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
(Abridged) The two high-redshift gamma-ray bursts, GRB 080913 at z=6.7 and
GRB 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 should
they have occurred at z <= 1. We perform a more thorough investigation on two
physically distinct types (Type I/II) of cosmological GRBs and their
observational characteristics. We reiterate the definitions of Type I/II GRBs
and review the observational criteria and their physical motivations. Contrary
to the traditional approach of assigning the physical category based on the
gamma-ray properties (duration, hardness, and spectral lag), we take an
alternative approach to define the Type I and Type II Gold Samples using
several criteria that are more directly related to the GRB progenitors, and
study the properties of the two Gold Samples and compare them with the
traditional long/soft and short/hard samples. We find that the Type II Gold
Sample reasonably tracks the long/soft population, although it includes several
intrinsically short (shorter than 1s in the rest frame) GRBs. The Type I Gold
Sample only has 5 GRBs, 4 of which are not strictly short but have extended
emission. Other short/hard GRBs detected in the Swift era represent the BATSE
short/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 GRBs
instead belong to Type II. We suggest that GRB 080913 and GRB 090423 are more
likely Type II events. We re-emphasize the importance of invoking multiple
observational criteria, and cautiously propose an operational procedure to
infer the physical origin of a given GRB with available multiple observational
criteria, with various caveats laid out.Comment: 32 pages, ApJ, in press. The strengths and weaknesses of physical
classification and its relation to phenomenological classification are fully
discussed in a newly added section 3. Discussions on GRBs 090423, 090426, and
090510 are include