Constraining GRB Initial Lorentz Factor with the Afterglow Onset Feature and Discovery of a Tight Gamma_0-E_iso Correlation

Understand

The onset of GRB afterglow is characterized by a smooth bump in the early afterglow lightcurve.

• We make an extensive search for such a feature.
• Twenty optically selected GRBs and 12 X-ray selected GRBs are found, among which 17 optically selected GRBs and 2 X-ray-selected GRBs have redshift measurements.
• We fit the lightcurves with a smooth broken power-law and measure the temporal characteristic timescales of the bumps at FWHM.