Prolonged activity of the central engine of Gamma Ray Bursts

TL;DR

This paper investigates the prolonged activity of the central engine in Gamma Ray Bursts by analyzing optical and X-ray afterglow data, revealing evidence of extended engine activity likely powered by fallback material.

Contribution

It introduces a composite model combining standard forward shock emission with an additional component to explain afterglow behaviors, suggesting longer central engine activity.

Findings

Evidence of long-lasting central engine activity

The need for a two-component model to fit afterglow data

Indications of fallback material powering extended engine activity

Abstract

We call "prompt" emission of Gamma Ray Bursts (GRBs) the erratic and violent phase of hard X-ray and soft gamma-ray emission, usually lasting for tens of seconds in long GRBs. However, the central engine of GRBs may live much longer. Evidence of it comes from the strange behaviour of the early "afterglow", seen especially in the X-ray band, characterised by a "steep-flat-steep" light curve, very often not paralleled by a similar behaviour in the optical band. This difference makes it hard to explain both the optical and the X-ray emission with a unique component. Two different mechanisms seem to be required. One can well be the standard emission from the forward shock of the fireball running through the interstellar medium. The second one is more elusive, and to characterise its properties we have studied those GRBs with well sampled data and known redshift, fitting at the same time the optical and the X-ray light curves by means of a composite model, i.e. the sum of the standard forward shock emission and a phenomenological additional component with a minimum of free parameters. Some interesting findings have emerged, pointing to a long activity of the central engine powered by fallback material.