decline with Δm₁₅(B) = 1.82. Correcting for a Galactic extinction E(B - V)_MW = 0.045 and host galaxy extinction of E(B - V)_host = 0.17, the absolute V-band magnitude peaks at M_V = -17.23 ± 0.20. The bolometric light curve indicates that ∼0.09 M_⊙ of ⁵⁶Ni was synthesized in the explosion. The earliest spectrum (-13 d) shows the presence of He I 5876 Å feature at a velocity of ∼15 000 km s^-1, which falls rapidly by the time the SN approaches the epoch of B-band maximum. The photospheric velocity near maximum light, as indicated by the Fe II 5169 Å feature, is ∼9000 km s^-1. The estimate for the ⁵⁶Ni mass, together with the estimates for the ejected mass (M_ej) and kinetic energy of the explosion (E_k) indicate that iPTF13bvn is a low-luminosity Type Ib supernova, with a lower than average ejected mass and kinetic energy. Our results suggest that the progenitor of iPTF13bvn is inconsistent with a single Wolf-Rayet star.