Evidence for a Canonical GRB Afterglow Light Curve in the Swift/XRT Data

Technical Report · Wed Aug 17 04:00:00 EDT 2005

DOI:https://doi.org/10.2172/878781· OSTI ID:878781

Nousek, J A; Kouveliotou, C; Grupe, D; Page, K; Granot, J; Ramirez-Ruiz, E; Patel, S K; Burrows, D N; Mangano, V; Barthelmy, S; Beardmore, A P; Campana, S; Capalbi, M; Chincarini, G; Cusumano, G; Falcone, A D; Gehrels, N; Giommi, P; Goad, M; Godet, O ; Hurkett, C

We present new observations of the early X-ray afterglows of the first 27 gamma-ray bursts (GRBs) detected with the Swift X-ray Telescope (XRT). The early X-ray afterglows show a canonical behavior, where the light curve broadly consists of three distinct power law segments: (1) an initial very steep decay ({infinity} t{sup -a} with 3 {approx}< a{sub 1} {approx}< 5) , followed by (2) a very shallow decay (0.2 {approx}< a{sub 2} {approx}< 0.8), and finally (3) a somewhat steeper decay (1 {approx}< a{sub 3} {approx}< 1.5). These power law segments are separated by two corresponding break times, 300 s {approx}< t{sub break,1} {approx}< 500 s and 10{sup 3} s {approx}< t{sub break,2} {approx}< 10{sup 4} s. On top of this canonical behavior of the early X-ray light curve, many events have superimposed X-ray flares, which are most likely caused by internal shocks due to long lasting sporadic activity of the central engine, up to several hours after the GRB. We find that the initial steep decay is consistent with it being the tail of the prompt emission, from photons that are radiated at large angles relative to our line of sight. The first break in the light curve (t{sub break,1}) takes place when the forward shock emission becomes dominant, with the intermediate shallow flux decay (a{sub 2}) likely caused by the continuous energy injection into the external shock. When this energy injection stops, a second break is then observed in the light curve (t{sub break,2}). This energy injection increases the energy of the afterglow shock by at least a factor of f {approx}> 4, and augments the already severe requirements for the efficiency of the prompt gamma-ray emission.

• Research Organization: Stanford Linear Accelerator Center (SLAC)
• Sponsoring Organization: USDOE
• DOE Contract Number: AC02-76SF00515
• OSTI ID: 878781
• Report Number(s): SLAC-PUB-11428; astro-ph/0508332
• Country of Publication: United States
• Language: English

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