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Abstract

We report observations and analysis of the nearby gamma-ray burst GRB 161219B (redshift z = 0.1475) and the associated Type Ic supernova (SN) 2016jca. GRB 161219B had an isotropic gamma-ray energy of ∼1.6 × 10⁵⁰ erg. Its afterglow is likely refreshed at an epoch preceding the first photometric points (0.6 d), 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 ∼42° ± 3°). Our spectral series shows broad absorption lines typical of GRB supernovae (SNe), which testify to the presence of material with velocities up to ∼0.25c. The spectrum at 3.73 d allows for the very early identification of an SN associated with a GRB. Reproducing it requires a large photospheric velocity (⁠35000±7000 km s⁻¹). The kinetic energy of the SN is estimated through models to be E_kin≈4 × 10⁵² erg in spherical symmetry. The ejected mass in the explosion was M_ej≈6.5 ± 1.5 M_⊙, much less than that of other GRB-SNe, demonstrating diversity among these events. The total amount of ⁵⁶Ni in the explosion was 0.27 ± 0.05 M_⊙. The observed spectra require the presence of freshly synthesized ⁵⁶Ni at the highest velocities, at least three times more than a standard GRB-SN. We also find evidence for a decreasing ⁵⁶Ni abundance as a function of decreasing velocity. This suggests that SN 2016jca was a highly aspherical explosion viewed close to on-axis, powered by a compact remnant. Applying a typical correction for asymmetry, the energy of SN 2016jca was ∼(1–3) × 10⁵² erg, confirming that most of the energy produced by GRB-SNe goes into the kinetic energy of the SN ejecta.