A Morphological Analysis of Gamma-Ray Burst Early-optical Afterglows
Abstract
Within the framework of the external shock model of gamma-ray burst (GRB) afterglows, we perform a morphological analysis of the early-optical light curves to directly constrain model parameters. We define four morphological types, i.e., the reverse shock-dominated cases with/without the emergence of the forward shock peak (Type I/Type II), and the forward shock-dominated cases without/with νm crossing the band (Type III/IV). We systematically investigate all of the Swift GRBs that have optical detection earlier than 500 s and find 3/63 Type I bursts (4.8%), 12/63 Type II bursts (19.0%), 30/63 Type III bursts (47.6%), 8/63 Type IV bursts (12.7%), and 10/63 Type III/IV bursts (15.9%). We perform Monte Carlo simulations to constrain model parameters in order to reproduce the observations. We find that the favored value of the magnetic equipartition parameter in the forward shock ({ɛ }B{{f}}) ranges from 10-6 to 10-2, and the reverse-to-forward ratio of ɛB ({{R}}B) is about 100. The preferred electron equipartition parameter {ɛ }{{e}}{{r},{{f}}} value is 0.01, which is smaller than the commonly assumed value, e.g., 0.1. This could mitigate the so-called “efficiency problem” for the internal shock model, if ɛe during the prompt emission phase (in the internal shocks) is large (say, ∼0.1). The preferred {{R}}B value is in agreement with the results in previous works that indicate a moderately magnetized baryonic jet for GRBs.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- September 2015
- DOI:
- 10.1088/0004-637X/810/2/160
- arXiv:
- arXiv:1507.01303
- Bibcode:
- 2015ApJ...810..160G
- Keywords:
-
- gamma-ray burst: general;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- 13 pages, 5 figures, 3 tables, accepted for publication in ApJ