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(Abridged) The two high-redshift gamma-ray bursts, GRB 080913 at z=6.7 andGRB 090423 at z=8.3, recently detected by Swift appear as intrinsically short,hard GRBs. They could have been recognized by BATSE as short/hard GRBs shouldthey have occurred at z <= 1. We perform a more thorough investigation on twophysically distinct types (Type I/II) of cosmological GRBs and theirobservational characteristics. We reiterate the definitions of Type I/II GRBsand review the observational criteria and their physical motivations. Contraryto the traditional approach of assigning the physical category based on thegamma-ray properties (duration, hardness, and spectral lag), we take analternative approach to define the Type I and Type II Gold Samples usingseveral criteria that are more directly related to the GRB progenitors, andstudy the properties of the two Gold Samples and compare them with thetraditional long/soft and short/hard samples. We find that the Type II GoldSample reasonably tracks the long/soft population, although it includes severalintrinsically short (shorter than 1s in the rest frame) GRBs. The Type I GoldSample only has 5 GRBs, 4 of which are not strictly short but have extendedemission. Other short/hard GRBs detected in the Swift era represent the BATSEshort/hard sample well, but it is unclear whether all of them belong to Type I.We suggest that some (probably even most) high-luminosity short/hard GRBsinstead belong to Type II. We suggest that GRB 080913 and GRB 090423 are morelikely Type II events. We re-emphasize the importance of invoking multipleobservational criteria, and cautiously propose an operational procedure toinfer the physical origin of a given GRB with available multiple observationalcriteria, with various caveats laid out.

astrophysical journal/the astrophysical journal

Discerning the physical origins of cosmological Gamma-ray bursts based  on multiple observational criteria: the cases of z=6.7 GRB 080913, z=8.3 GRB  090423, and some short/hard GRBs