The Swift/Fermi GRB 080928 from 1 eV to 150 keV

TL;DR

This study provides a comprehensive multi-wavelength analysis of GRB 080928, revealing synchrotron emission during prompt phase, complex energy injections shaping the afterglow, and insights into the tail emission and host galaxy.

Contribution

It offers the first detailed spectral energy distribution from 1 eV to 150 keV during the prompt phase and models the afterglow with multiple energy injections and a nonstandard large-angle emission.

Findings

SED during prompt phase matches synchrotron radiation

Afterglow shaped by multiple energy injections

Tail emission requires nonstandard large-angle emission model

Abstract

We present the results of a comprehensive study of the gamma-ray burst 080928 and of its afterglow. GRB 080928 was a long burst detected by Swift/BAT and Fermi/GBM. It is one of the exceptional cases where optical emission had already been detected when the GRB itself was still radiating in the gamma-ray band. For nearly 100 seconds simultaneous optical, X-ray and gamma-ray data provide a coverage of the spectral energy distribution of the transient source from about 1 eV to 150 keV. In particular, we show that the SED during the main prompt emission phase agrees with synchrotron radiation. We constructed the optical/near-infrared light curve and the spectral energy distribution based on Swift/UVOT, ROTSE-IIIa (Australia), and GROND (La Silla) data and compared it to the X-ray light curve retrieved from the Swift/XRT repository. We show that its bumpy shape can be modeled by multiple energy-injections into the forward shock.Furthermore, we investigate whether the temporal and spectral evolution of the tail emission of the first strong flare seen in the early X-ray light curve can be explained by large-angle emission (LAE). We find that a nonstandard LAE model is required to explain the observations. Finally, we report on the results of our search for the GRB host galaxy, for which only a deep upper limit can be provided.