\begin{table}%t3
\caption{\label{table:4}Grouping definition used to study the time dependence of the EPIC~flux cross-calibration.}
\par
\small%\centerline
 {
\begin{tabular}{c c}         
\hline\hline                  
%& \\[-8pt]
Revolution$^a$ &  $N_{{\rm obs}}^{b}$ \\
%& \\[-8pt]
\hline                         
 28 --290    & 730  \\
 291--552    & 919  \\
 553--814    & 901  \\
 815--1076   & 739  \\
 1077--1338   & 202  \\
\hline                                    
\end{tabular}}
\medskip
$^a$ Range of XMM-{\it Newton} revolutions in group; $^b$~total number of observations with revolution in group.
\end{table}