SN 2013dx associated with GRB 130702A: a detailed photometric and spectroscopic monitoring and a study of the environment

Aims. Long-duration gamma-ray bursts (GRBs) and broad-line, type Ic supernovae (SNe) are strongly connected. We aim at characterizing SN 2013dx, which is associated with GRB130702A, through a sensitive and extensive ground-based observational campaign in the optical-IR band. Methods. We monitored the field of the Swift GRB 130702A (redshift z = 0.145) using the 8.2 m VLT, the 3.6 m TNG and the 0.6 m REM telescopes during the time interval between 4 and 40 days after the burst. Photometric and spectroscopic observations revealed the associated type Ic SN 2013dx. Our multiband photometry allowed constructing a bolometric light curve. Results. The bolometric light curve of SN 2013dx resembles that of 2003dh (associated with GRB030329), but is similar to 10% faster and similar to 25% dimmer. From this we infer a synthesized Ni-56 mass of similar to 0.2 M-circle dot. The multi-epoch optical spectroscopy shows that the SN 2013dx behavior is best matched by SN 1998bw, among the other well-known low-redshift SNe associated with GRBs and XRFs, and by SN 2010ah, an energetic type Ic SN not associated with any GRB. The photospheric velocity of the ejected material declines from similar to 2.7 x 10(4) km s(-1) at 8 rest frame days from the explosion, to similar to 3.5 x 10(3) km s(-1) at 40 days. These values are extremely close to those of SN1998bw and 2010ah. We deduce for SN 2013dx a kinetic energy of similar to 35 x 10(51) erg and an ejected mass of similar to 7 M-circle dot. This suggests that the progenitor of SN2013dx had a mass of similar to 25-30 M-circle dot, which is 15-20% less massive than that of SN 1998bw. Finally, we studied the SN 2013dx environment through spectroscopy of the closeby galaxies: 9 out of the 14 inspected galaxies lie within 0.03 in redshift from z = 0 : 145, indicating that the host of GRB130702A/SN 2013dx belongs to a group of galaxies, an unprecedented finding for a GRB-associated SN and, to our knowledge, for long GRBs in general.