Gamma-ray burst taxonomy: Looking for the third class on the spectral peak energy-duration plane in the rest frame

¹ Dipartimento di Fisica, Università degli Studi di Cagliari, SP Monserrato-Sestu, km 0.7, I-09042 Monserrato, Italy
² Ioffe Institute, Politekhnicheskaya 26, 194021 St. Petersburg, Russia
³ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Piero Gobetti 93/3, 40129 Bologna, Italy
⁴ Department of Physics and Earth Science, University of Ferrara, Via Saragat 1, I-44122 Ferrara, Italy
⁵ INFN – Sezione di Ferrara, Via Saragat 1, 44122 Ferrara, Italy
⁶ INAF, Osservatorio Astronomico d’Abruzzo, Via Mentore Maggini snc, 64100 Teramo, Italy
⁷ INFN, Sezione di Cagliari, Cittadella Universitaria, 09042 Monserrato, CA, Italy
⁸ INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica di Palermo, Via U. La Malfa 153, 90146 Palermo, Italy
⁹ INAF – Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius, CA, Italy
¹⁰ Dipartimento di Fisica e Chimica, Università degli Studi di Palermo, Via Archirafi 36, 90123 Palermo, Italy
¹¹ Alferov Academic University, Khlopina St. 8/3, 194021 St. Petersburg, Russia

^⋆ Corresponding author: tsvetkova.lea@gmail.com

Received: 19 October 2024
Accepted: 22 April 2025

Abstract

Context. Two classes of gamma-ray bursts (GRBs) corresponding to the short-hard and the long-soft events, with a putative intermediate class, are typically considered in the observer frame. However, when considering GRB characteristics in the cosmological rest frame, the boundary between the classes becomes blurred.

Aims. The goal of this research is to check for evidences of a third ‘intermediate’ class of GRBs and investigate how the transformation from the observer to the rest frame affects the hardness-duration-based classification.

Methods. We applied fits with skewed and non-skewed (symmetric) Gaussian and Student distributions to a sample of 409 GRBs with reliably measured redshifts to cluster the bursts on the hardness (E_p) – duration (T₉₀) plane.

Results. We find that based on AIC/BIC criteria, the statistically preferred number of clusters on the GRB rest-frame hardness-duration plane does not exceed two. We also assessed the robustness of the clustering technique.

Conclusions. We did not find any solid evidence of an intermediate GRB class on the rest-frame hardness-duration plane.