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Paper text:
grbs detected by batse @xcite show a bimodality in the distribution of the duration ( usually characterized with @xmath0 ) @xcite .
classification in two classes , short ( @xmath2s ) and long ( @xmath3s ) was also supported by the hardness ratio ( @xmath4 ) showing the short bursts to be harder than long ones @xcite .
this classification in short and long bursts is widely accepted , as well as the understanding that they represent two different physical phenomena .
batse and _ swift _ data also show some evidence for a third , intermediate group ( @xcite , @xcite ) . for simplicity , we here classify bursts only to short and long . here
we review the @xmath0 and hardness ratio of _ swift _ grbs detected between dec 2004 and dec 2009 .
our sample consists of 436 grbs , among which there are 145 with known redshift @xmath5 .
all data were taken from the _ swift _ archive and the gamma ray bursts coordinates network .
the ratio between the number of short and long bursts in the _ swift _ sample is roughly 1:10 , while in the batse sample it was about 1:3 .
the duration distribution of bursts in the first _ swift _ bat grb catalogue does not show clear bimodality due to the smaller sample of short duration bursts @xcite .
our results on number of grbs ( @xmath6 ) vs @xmath0 confirm this finding . to compare grbs in their rest frames , we focus on grbs with known redshift and take into account the cosmological time dilatation . fig .
[ fig01 ] shows @xmath6 vs. @xmath7 .
there is a gap between short and long grbs at @xmath8s .
this is consistent with the gap at @xmath9s seen in the observer frame for batse bursts , but shifted to the average rest frame , i.e. to the average redshift of bursts in our sample : @xmath10 .
we will use this gap in the final part of our analysis for classifying bursts into intrinsically short and long .
( taking into account the correlation between burst duration and energy @xcite would give @xmath11 , @xmath12s and same grbs falling into the category of short / long bursts , therefore giving the same results as those presented below . ) +    .hardness ratios ( @xmath4s ) of _ swift _ grbs for different energy channels ( for details see text ) .
[ cols= " < " , ]     @xmath13 in some cases , @xmath14 exceeds the mean value due to large data scatter .
@xmath4s can have only non - negative values .
for hardness ratio analysis we use the following energy channels : channel 1 : 15 - 25  kev , channel 2 : 25 - 50  kev , channel 3 : 50 - 100  kev , and channel 4 : 100 - 350 kev .
we define @xmath15 as the ratio between fluence @xmath16 in channel @xmath17 and @xmath18 , and @xmath19 .
we note that @xmath20 in @xcite roughly corresponds to our @xmath21 .
results are presented in table  [ hr ] and fig .
[ fig02 ] and depend on the channels used ( @xmath17 , @xmath18 ) .
first we analyze all grbs and a subgroup of those with known redshift in the observer frame .
we find the results for both groups to be consistent and the difference in @xmath4s among short and long bursts not very pronounced .
then we make a rough approximation by assuming that the hardness ratio of an individual burst is the same in the observer and in the rest frame ( the transformation of fluences cancels out in the fluence ratio ) . in the transformation to the rest frame
, only the burst duration @xmath0 is changed .
if we take @xmath22s to separate bursts into short and long , we see that the difference in @xmath4s between long and short burst decreases . in some cases
the median value of @xmath4s is even higher for long bursts than for short ones . on the other hand , by taking @xmath23s , we obtain results that are similar to the results in the observer frame for the same sample .
we review the @xmath0 duration and hardness ratio of _ swift _ gamma ray bursts ( grbs ) . 
 we focus on grbs with known redshift and review their gamma properties in the grbs rest frames . 
 we find that grbs number vs. @xmath1 distribution shows a separation between two classes at 0.65  s. furthermore , we find that the difference in hardness ratio between short and long bursts is not very pronounced and depends on energy channels used for comparison .