Gamma-Ray Burst ( GRB ) afterglows are commonly interpreted as synchrotron emission from a relativistic blast wave produced by a point explosion in an ambient medium , plausibly the interstellar medium of galaxies .
We calculate the amplitude of flux fluctuations in the lightcurve of afterglows due to inhomogeneities in the surrounding medium .
Such inhomogeneities are an inevitable consequence of interstellar turbulence , but could also be generated by variability and anisotropy in a precursor wind from the GRB progenitor .
Detection of their properties could provide important clues about the environments of GRB sources .
We apply our calculations to GRB990510 , where an rms scatter of \sim 2 \% was observed for the optical flux fluctuations on the 0.1–2 hour timescale during the first day of the afterglow , consistent with it being entirely due to photometric noise ( Stanek et al .
1999 ) .
The resulting upper limits on the density fluctuations on scales of \sim 20 – 200 AU around the source of GRB990510 , are lower than the inferred fluctuation amplitude on similar scales in the Galactic interstellar medium .
Hourly monitoring of future optical afterglows might therefore reveal fractional flux fluctuations at the level of a few percent .