Outliers in the Ep,z – Eγ relation of Fermi-GBM long-duration gamma-ray bursts

Abstract

ABSTRACT Long gamma-ray bursts (GRBs) are typically associated with massive star core collapse, while the short GRBs are associated with compact binary mergers. However, recent evidence indicates that some peculiar long-duration bursts may correspond to compact binary mergers origins. In this paper, we use the data of the Fermi Gamma-ray Burst Monitor to search for peculiar long-duration bursts which may be from compact binary mergers based on outlier events in the $E_{p,z}\!-\!E_{\gamma ,\rm iso}$ relation. We obtained 10 outlier events by systematically analysing bursts with $T_{90}\gt 4.2 \rm \ s$ from 2008 August to 2021 July. In order to determine whether these outlier events were from compact binary mergers, we analysed their properties, including spectral lag, hardness ratio, and energy-hardness parameter. Based on the distributions of T90 − HR and T90 − Ep, we calculated the probability of outlier events belonging to the short GRBs. Our analysis indicates that GRB 120304B is likely to arise from the merger of a neutron star and a massive white dwarf. GRB 150210A is likely to arise from massive star core collapse. The other eight GRBs are fuzzy bursts that have both long and short GRBs properties. Additionally, we find that outlier samples have relatively high Ep and low fluences.