Classifying GRB170817A / GW170817 in a Fermi duration - hardness plane
  GRB170817A, associated with the LIGO-Virgo GW170817 neutron-star merger
event, lacks the short duration and hard spectrum of a Short gamma-ray burst
(GRB) expected from long-standing classification models. Correctly identifying
the class to which this burst belongs requires comparison with other GRBs
detected by the Fermi GBM. The aim of our analysis is to classify Fermi GRBs
and to test whether or not GRB170817A belongs - as suggested - to the Short GRB
class. The Fermi GBM catalog provides a large database with many measured
variables that can be used to explore gamma-ray burst classification. We use
statistical techniques to look for clustering in a sample of 1298 gamma-ray
bursts described by duration and spectral hardness. Classification of the
detected bursts shows that GRB170817A most likely belongs to the Intermediate,
rather than the Short GRB class. We discuss this result in light of theoretical
neutron-star merger models and existing GRB classification schemes. It appears
that GRB classification schemes may not yet be linked to appropriate
theoretical models, and that theoretical models may not yet adequately account
for known GRB class properties. We conclude that GRB170817A may not fit into a
simple phenomenological classification scheme.
