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

TL;DR

This study identifies a subset of long gamma-ray bursts with unique temporal properties that suggest they originate from compact object mergers, indicating a potentially larger hidden population of such events.

Contribution

The paper introduces a new diagnostic method based on temporal properties to distinguish long-duration compact object mergers from collapsar GRBs, revealing a larger hidden population.

Findings

Identified 9 long GRB candidates with properties similar to known COMs.

Found at least two outliers in the Ep,i-Eiso relation with high confidence.

MVTs of candidates align more with COM than collapsar populations.

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 Ep,i-Eiso plane as a function of redshift. We then evaluated the joint likelihood of their compatibility with the Ep,i-Eiso 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 9 COM candidates with unknown redshifts and demonstrated that there are at least two outliers of the Ep,i-Eiso relation with 3.1 sigma (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 may be larger than previously assumed.