On the ultra-relativistic Prompt Emission (UPE), the Hard and Soft X-ray Flares, and the extended thermal emission (ETE) in GRB 151027A

TL;DR

This paper models the complex emission features of GRB 151027A within a binary-driven hypernova framework, explaining the ultra-relativistic prompt emission, X-ray flares, and thermal emission as different viewing angles of the same black hole formation process.

Contribution

It applies a comprehensive hypernova model to interpret multiple emission components in GRB 151027A, integrating hydrodynamical simulations and viewing angle effects for the first time.

Findings

Confirmed the ultra-relativistic nature of the UPE.

Identified the soft X-ray flare as mildly relativistic.

Linked the extended thermal emission to the SN-HN transition.

Abstract

We analyze GRB 151027A within the binary-driven hypernova (BdHN) approach, with progenitor a carbon-oxygen core on the verge of a supernova (SN) explosion and a binary companion neutron star (NS). The hypercritical accretion of the SN ejecta onto the NS leads to its gravitational collapse into a black hole (BH), to the emission of the GRB and to a copious $e^+e^-$ plasma. The impact of this $e^+e^-$ plasma on the SN ejecta explains {the} early soft X-ray flare observed in long GRBs. We here apply this approach to the UPE and to the hard X-ray flares. We use GRB 151027A as a prototype. From the time-integrated and the time-resolved analysis we identify a double component in the UPE and confirm its ultra-relativistic nature. We confirm the mildly-relativistic nature of the soft X-ray flare, of the hard X-ray flare and of the ETE. We show that the ETE identifies the transition from a SN to the HN. We then address the theoretical justification of these observations by integrating the hydrodynamical propagation equations of the $e^+ e^-$ into the SN ejecta, the latter independently obtained from 3D smoothed-particle-hydrodynamics simulations. We conclude that the UPE, the hard X-ray flare and the soft X-ray flare do not form a causally connected sequence: Within our model they are the manifestation of \textbf{the same} physical process of the BH formation as seen through different viewing angles, implied by the morphology and the $\sim 300$~s rotation period of the HN ejecta.