The optical SN2012bz associated with the long GRB120422A

Aims. The association of Type Ic supernovae (SNe) with long-duration gamma-ray bursts (GRBs) is well established. We endeavor, through accurate ground-based observational campaigns, to characterize these SNe at increasingly high redshifts. Methods. We obtained a series of optical photometric and spectroscopic observations of the Type Ic SN2012bz associated with the Swift long-duration GRB120422A (redshift z = 0.283) using the 3.6-m TNG and the 8.2-m VLT telescopes during the time interval between 4 and 36 days after the burst. Results. The peak times of the light curves of SN2012bz in various optical filters differ, with the B-band and i'-band light curves reaching maximum at 9 +/- 4 and 23 +/- 3 rest-frame days, respectively. The bolometric light curve has been derived from individual bands photometric measurements, but no correction for the unknown contribution in the near-infrared (probably around 10-15%) has been applied. Therefore, the present light curve should be considered as a lower limit to the actual UV-optical-IR bolometric light curve. This pseudo-bolometric curve reaches its maximum (M-bol = -18.56 +/- 0.06) at 13 +/- 1 rest-frame days; it is similar in shape and luminosity to the bolometric light curves of the SNe associated with z < 0.2 GRBs and more luminous than those of SNe associated with X-ray flashes (XRFs). A comparison with the model generated for the bolometric light curve of SN2003dh suggests that SN2012bz produced only about 15% less Ni-56 than SN2003dh, about 0.35 M-circle dot. Similarly the VLT spectra of SN2012bz, after correction for Galactic extinction and for the contribution of the host galaxy, suggest comparable explosion parameters with those observed in SN2003dh (E-K similar to 3.5 x 10(52) erg, M-ej similar to 7 M-circle dot) and a similar progenitor mass (similar to 25-40 M-circle dot). GRB120422A is consistent with the E-peak -E-iso and the E-X,E-iso -E-gamma,E-iso -E-peak relations. GRB120422A / SN2012bz shows the GRB-SN connection at the highest redshift so far accurately monitored both photometrically and spectroscopically.