Letter to the Editor

The hidden population of long gamma-ray bursts from compact object mergers

¹ Department of Physics and Earth Science, University of Ferrara, Via Saragat 1, I-44122 Ferrara, Italy
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, Teramo, 64100 Teramo, Italy
⁵ Astrophysics Research Institute, LJMU, IC2, Liverpool Science Park, 146 Brownlow Hill, Liverpool L3 5RF, UK
⁶ INFN – Sezione di Bologna, Viale C. Berti Pichat 6/2 – 40126 Bologna, Italy
⁷ Dipartimento di Fisica, Università degli Studi di Cagliari, SP Monserrato-Sestu, km 0.7, I-09042 Monserrato, Italy
⁸ Ioffe Institute, Politekhnicheskaya 26, 194021 St. Petersburg, Russia

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 22 January 2026
Accepted: 16 March 2026

Abstract

Context. The prompt-emission time profiles of GRB 230307A and other long-duration compact object merger (COM) candidates exhibit a unique set of temporal properties, characterised by a deterministic evolution of waiting times and pulse widths.

Aims. We searched the Fermi/GBM catalogue for other unidentified long COM candidates exhibiting temporal properties similar to those observed in GRB 230307A.

Methods. We examined the temporal and spectral prompt-emission properties of GRBs featuring at least eight light-curve peaks. For candidates, all with unknown redshifts, that exhibited properties similar to GRB 230307A, we analysed their trajectories in the E_p, i − E_iso plane as a function of redshift. We then evaluated the joint likelihood of their compatibility with the E_p, i − E_iso relation satisfied by the bulk of long GRBs. Furthermore, we calculated their minimum variability timescales (MVTs) for comparison against known COM and collapsar populations.

Results. We identified nine COM candidates with unknown redshifts and demonstrated that there are at least two outliers of the E_p, i − E_iso relation with 3.1σ (Gaussian) confidence level. Furthermore, their MVTs are more consistent with those of COM than with collapsar GRBs.

Conclusions. These results indicate that this specific set of temporal properties can serve as a diagnostic tool to distinguish long-duration COMs from the broader collapsar population. Furthermore, our findings suggest that the fraction of unidentified COMs among long GRBs might be larger than previously assumed.