The rapid decay phase of the afterglow as the signature of the Blandford-Znajek mechanism

TL;DR

This paper investigates the exponential decay phase in long-duration GRB afterglows, linking it to the Blandford-Znajek mechanism, and estimates progenitor magnetic fields consistent with Wolf-Rayet stars.

Contribution

It provides observational evidence of exponential decay in GRB afterglows and connects it to the Blandford-Znajek mechanism, estimating progenitor magnetic fields.

Findings

Approximately 30% of XRT lightcurves show exponential decay.

Estimated progenitor magnetic fields are consistent with Wolf-Rayet stars.

The decay phase supports the Blandford-Znajek energy extraction model.

Abstract

Gamma-ray bursts (GRBs) are believed to be powered by the electromagnetic extraction of spin energy from a black hole endowed with a magnetic field supported by electric currents in a surrounding disk (Blandford & Znajek 1977). A generic feature of this mechanism is that, under certain fairly general assumptions, the energy loss rate decays exponentially. In this work, we are looking precisely for such exponential decay in the lightcurves of long duration GRBs observed with the XRT instrument on the Swift satellite. We found out that almost 30 % of XRT lightcurves show such behavior before they reach the afterglow plateau. According to Blandford & Znajek, the duration of the burst depends on the magnetic flux accumulated on the event horizon. This allows us to estimate the surface magnetic field of a possible progenitor. Our estimations are consistent with magnetic fields observed in Wolf-Rayet stars.