All Figures

$\begin{figure} \par\includegraphics[width=7.2cm,clip]{7157fig1.eps}\hspace*{4mm} \includegraphics[width=7cm,clip]{7157fig2.eps} \end{figure}$ Figure 1: Redshift distribution ( left panel) and cumulative redshift distributions of Swift (solid line) and BeppoSAX+HETE2 (dashed line) GRBs.
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In the text

$\begin{figure} \par\includegraphics[width=7cm,clip]{7157fig3.eps}\hspace*{3mm} \includegraphics[width=7.2cm,clip]{7157fig4.eps} \end{figure}$ Figure 2: peak flux cumulative distributions of the Swift (solid line) and BeppoSAX+HETE2 (dashed line) GRBs. a), left panel, 15-150 keV band; b), right panel, 2-26 keV band.
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In the text

$\begin{figure} \par\includegraphics[width=7.3cm,clip]{7157fig5.eps}\hspace*{3mm} \includegraphics[width=7cm,clip]{7157fig6.eps} \end{figure}$ Figure 3: a), left panel, $N_{\rm H}$ cumulative distributions of the Swift (solid line) and BeppoSAX (dashed lines) GRBs. The leftmost BeppoSAX curve assumes $N_{\rm H}=N_{\rm H}{Galactic}$ for the GRBs with a best-fit intrinsic $N_{\rm H}$ consistent with zero. The rightmost BeppoSAX curve assumes for these GRBs the 90% upper limit. b), right panel, $N_{\rm H}$ histograms of the Swift GRB with (shadow histogram) and without redshift (black histogram).
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In the text

$\begin{figure} \par\includegraphics[width=6.8cm,clip]{7157fig7.eps} \end{figure}$ Figure 4: The R magnitude of the optical afterglow at the time of its discovery as a function of this time. Filled symbols are GRBs with reliable redshift determination. The size of the symbol is proportional to the redshift (the larger the symbol, the higher the redshift). Circles = Swift GRBs; squares = BeppoSAX GRBs; triangles = HETE2 GRBs.
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In the text

$\begin{figure} \par\includegraphics[width=6.5cm,clip]{7157fig8.eps}\hspace*{3mm} \includegraphics[width=6.5cm,clip]{7157fig9.eps} \end{figure}$ Figure 5: a), left panel, the distribution of the R magnitude 10ks after the GRB events for the Swift (solid histogram) and the BeppoSAX+HETE2 (dashed histogram) GRB samples. b), right panel, the distribution of the $\gamma$ -ray (15-150 keV) to optical (R band) flux ratio for the Swift (solid histogram), and BeppoSAX+HETE2 (red histogram) GRB samples.
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In the text

$\begin{figure} \par\includegraphics[width=7.4cm,clip]{7157fig10.eps}\hspace*{4mm} \includegraphics[width=7cm,clip]{7157fig11.eps} \end{figure}$ Figure 6: The redshift as a function of the 15-150 keV peak flux a), left panel, and of the observer frame $N_{\rm H}$ b), right panel, for the Swift (circles), BeppoSAX (triangles) and HETE2 (squares) GRBs. GRB without a measured redshift are plotted at a constant z as empty symbols. The left dashed line in Fig. 6b is the expectation for a constant, rest-frame column density of $\log N_{\rm H}=22$ , and the right dashed line is the expectation for $\log N_{\rm H}=23$ .
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In the text

$\begin{figure} \par\includegraphics[width=7cm,clip]{7157fig12.eps} \end{figure}$ Figure 7: The average cumulative redshift distribution of a subsample of Swift GRBs having the same peak flux, $N_{\rm H}$ and R mag (at 10 ks) distributions of the joined BeppoSAX+HETE2 samples (thin solid line) compared with the Swift (thick solid line) and BeppoSAX+HETE2 (dashed line) total redshift distributions. The thin dotted lines mark the redshift range covered by 100 random extractions and give an idea of the statistical uncertainty associated to a single extraction of a redshift distribution of 26 GRBs from a parent population.
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In the text

$\begin{figure} \par\includegraphics[width=7.2cm,clip]{7157fig1.eps}\hspace*{4mm} \includegraphics[width=7cm,clip]{7157fig2.eps} \end{figure}$	Figure 1: Redshift distribution ( left panel) and cumulative redshift distributions of Swift (solid line) and BeppoSAX+HETE2 (dashed line) GRBs.
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$\begin{figure} \par\includegraphics[width=7cm,clip]{7157fig3.eps}\hspace*{3mm} \includegraphics[width=7.2cm,clip]{7157fig4.eps} \end{figure}$	Figure 2: peak flux cumulative distributions of the Swift (solid line) and BeppoSAX+HETE2 (dashed line) GRBs. a), left panel, 15-150 keV band; b), right panel, 2-26 keV band.
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$\begin{figure} \par\includegraphics[width=6.8cm,clip]{7157fig7.eps} \end{figure}$	Figure 4: The R magnitude of the optical afterglow at the time of its discovery as a function of this time. Filled symbols are GRBs with reliable redshift determination. The size of the symbol is proportional to the redshift (the larger the symbol, the higher the redshift). Circles = Swift GRBs; squares = BeppoSAX GRBs; triangles = HETE2 GRBs.
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$\begin{figure} \par\includegraphics[width=6.5cm,clip]{7157fig8.eps}\hspace*{3mm} \includegraphics[width=6.5cm,clip]{7157fig9.eps} \end{figure}$	Figure 5: a), left panel, the distribution of the R magnitude 10ks after the GRB events for the Swift (solid histogram) and the BeppoSAX+HETE2 (dashed histogram) GRB samples. b), right panel, the distribution of the $\gamma$ -ray (15-150 keV) to optical (R band) flux ratio for the Swift (solid histogram), and BeppoSAX+HETE2 (red histogram) GRB samples.
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