The fast evolution of SN 2010bh associated with XRF 100316D

Context. The first observational evidence of a connection between supernovae (SNe) and gamma-ray bursts (GRBs) was found about a decade ago. Since then, only half a dozen spectroscopically confirmed associations have been discovered and XRF 100316D/SN 2010bh is among the latest. Aims. We constrain the progenitor radius, the host-galaxy extinction, and the physical parameters of the explosion of XRF 100316D and its associated SN 2010bh at z = 0.059. We study the brightness and colours of SN 2010bh in the context of GRB-SNe. Methods. We began observations 12 h after the GRB trigger and continued until 80 days after the burst. The Gamma-Ray burst Optical and Near-infrared Detector (GROND) provided excellent photometric data of XRF 100316D/SN 2010bh in six filter bands covering a wavelength range from approximately 350 to 1800 nm, significantly expanding the pre-existing data set for this event. Combining GROND and Swift data, the early broad-band spectral energy distribution (SED) is modelled with a blackbody and afterglow component attenuated by dust and gas absorption. The temperature and radius evolution of the thermal component are analysed and combined with earlier measurements available from the literature. Templates of SN 1998bw are fitted to the SN itself to directly compare the light-curve properties. Finally, a two-component parametrised model is fitted to the quasi-bolometric light curve, which delivers physical parameters of the explosion. Results. The best-fit models to the broad-band SEDs imply moderate reddening along the line of sight through the host galaxy (AV,host = 1.2 ± 0.1 mag). Furthermore, the parameters of the blackbody component reveal a cooling envelope at an apparent initial radius of 7 × 10 11 cm, which is compatible with a dense wind surrounding a Wolf-Rayet star. A multicolour comparison shows that SN 2010bh is 60–70% as bright as SN 1998bw. It proves to be the most rapidly evolving GRB-SNe to date, reaching maximum brightness at 8–9 days after the burst in the blue bands. Modelling of the quasi-bolometric light curve yields MNi = 0.21 ± 0.03 M� and Mej = 2.6 ± 0.2 M� , typical of values within the GRB-SN population. The kinetic energy is Ek = (2.4 ± 0.7) × 10 52 erg, which is making this SN the second most energetic GRB-SN after SN 1998bw. Conclusions. This supernova has one of the earliest peaks ever recorded and thereafter fades more rapidly than other GRB-SNe, hypernovae, or typical type-Ic SNe. This implies that a thin envelope is possibly expanding at very high velocities and is, therefore, unable to retain the γ-rays that would prolong the duration of the SN event.