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Register a new userOn a GRB afterglow model consistent with hypernovae observations
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We describe the afterglows of the long gamma-ray-burst (GRB) 130427A within the context of a binary-driven hypernova (BdHN). The afterglows originate from the interaction between a newly born neutron star ($ u$NS), created by an Ic supernova (SN), and a mildly relativistic ejecta of a hypernova (HN). Such a HN in turn results from the impact of the GRB on the original SN Ic. The mildly relativistic expansion velocity of the afterglow ($Gamma sim 3$) is determined, using our model independent approach, from the thermal emission between $196$~s 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 $ u$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 $5.18times 10^6$~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 $ u$NS, for which we adopt an initial rotation period of $2$~ms and a dipole plus quadrupole magnetic field of $lesssim ! 7times 10^{12}$~G or $sim ! 10^{14}$~G. This scenario with a progenitor composed of a CO$_{rm core}$ and a NS companion differs from the traditional ultra-relativistic-jetted treatments of the afterglows originating from a single black hole.
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The complex multiwavelength emission of GRB afterglow 130427A (monitored in the radio up to 10 days, in the optical and X-ray until 50 days, and at GeV energies until 1 day) can be accounted for by a hybrid reverse-forward shock synchrotron model, with inverse-Compton emerging only above a few GeV. The high ratio of the early optical to late radio flux requires that the ambient medium is a wind and that the forward-shock synchrotron spectrum peaks in the optical at about 10 ks. The latter has two consequences: the wind must be very tenuous and the optical emission before 10 ks must arise from the reverse-shock, as suggested also by the bright optical flash that Raptor has monitored during the prompt emission phase (<100 s). The VLA radio emission is from the reverse-shock, the Swift X-ray emission is mostly from the forward-shock, but the both shocks give comparable contributions to the Fermi GeV emission. The weak wind implies a large blast-wave radius (8 t_{day}^{1/2} pc), which requires a very tenuous circumstellar medium, suggesting that the massive stellar progenitor of GRB 130427A resided in a super-bubble.
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