The detection of delayed emission in X-ray, optical, and radio wavelen
gth, or ''afterglow,'' following a gamma-ray burst can be described as
the emission of a relativistic shell decelerating upon collision with
the interstellar medium. We show that the observed radiation surface
has well-defined bright edges. We derive an explicit expression for th
e size as a function of time, and obtain the surface brightness distri
bution. This might be directly observed if the burst occurs at a small
redshift so that its radio signal can be resolved. The size and shape
are relevant for detailed analysis of scintillation or microlensing.
We show that the effective Lorentz factor depends on the observed freq
uency and that it is higher for frequencies above the synchrotron typi
cal frequency (optical and X-ray) than for low frequencies (radio). Co
nsequently, transition to nonrelativistic evolution will be observed f
irst in low frequencies and only a factor of similar to 2 later in the
high frequencies.