\begin{table}%t4
\caption{\label{burstrate}Average burst recurrence time estimates for various instruments during three periods.}
%\centering
\par
\begin{tabular}{ccccccccccccccccccc}
\hline
\hline\noalign{\smallskip}
\multicolumn{4}{c}{1996--2000 flaring period (I)} & 
\multicolumn{4}{c}{2001--2002 calm period (II)} & 
\multicolumn{4}{c}{2003--2007 flaring period (III)} \\ 
\multicolumn{1}{c}{Inst} & \multicolumn{1}{c}{$t_{\rm exp}$} & \multicolumn{1}{c}{$n$\tablefootmark{a}} & \multicolumn{1}{c}{$\delta t_{\rm b}$\tablefootmark{b}} &
\multicolumn{1}{c}{Inst} & \multicolumn{1}{c}{$t_{\rm exp}$} & \multicolumn{1}{c}{$n$} & \multicolumn{1}{c}{$\delta t_{\rm b}$} &
\multicolumn{1}{c}{Inst} & \multicolumn{1}{c}{$t_{\rm exp}$} & \multicolumn{1}{c}{$n$} & \multicolumn{1}{c}{$\delta t_{\rm b}$} \\
\multicolumn{1}{c}{~} & \multicolumn{1}{c}{(d)} & \multicolumn{1}{c}{~} & \multicolumn{1}{c}{(d)} &
\multicolumn{1}{c}{~} & \multicolumn{1}{c}{(d)} & \multicolumn{1}{c}{~} & \multicolumn{1}{c}{(d)} &
\multicolumn{1}{c}{~} & \multicolumn{1}{c}{(d)} & \multicolumn{1}{c}{~} & \multicolumn{1}{c}{(d)} \\
\hline
WFC     & 24.9 & 1   & 11--83 & WFC     &  2.3       & 0   &  $>$5   & WFC     &  --         & --   &  --   \\
ASM     & 27.5 & 2   & 7--31  & ASM     &  9.6       & 0   &  $>$19  & ASM     &  20.5       &  4   & 3--9  \\ 
PCA     &  9.6 & 0   & $>$19  & PCA     &  3.7       & 1   &  2--12  & PCA     &   9.4       &  0   & $>$19 \\
FREGATE &  --  & --  & --     & FREGATE & $\simeq$65 & 2   & 17--72  & FREGATE & $\simeq$129 & 11   & 9--16 \\
ISGRI   &  --  & --  & --     & ISGRI   & --         & --  & --      & ISGRI   & 25.3        &  2   & 7--28 \\
\hline
All     & $\simeq$62 & 3 & 12--39 & All  & $\simeq$81 & 2\tablefootmark{c} & 22--90 & All    & $\simeq$184 & 17   & 9--14 \\
\hline
\end{tabular}
\tablefoot {\tablefoottext{a}{$n$ = number of bursts observed in the considered period;}
\tablefoottext{b}{$\delta t_{\rm b}$~= expected average burst recurrence time based on Poisson likelihood 1$\sigma$~limits, see~text;} 
\tablefoottext{c}{Note that PCA and FREGATE observed one of the bursts simultaneously, see Sect.~\ref{intermediate}.}}
\vspace*{2mm}
\end{table}