GRB afterglow light curves from realistic density profiles

TL;DR

This paper models GRB afterglow light curves in complex, realistic density environments shaped by stellar winds, revealing features like flattening, breaks, and potential GeV flares that differ from simpler models.

Contribution

It introduces detailed simulations of GRB blast waves in realistic wind-shaped media, highlighting new features in afterglow light curves and potential high-energy flares.

Findings

Afterglow light curves show flattening and shallow breaks in wind-shaped media.

Jet breaks can be very steep, with declines as steep as t^-5.

Inverse Compton scattering produces detectable GeV flares.

Abstract

The afterglow emission that follows gamma-ray bursts (GRBs) contains valuable information about the circumburst medium and, therefore, about the GRB progenitor. Theoretical studies of GRB blast waves, however, are often limited to simple density profiles for the external medium (mostly constant density and power-law R^{-k} ones). We argue that a large fraction of long-duration GRBs should take place in massive stellar clusters where the circumburst medium is much more complicated. As a case study, we simulate the propagation of a GRB blast wave in a medium shaped by the collision of the winds of O and Wolf-Rayet stars, the typical distance of which is d /sim 0.1 - 1 pc. Assuming a spherical blast wave, the afterglow light curve shows a flattening followed by a shallow break on a timescale from hours up to a week after the burst, which is a result of the propagation of the blast wave through the shocked wind region. If the blast wave is collimated, the jet break may, in some cases, become very pronounced with the post break decline of the light curve as steep as t-5. Inverse Compton scattering of ultra-violet photons from the nearby star off energetic electrons in the blast wave leads to a bright \simGeV afterglow flare that may be detectable by Fermi.