The afterglow onset for GRB060418 and GRB060607A

TL;DR

This paper analyzes early-time near-infrared afterglow observations of two gamma-ray bursts to directly determine their initial Lorentz factors, finding values around 400, which enhances understanding of relativistic outflows in GRBs.

Contribution

It provides the first direct measurement of the initial Lorentz factor for GRBs using early afterglow peaks, improving upon previous ambiguous estimates.

Findings

Initial Lorentz factors around 400 for both GRBs

Early afterglow peaks can be used to measure outflow Lorentz factors

Supports relativistic outflow models in GRB physics

Abstract

Gamma-ray burst are thought to be produced by highly relativistic outflows. Although upper and lower limits for the outflow initial Lorentz factor $\Gamma_0$ are available, observational efforts to derive a direct determination of $\Gamma_0$ have so far failed or provided ambiguous results. As a matter of fact, the shape of the early-time afterglow light curve is strongly sensitive on $\Gamma_0$ which determines the time of the afterglow peak, i.e. when the outflow and the shocked circumburst material share a comparable amount of energy. We now comment early-time observations of the near-infrared afterglows of GRB 060418 and GRB 060607A performed by the REM robotic telescope. For both events, the afterglow peak was singled out and allowed us to determine the initial fireball Lorentz, $\Gamma_0\sim 400$.