Letter to the Editor

Distribution of the number of peaks within a long gamma-ray burst: The full Fermi/GBM catalogue

¹ Department of Physics and Earth Science, University of Ferrara, via Saragat 1, 44122 Ferrara, Italy
e-mail: mccrnl@unife.it
² INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Piero Gobetti 101, 40129 Bologna, Italy
³ INFN – Sezione di Ferrara, via Saragat 1, 44122 Ferrara, Italy
⁴ INAF, Osservatorio Astronomico d’Abruzzo, via Mentore Maggini snc, 64100 Teramo, Italy
⁵ Department of Physics, University of Cagliari, SP Monserrato-Sestu, km 0.7, 09042 Monserrato, Italy
⁶ Ioffe Institute, Politekhnicheskaya 26, 194021 St. Petersburg, Russia
⁷ Astronomical Observatory of the Autonomous Region of the Aosta Valley (OAVdA), Loc. Lignan 39, 11020 Nus (Aosta Valley), Italy

Received: 9 May 2024
Accepted: 3 July 2024

Abstract

Context. The dissipation process responsible for the long gamma-ray burst (GRB) prompt emission and the kind of dynamics that drives the release of energy as a function of time are still key open issues. We recently found that the distribution of the number of peaks per GRB is described by a mixture of two exponentials, suggesting the existence of two behaviours that turn up as peak-rich and peak-poor time profiles.

Aims. Our aims are to study the distribution of the number of peaks per GRB of the entire catalogue of about 3000 GRBs observed by the Fermi Gamma-ray Burst Monitor (GBM) and to make a comparison with previous results obtained from other catalogues.

Methods. We identified GRB peaks using the MEPSA code and modelled the resulting distribution following the same procedure that was adopted in the previous analogous investigation.

Results. We confirm that only a mixture of two exponentials can model the distribution satisfactorily, with model parameters that fully agree with those found from previous analyses. In particular, we confirm that (21 ± 4)% of the observed GRBs are peak-rich (8 ± 1 peaks per GRB on average), while the remaining 80% are peak-poor (2.12 ± 0.10 peaks per GRB on average).

Conclusions. We confirm the existence of two different components, peak-poor and peak-rich GRBs, that make up the observed GRB populations. Together with previous analogous results from other GRB catalogues, these results provide compelling evidence that GRB prompt emission is governed by two distinct regimes.