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GRB170817A, associated with the LIGO-Virgo GW170817 neutron-star mergerevent, lacks the short duration and hard spectrum of a Short gamma-ray burst(GRB) expected from long-standing classification models. Correctly identifyingthe class to which this burst belongs requires comparison with other GRBsdetected by the Fermi GBM. The aim of our analysis is to classify Fermi GRBsand to test whether or not GRB170817A belongs - as suggested - to the Short GRBclass. The Fermi GBM catalog provides a large database with many measuredvariables that can be used to explore gamma-ray burst classification. We usestatistical techniques to look for clustering in a sample of 1298 gamma-raybursts described by duration and spectral hardness. Classification of thedetected bursts shows that GRB170817A most likely belongs to the Intermediate,rather than the Short GRB class. We discuss this result in light of theoreticalneutron-star merger models and existing GRB classification schemes. It appearsthat GRB classification schemes may not yet be linked to appropriatetheoretical models, and that theoretical models may not yet adequately accountfor known GRB class properties. We conclude that GRB170817A may not fit into asimple phenomenological classification scheme.

Classifying GRB170817A / GW170817 in a Fermi duration - hardness plane