Gamma-ray burst interaction with the circumburst medium: The CBM phase of GRBs

TL;DR

This paper explores the interaction between gamma-ray burst jets and their surrounding nebulae, revealing a distinct circumburst medium phase characterized by a bright synchrotron flash occurring around 100 seconds post-explosion.

Contribution

It introduces the concept of a CBM phase in GRBs, explaining a delayed synchrotron emission as a result of jet interaction with the circumburst environment, distinct from prompt and afterglow phases.

Findings

Weak interaction between jet blastwave and wind termination shock in many cases.

Bright synchrotron flash occurs at ~100 seconds after initial burst.

CBM phase may explain observed GRBs with precursors followed by intense emission.

Abstract

Progenitor stars of long gamma-ray bursts (GRBs) could be surrounded by a significant and complex nebula structure lying at a parsec scale distance. After the initial release of energy from the GRB jet, the jet will interact with this nebula environment. We show here that for a large, plausible parameter space region, the interaction between the jet blastwave and the wind termination (reverse) shock is expected to be weak, and may be associated with a precursor emission. As the jet blast wave encounters the contact discontinuity separating the shocked wind and the shocked interstellar medium, we find that a bright flash of synchrotron emission from the newly-formed reverse shock is produced. This flash is expected to be observed at around ~100 s after the initial explosion and precursor. Such a delayed emission thus constitutes a circumburst medium (CBM) phase in a GRB, having a physically distinct origin from the preceding prompt phase and the succeeding afterglow phase. The CBM phase emission may thus provide a natural explanation to bursts observed to have a precursor followed by an intense, synchrotron-dominated main episode that is found in a substantial minority, ~10% of GRBs. A correct identification of the emission phase is thus required to infer the properties of the flow and of the immediate environment around GRB progenitors.