2010ApJ...725.1965L

Recent Swift observations suggest that the traditional long versus short gamma-ray burst (GRB) classification scheme does not always associate GRBs to the two physically motivated model types, i.e., Type II (massive star origin) versus Type I (compact star origin). We propose a new phenomenological classification method of GRBs by introducing a new parameter ε = E _γ,iso,52/E ^5/3_p,z,2, where E_γ,iso is the isotropic gamma-ray energy (in units of 10⁵² erg) and E_p,z is the cosmic rest-frame spectral peak energy (in units of 100 keV). For those short GRBs with "extended emission," both quantities are defined for the short/hard spike only. With the current complete sample of GRBs with redshift and E_pmeasurements, the ε parameter shows a clear bimodal distribution with a separation at ε ∼ 0.03. The high-ε region encloses the typical long GRBs with high luminosity, some high-z "rest-frame-short" GRBs (such as GRB 090423 and GRB 080913), as well as some high-z short GRBs (such as GRB 090426). All these GRBs have been claimed to be of Type II origin based on other observational properties in the literature. All the GRBs that are argued to be of Type I origin are found to be clustered in the low-ε region. They can be separated from some nearby low-luminosity long GRBs (in 3σ) by an additional T₉₀criterion, i.e., T_90,z ≲ 5 s in the Swift/BAT band. We suggest that this new classification scheme can better match the physically motivated Type II/I classification scheme.