Optical observations of the fast declining type Ib supernova iPTF13bvn

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We present optical UBVRI photometry and medium resolution spectroscopy of the type Ib supernova iPTF13bvn, spanning a phase of -13d to +71d 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)=0.045 and host galaxy extinction of E(B-V)=0.17, the absolute V-band magnitude peaks at Mᵥ=-17.23 ± 0.20. The bolometric light curve indicates that 0.09 M of ⁵⁶Ni was synthesized in the explosion. The earliest spectrum (-13d) shows the presence of He i 5876 Å feature at a velocity of 15000 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 (Mej) and kinetic energy of the explosion (Ek) 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.