X-ray Flares in Gamma-Ray Bursts

Author:

Morris, David C.

Graduate Program:

Astronomy and Astrophysics

Degree:

Doctor of Philosophy

Document Type:

Dissertation

Date of Defense:

January 21, 2008

Committee Members:

David Nelson Burrows, Committee Chair/Co-Chair
Derek Brindley Fox, Committee Member
Robert Graham Melton, Committee Member
Peter Istvan Meszaros, Committee Member
John Andrew Nousek, Committee Member

Keywords:

X-rays
flares
GRBs

Abstract:

Data from the Swift mission have now shown that flares are a common component of Gamma-Ray Burst afterglows, appearing in roughly 50% of GRBs to which Swift slews promptly and in all phases of GRBs. Much has been learned from analysis of individual flares and from the recent first GRB flare surveys (cite{Falcone07, Chincarini07}) which have focused primarily on properties of the X-ray emission from flares. The broadband spectral properties of flares, however, particularly at UV and optical wavelengths, have yet to be systematically studied. In this thesis, I discuss results from a multiwavelength survey of bright X-ray selected flares seen in Swift GRBs. Using simultaneous data from the UVOT, XRT and BAT, I have produced SEDs of flares from 0.002 keV to 150 keV and fit them using several different spectral models. My results show that a simple absorbed powerlaw is unable to fit flare spectra in the 0.002 keV to 150 keV energy range due, in large part, to a very low UV/X-ray emission ratio. I furthermore investigate the applicability to the data of several models for GRB flare prodction from the literature. I find that the internal shock model of flare production is the most likely model to explain any given flare, but that no single model is able to explain the complete taxonomy of GRB X-ray flares. I determine the approximate fraction of flares which are explainable by each mechanism. I also use the flares in my sample to investigate the bulk Lorentz factor of GRB flares and to compare it to the bulk Lorentz factor of the prompt GRB emission. I find a likely range for the Lorentz factor of flares of 10 < Gamma < 30, significantly lower than the canonical value for the prompt emission of Gamma < 300. Finally, I also investigate a discovered trend between the amount of flaring activity in GRBs and redshift.