All Figures

	Figure 1: Intrinsic distribution of Ti90 durations observed by Swift-BAT corrected for the cosmological time dilation (redshift). The dashed-line histogram shows the observed distribution (Ti90), the continuous-line histogram shows the same distribution shifted at z=5. The vertical dotted line marks our reference constraint T90>60 s.
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In the text

Figure 2: Redshift distribution of the complete sample of 45 Swift GRBs versus their T90, i.e. the time containing of the BAT 15-350 keV fluence. The horizontal line marks our selection criteria for long-distant GRBs, and the two vertical dashed lines indicate the selected redshift interval. Basically, by requiring T90>60 s we mainly select closeby GRBs with long durations (like GRB 060218 or GRB 060614) or distant (z>3) GRBs.

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In the text

Figure 3: Distribution of UVOT V magnitudes for all Swift GRBs from March 2006 versus their T90 duration. Circles mark GRBs with a detection with UVOT (in the V band). Filled circles are GRBs with a spectroscopic redshift. Squares instead mark GRBs without a UVOT detection. Filled squares are GRBs with measured redshift. The star marks GRB 060522 with a non-detection in the V band but a detection in white-light with UVOT. The combination of T90>60 s and absence of UVOT counterpart can be used to single out high-redshift GRBs. We have 4 objects in this space of parameters: two of them do not have a secure redshift (they have strong upper limits in the R-band and one has a K band detection), and a spectroscopic redshift with has been measured for two of them. We note that GRB 050904 at z=6.29 ( s, no optical counterpart with V>20.1, and E(B-V)=0.06) satisfies all our selection criteria, but being observed before March 2006 is not included in the present sample.

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In the text