Analytic expressions for the surface brightness profile of gamma-ray burst afterglow images

The exact profile of a gamma-ray burst afterglow image on the plane of the sky can provide important constraints on the underlying physics. In particular, it can test whether the magnetic field in the emitting shocked external medium decreases significantly with the distance behind the shock front, or remains roughly constant. Moreover, it enables more accurate measurements of the afterglow image size and the expected scintillation properties. In this work, analytic expressions are derived for the afterglow image in power-law segments (PLSs) of the afterglow synchrotron spectrum in which the emission originates from a very thin layer just behind the shock, while simple semi-analytic expressions are derived for the remaining PLSs in which the emission arises from the bulk of the shocked fluid. In all cases, the expressions are for a general power-law external density profile, and are convenient to use in afterglow studies.