Classifying GRB 170817A/GW170817 in a Fermi duration–hardness plane

Abstract

GRB 170817A, associated with the LIGO-Virgo GW170817 neutron-star merger event, lacks the short duration and hard spectrum of a Short gammaray 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 GRB 170817A 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 GRB 170817A 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 GRB 170817A may not fit into a simple phenomenological classification scheme. I. Horváth B.G. Tóth J. Hakkila

Transient detection capabilities of small satellite gamma‐ray detectors

“…Using similar algorithms, we can compare the overall detector efficiencies: for this, we used 776 GRB's from the Fermi GBM database with CTIME and DRM information, all of them with T 90 < 8 s (GRB 170817A/GW170817 was classified as an intermediate GRB [Horváth et al ]). Summing the counts for the energy channels between 10 keV and 960 keV, we obtained the ligh curves.…”

Multi‐messenger studies of γ‐ray bursts and their cosmic environment

“…The gravitational wave (GW) event GW170817 had an electromagnetic (EM) counterpart in form of a GRB with z ≈0.1, which originated from the merging two compact objects of 1.36 and 2.26 M ⊙ . Though the Fermi collaboration (Goldstein et al ), which analyzed the GRB170814A Fermi GRB data, later Horváth et al () classified this event as an intermediate‐duration object, and pointed out the possibility that the intermediate GRBs originate from NS–NS merger.…”

Gaussian-mixture-model-based cluster analysis of gamma-ray bursts in the BATSE catalog

“…An intermediate class has similarly been identified in Beppo-SAX (Horváth 2009) and Swift data (Zhang & Choi 2008;Horváth et al 2008;Huja et al 2009;Horváth et al 2010;Koen & Bere 2012;Tsutsui & Shigeyama 2014;Horváth & Tóth 2016;Tarnopolski 2016), even though Beppo-SAX had a smaller effective area and though Swift works in a different energy range than BATSE. The problem of force-assigning each GRB to either the long or short class became apparent with GRB171081A (associated with gravitational wave source GW170817), a burst with intermediate class attributes that physically fits into the merging neutron star scenario (Horváth et al 2018). In addition to this ambiguous individual case, the two-class scheme fails to recognize that instrumental and sampling biases are capable of creating additional clusters in the observational data, even when physical mechanisms might not be responsible.…”