Classifying GRB170817A / GW170817 in a Fermi duration - hardness plane

TL;DR

This study analyzes Fermi GBM gamma-ray bursts to classify GRB170817A, revealing it likely belongs to the Intermediate class, challenging traditional short/long GRB classifications and highlighting gaps between models and observations.

Contribution

The paper introduces a statistical classification method for Fermi GRBs and applies it to GRB170817A, suggesting it belongs to the Intermediate class rather than Short.

Findings

GRB170817A most likely belongs to the Intermediate class.

Traditional short/long GRB classifications may be insufficient.

Theoretical models may not fully explain observed GRB properties.

Abstract

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.