GRB 090313 and the Origin of Optical Peaks in GRB Light Curves: Implications for Lorentz Factors and Radio Flares

TL;DR

This study analyzes optical and radio data from 19 GRBs to test the fireball model, estimate Lorentz factors, and predict radio flares, providing insights into GRB physics and guiding future radio observations.

Contribution

It introduces a method to constrain GRB parameters using optical light curves and predicts radio flares, enhancing understanding of GRB emission mechanisms.

Findings

Derived Lorentz factors range from 40 to 450.

Optical data constrain microphysical parameters consistent with previous studies.

Predicted radio light curves match observed data for GRB 090313.

Abstract

We use a sample of 19 Gamma Ray Bursts (GRBs) that exhibit single-peaked optical light curves to test the standard fireball model by investigating the relationship between the time of the onset of the afterglow and the temporal rising index. Our sample includes GRBs and X-ray flashes for which we derive a wide range of initial Lorentz factors ($40 < \Gamma < 450$). Using plausible model parameters the typical frequency of the forward shock is expected to lie close to the optical band; within this low typical frequency framework, we use the optical data to constrain $\epsilon_e$ and show that values derived from the early time light curve properties are consistent with published typical values derived from other afterglow studies. We produce expected radio light curves by predicting the temporal evolution of the expected radio emission from forward and reverse shock components, including synchrotron self-absorption effects at early time. Although a number of the GRBs in this sample do not have published radio measurements, we demonstrate the effectiveness of this method in the case of {\it Swift} GRB 090313, for which millimetric and centrimetric observations were available, and conclude that future detections of reverse-shock radio flares with new radio facilities such as the EVLA and ALMA will test the low frequency model and provide constraints on magnetic models.