  We present our broadband study of GW170817 from radio to hard X-rays,
including NuSTAR and Chandra observations up to 165 days after the merger, and
a multi-messenger analysis including LIGO constraints. The data are compared
with predictions from a wide range of models, providing the first detailed
comparison between non-trivial cocoon and jet models. Homogeneous and power-law
shaped jets, as well as simple cocoon models are ruled out by the data, while
both a Gaussian shaped jet and a cocoon with energy injection can describe the
current dataset for a reasonable range of physical parameters, consistent with
the typical values derived from short GRB afterglows. We propose that these
models can be unambiguously discriminated by future observations measuring the
post-peak behaviour, with slope -1.0 for the cocoon and -2.5 for the jet model.
