2015ApJ...810..160G

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 ν_m 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 (ε_B^f) ranges from 10^–6 to 10^–2, and the reverse-to-forward ratio of ∈_B(R_B) is about 100. The preferred electron equipartition parameter ε_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 ∈_e 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.