What did we learn from gamma-ray burst 080319B ?

Abstract

The optical and gamma-ray observations of GRB 080319B allow us to determine a fairly complete physical picture for this remarkable burst. The data indicate that the prompt optical and gamma-ray photons were produced at the same location but by different radiation processes: synchrotron and synchrotron self-Compton, respectively. The burst emission was produced at a distance of 10^{16.5} cm from the center of explosion by an ultra-relativistic source moving at Lorentz factor of ~500. A straightforward inference is that about 10 times more energy must have been radiated at tens of GeV than that released at ~1 MeV. Assuming that the GRB outflow was baryonic and that the gamma-ray source was shock-heated plasma, the collimation-corrected kinetic energy of the jet powering GRB 080319B was larger than 10^{52.3} erg. The decay of the early afterglow optical emission (up to 1 ks) is too fast to be attributed to the reverse shock crossing the GRB ejecta but is consistent with the expectations for the "large-angle emission" released during the burst. The pure power-law decay of the optical afterglow flux from 1 ks to 10 day is most naturally identified with the (synchrotron) emission from the shock propagating into a wind-like medium. However, the X-ray afterglow requires a departure from the standard blast-wave model.