The GRB afterglow onset observed by REM: fireball Lorentz factor and afterglow fluence

TL;DR

This study observed early afterglow peaks of two GRBs using REM, enabling direct measurement of their initial Lorentz factors and quantification of afterglow fluence relative to prompt emission.

Contribution

It provides the first direct measurement of the fireball Lorentz factor at afterglow onset for these GRBs, confirming their ultrarelativistic speeds.

Findings

Initial Lorentz factor ~400 for both GRBs

Afterglow fluence is 16% of prompt emission for GRB 060418

Detected early afterglow peaks confirming fireball deceleration

Abstract

We report observations of the early light curves of GRB 060418 and GRB 060607A, carried out with the pink robotic telescope REM. A clear peak is detected for both events, which is interpreted as the onset of the afterglow, that is the time at which the fireball starts decelerating. This detection allows to directly measure the initial fireball Lorentz factor, which was found to be Gamma_0 ~ 400 for both events, fully confirming the ultrarelativistic nature of gamma-ray burst fireballs. Sampling the light curve before the peak also allows to compute the bolometric fluence of the afterglow, which is 16% of the prompt one in the case of GRB 060418.