Diversity of gamma-ray burst energetics vs. supernova homogeneity SN 2013cq associated with GRB 130427A

<BR /> Aims Long-duration gamma-ray bursts (GRBs) have been found to be associated with broad-lined type-Ic supernovae (SNe), but only a handful of cases have been studied in detail. Prompted by the discovery of the exceptionally bright, nearby GRB 130427A (redshift z = 0.3399), we aim at characterising the properties of its associated SN 2013cq. This is the first opportunity to test the progenitors of high-luminosity GRBs directly. <BR /> Methods We monitored the field of the Swift long-duration GRB 130427A using the 3.6 m TNG and the 8.2 m VLT during the time interval between 3.6 and 51.6 days after the burst. Photometric and spectroscopic observations revealed the presence of the type Ic SN 2013cq. <BR /> Results Spectroscopic analysis suggests that SN 2013cq resembles two previous GRB-SNe, SN 1998bw and SN 2010bh, associated with GRB 980425 and X-ray flash (XRF) 100316D, respectively. The bolometric light curve of SN 2013cq, which is significantly affected by the host galaxy contribution, is systematically more luminous than that of SN 2010bh (~2 mag at peak), but is consistent with SN 1998bw. The comparison with the light curve model of another GRB-connected SN 2003dh indicates that SN 2013cq is consistent with the model when brightened by 20%. This suggests a synthesised radioactive <SUP>56</SUP>Ni mass of ~0.4M<SUB>&sun;</SUB>. GRB 130427A/SN 2013cq is the first case of low-z GRB-SN connection where the GRB energetics are extreme (E<SUB>gamma,iso</SUB> ~ 10<SUP>54</SUP> erg). We show that the maximum luminosities attained by SNe associated with GRBs span a very narrow range, but those associated with XRFs are significantly less luminous. On the other hand the isotropic energies of the accompanying GRBs span 6 orders of magnitude (10<SUP>48</SUP> erg <E<SUB>gamma,iso</SUB>< 10<SUP>54</SUP> erg), although this range is reduced when corrected for jet collimation. The GRB total radiated energy is in fact a small fraction of the SN energy budget. Based on observations made with the VLT, operated on the mountain of Cerro Paranal in Chile under programme 091.D-0291(A) and with the TNG, operated on the island of La Palma by the Fundacin Galileo Galilei of the Instituto Nazionale di Astrofisica (INAF) at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofsica de Canarias under programme A27TAC_5.Table 3 is available in electronic form at <A href="http //www.aanda.org/10.1051/0004-6361/201423572/olm">http //www.aanda.org</A>