Early afterglows from radially structured outflows and the application to X-ray shallow decays

Abstract

In the fireball model, it is more physically realistic that gamma-ray burst (GRB) ejecta have a range of bulk Lorentz factors (assuming $M\propto {\Gamma }^{-s}$). The low Lorentz factor part of the ejecta will catch up with the high Lorentz factor part when the latter is decelerated by the surrounding medium to a comparable Lorentz factor. Such a process will develop a long-lasting weak reverse shock until the whole ejecta are shocked. Meanwhile, the forward shocked materials are gradually supplied with energy from the ejecta that are catching-up, and thus the temporal decay of the forward shock emission will be slower than that without an energy supply. However, the reverse shock may be strong. Here, we extend the standard reverse-forward shock model to the case of radially nonuniform ejecta. We show that this process can be classified into two cases: the thick shell case and the thin shell case. In the thin shell case, the reverse shock is weak and the temporal scaling law of the afterglow is the same as that in Sari & Mesz (2000). However, in the thick shell case, the reverse shock is strong and thus its emission dominates the afterglow in the high energy band. Our results also show slower decaying behavior of the afterglow due to the energy supply by low Lorentz factor materials, which may help the understanding of the plateau observed in the early optical and X-ray afterglows.

References in corpus (10)

• The early X-ray emission from GRBs
• Gamma-Ray Bursts in the Swift Era
• REM observations of GRB 060418 and GRB 060607A: the onset of the afterglow and the initial fireball Lorentz factor determination
• Evidence for chromatic X-ray light-curve breaks in Swift GRB afterglows and their theoretical implications
• On the Mechanism of Gamma-Ray Burst Afterglows
• Can the early X-ray afterglow of GRBs be explained by a contribution from the reverse shock?
• On the Redshift Distribution of Gamma Ray Bursts in the Swift Era
• Continuous optical monitoring during the prompt emission of GRB 060111B
• The bright optical flash from GRB 060117
• The initial Lorentz factors of fireballs inferred from the early X-ray data of SWIFT GRBs

Cited by in corpus (1)

• Reverse Shock Emission in Gamma-ray Bursts Revisited