A GRB Afterglow Model Consistent with Hypernova Observations

Abstract

We describe the afterglows of the long gamma-ray-burst (GRB) 130427A within the context of a binary-driven hypernova. The afterglows originate from the interaction between a newly born neutron star ( ν NS), created by an Ic supernova (SN), and a mildly relativistic ejecta of a hypernova (HN). Such an HN in turn results from the impact of the GRB on the original SN Ic. The mildly relativistic expansion velocity of the afterglow (Γ ∼ 3) is determined, using our model-independent approach, from the thermal emission between 196 and 461 s. The power law in the optical and X-ray bands of the afterglow is shown to arise from the synchrotron emission of relativistic electrons in the expanding magnetized HN ejecta. Two components contribute to the injected energy: the kinetic energy of the mildly relativistic expanding HN and the rotational energy of the fast-rotating highly magnetized ν NS. We reproduce the afterglow in all wavelengths from the optical (10 14 Hz) to the X-ray band (10 19 Hz) over times from 604 s to 5.18 × 10 6 s relative to the Fermi -GBM trigger. Initially, the emission is dominated by the loss of kinetic energy of the HN component. After 10 5 s the emission is dominated by the loss of rotational energy of the ν NS, for which we adopt an initial rotation period of 2 ms and a dipole plus quadrupole magnetic field of ≲7 × 10 12 G or ∼10 14 G. This scenario with a progenitor composed of a CO core and an NS companion differs from the traditional ultra-relativistic-jetted treatments of the afterglows originating from a single black hole.