GRB161219B/SN2016jca: a powerful stellar collapse

We report observations and analysis of the nearby gamma-ray burst GRB161219B (redshift z= 0.1475) and the associated Type Ic supernova (SN) 2016jca. GRB161219B had an isotropic gamma-ray energy of similar to 1.6x10(50)erg. Its afterglow is likely refreshed at an epoch preceding the first photometric points (0.6d), which slows down the decay rates. Combined analysis of the SN light curve and multiwavelength observations of the afterglow suggest that the GRB jet was broad during the afterglow phase (full opening angle similar to 42 degrees +/- 3 degrees). Our spectral series shows broad absorption lines typical of GRB supernovae (SNe), which testify to the presence of material with velocities up to similar to 0.25c. The spectrum at 3.73d allows for the very early identification of an SN associated with a GRB. Reproducing it requires a large photospheric velocity (kms(-1)). The kinetic energy of the SN is estimated through models to be E-kin approximate to 4x10(52)erg in spherical symmetry. The ejected mass in the explosion was M-ej approximate to 6.5 +/- 1.5M(circle dot), much less than that of other GRB-SNe, demonstrating diversity among these events. The total amount of Ni-56 in the explosion was 0.27 +/- 0.05M(circle dot). The observed spectra require the presence of freshly synthesized Ni-56 at the highest velocities, at least threetimes more than a standard GRB-SN. We also find evidence for a decreasing Ni-56 abundance as a function of decreasing velocity. This suggests that SN2016jca was a highly aspherical explosion viewed close to on-axis, powered by a compact remnant. Applying a typical correction for asymmetry, the energy of SN2016jca was similar to(1-3)x10(52)erg, confirming that most of the energy produced by GRB-SNe goes into the kinetic energy of the SN ejecta.