\begin{table}%t4
\caption{Mean parameters of each sub-sample from this paper. We include the decay index ($\delta$), the spectral index ($\alpha$), the flux at 11~h after the burst and the observation start time. We also recall the main parameters from the BeppoSAX sample \citep{dep05}. All errors are quoted at a 68\% confidence level.}             % title of Table
\label{table_param}      % is used to refer this table in the text
%\centering
                          % used for centering table
\begin{tabular}{lcccc}        % centered columns
\hline\hline                 % inserts double horizontal lines
            & Chandra         & Chandra         &   XMM           & BeppoSAX  \\ 
            & Imaging         & Grating         &                 & \\
\hline                   
$\delta$    & $2.0 \pm 0.3$   & $1.4 \pm 0.1$   & $1.2 \pm 0.2$   & $1.3 \pm 0.1$ \\
$\alpha$    & $0.8 \pm 0.1$   & $1.0 \pm 0.1$   & $1.2 \pm 0.2$   & $1.2 \pm 0.1$  \\
Log(Flux)   & $-11.6 \pm 0.3$ & $-11.8 \pm 0.3$ & $-12.6 \pm 0.2$ & $-12.2 \pm 0.1$ \\
 @ 11 h &                 &                 &                 & \\
Obs. time   & 145.4           &  95.9           &  30.1           & 36.5\\
(ksec)      &                 &                 &                 & \\
\hline
\end{tabular}
\end{table}