Search for Type IL Gamma-Ray Bursts: Criterion, Results, Verification, and Physical Implication

Abstract As an interesting subclass of gamma-ray bursts (GRBs), Type IL GRBs (such as GRB 211211A and GRB 230307A) features a long-duration prompt emission but originating from a compact binary merger. The “long duration” emission of Type IL GRBs are dominantly composed of the main burst, rather than the extended emission, differentiating them from the traditional “long-short” GRB (e.g., GRB 060614). A previous study has reported several Type IL GRBs by visual inspection of their light curves. In this work, we established a detailed criterion to identify Type IL GRBs by light curve and then systematically searched the archival Fermi/Gamma-ray Burst Monitor data with this criterion, resulting in a sample of five type IL GRBs from 2014 January 1 to 2024 January 1, i.e., GRB 230307A, GRB 211211A, GRB 200914A, GRB 200311A, and GRB 170228A. Apart from the light-curve pattern, we find that the temporal and spectral properties of these five GRBs also support this classification. Interestingly, we find that the energy ratio between extended emission and main emission is almost constant (∼0.7, with small scattering) for these GRBs, which have strong implications on the mechanism of Type IL burst. We discuss theoretical models to interpret the progenitor, central engine, and extended emission of these Type IL bursts.