A MORPHOLOGICAL ANALYSIS of GAMMA-RAY BURST EARLY-OPTICAL AFTERGLOWS

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 () ranges from 10^-6 to 10^-2, and the reverse-to-forward ratio of _B () is about 100. The preferred electron equipartition parameter 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 value is in agreement with the results in previous works that indicate a moderately magnetized baryonic jet for GRBs.