Looking Into the Fireball: ROTSE-III and Swift Observations of Early GRB Afterglows

TL;DR

This study presents early optical and X-ray observations of 12 gamma-ray burst afterglows, analyzing their spectral and temporal properties to test fireball models and estimate initial outflow speeds.

Contribution

It provides a comprehensive dataset of early GRB afterglows with multi-wavelength observations and evaluates the fireball model's predictions against observed decay behaviors.

Findings

Optical and X-ray emissions originate from a common region in the fireball model.

The fireball model does not fully predict early afterglow decay indices.

Estimated initial Lorentz factors range from 100 to 1000.

Abstract

We report on a complete set of early optical afterglows of gamma-ray bursts (GRBs) obtained with the ROTSE-III telescope network from March 2005 through June 2007. This set is comprised of 12 afterglows with early optical and Swift/XRT observations, with a median ROTSE-III response time of 45 s after the start of gamma-ray emission (8 s after the GCN notice time). These afterglows span four orders of magnitude in optical luminosity, and the contemporaneous X-ray detections allow multi-wavelength spectral analysis. Excluding X-ray flares, the broadband synchrotron spectra show that the optical and X-ray emission originate in a common region, consistent with predictions of the external forward shock in the fireball model. However, the fireball model is inadequate to predict the temporal decay indices of the early afterglows, even after accounting for possible long-duration continuous energy injection. We find that the optical afterglow is a clean tracer of the forward shock, and we use the peak time of the forward shock to estimate the initial bulk Lorentz factor of the GRB outflow, and find 100<Gamma_0<1000, consistent with expectations.