2015A&A...577A.116D

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. We monitored the field of the Swift GRB 130702A (redshift z=0.145) using the 8.2m VLT, the 3.6m TNG and the 0.6m 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. The bolometric light curve of SN 2013dx resembles that of 2003dh (associated with GRB030329), but is ∼10% faster and ∼25% dimmer. From this we infer a synthesized ⁵⁶Ni mass of ∼0.2M_☉. 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 ∼2.7x10⁴km/s at 8 rest frame days from the explosion, to ∼3.5x10³km/s at 40-days. These values are extremely close to those of SN1998bw and 2010ah. We deduce for SN 2013dx a kinetic energy of ∼ 35x10⁵¹ erg and an ejected mass of ∼7M_☉. This suggests that the progenitor of SN2013dx had a mass of ∼25-30M_☉, 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.