The outflow structure of GW170817 from late-time broad-band observations.

We present our broad-band study of GW170817 from radio to hard X-rays, including <italic>NuSTAR</italic> and <italic>Chandra</italic> observations up to 165  d after the merger, and a multimessenger 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 data set 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 <italic>F</italic><italic>ν</italic> ∝ <italic>t</italic>∼−1.0 for the cocoon and <italic>Fν</italic>∝ <italic>t</italic>∼−2.5 for the jet model. [ABSTRACT FROM AUTHOR]