Early Optical-Infrared Emission from GRB 041219a: Neutron-rich Internal Shocks and a Mildly Magnetized External Reverse Shock

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Abstract
Very early optical and near-infrared emission was discovered to have accompanied the long gamma-ray burst GRB 041219a. Here we show that the optical-IR flash tracking the gamma-ray light curve during the prompt emission can be understood as emission from neutron-rich internal shocks, as has been suggested by Fan & Wei. The early Ks-band afterglow light curve after the prompt phase can be well modeled as the superposition of a reverse and a forward shock component. The radio data also support the reverse-shock interpretation.