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


 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 analyzing bursts with $T_{90}>4.2 \rm \ s$ from August 2008 to July 2021. In order to determine whether these outlier events were from compact binary mergers, we analyzed 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 8 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.