A new classification method for gamma-ray bursts

Abstract

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 <inf>γ,iso,52</inf>/E<inf>p,z,2</inf>^5/3, where E <inf>γ,iso</inf> is the isotropic gamma-ray energy (in units of 10 ⁵² erg) and E<inf>p,z</inf> 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<inf>p</inf> measurements, 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), aswell as some high-z shortGRBs (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 theGRBs 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 T<inf>90</inf> criterion, i.e., T<inf>90,z</inf> ≲ 5 s in the Swift/BAT band.We suggest that this newclassification scheme can bettermatch the physically motivated Type II/I classification scheme. © 2010. The American Astronomical Society. All rights reserved.