  We present analytic flux prescriptions for broadband spectra of self-absorbed
and optically thin synchrotron radiation from gamma-ray burst afterglows, based
on one-dimensional relativistic hydrodynamic simulations. By treating the
evolution of critical spectrum parameters as a power-law break between the
ultrarelativistic and non-relativistic asymptotic solutions, we generalize the
prescriptions to any observer time. Our aim is to provide a set of formulas
that constitutes a useful tool for accurate fitting of model-parameters to
observational data, regardless of the dynamical phase of the outflow. The
applicability range is not confined to gamma-ray burst afterglows, but includes
all spherical outflows (also jets before the jet-break) that produce
synchrotron radiation as they adiabatically decelerate in a cold, power-law
medium. We test the accuracy of the prescriptions and show that numerical
evidence suggests that typical relative errors in the derivation of physical
quantities are about 10 per cent. A software implementation of the presented
flux prescriptions combined with a fitting code is freely available on request
and on-line. Together they can be used in order to directly fit model
parameters to data.
