Late-Time X-ray Flares during GRB Afterglows: Extended Internal Engine Activity

A D Falcone

doi:10.1063/1.2207925

Abstract

Observations of gamma ray bursts (GRBs) with Swift produced the initially surprising result that many bursts have large X-ray flares superimposed on the underlying afterglow. These flares were sometimes intense, rapid, and late relative to the nominal prompt phase. The most intense of these flares was observed by XRT with a flux >500 the afterglow. This burst then surprised observers by flaring again after >10000 s. The intense flare can be most easily understood within the context of the standard fireball model, if the internal engine that powers the prompt GRB emission is still active at late times. Recent observations indicate that X-ray flares are detected in ~1/3 of XRT detected afterglows. By studying the properties of the varieties of flares (such as rise/fall time, onset time, spectral variability, etc.) and relating them to overall burst properties, models of flare production and the GRB internal engine can be constrained.

Highlights

Since its launch on 2004 November 20, Swift [1] has provided detailed measurements of numerous gamma ray bursts (GRBs) and their afterglows with unprecedented reaction times
By detecting burst afterglows promptly, and with high sensitivity, the properties of the early afterglow and extended prompt emission can be studied in detail for the first time
The rapid response of the pointed X-ray Telescope (XRT) instrument [2] on Swift has led to the discovery that large X-ray flares are common in GRBs and occur at times well after the initial prompt emission

Summary

INTRODUCTION

Since its launch on 2004 November 20, Swift [1] has provided detailed measurements of numerous gamma ray bursts (GRBs) and their afterglows with unprecedented reaction times. By detecting burst afterglows promptly, and with high sensitivity, the properties of the early afterglow and extended prompt emission can be studied in detail for the first time. GRB emission, the most commonly accepted model is that of a relativistically expanding fireball with associated internal and external shocks [3]. In this model, internal shocks produce the prompt GRB emission. Internal shocks produce the prompt GRB emission This emission typically has a timescale of ∼ 30 s for long bursts and ∼0.3 s for short bursts [4]. > 25% of all Swift-BAT detected bursts have significant X-ray flares, and > 43% of the bursts with a prompt XRT detection have significant X-ray flares

A FEW REMARKABLE FLARING GRBS

Findings

DISCUSSION AND CONCLUSIONS