\begin{table}%t1
\caption{\xmm~Log of observations of Mrk 421.}\label{tab1}
\small%%\centerline

{
\begin{tabular}{ccccccc}
\hline \hline
\multicolumn{1}{c}{Date} & \multicolumn{4}{c}{EPIC-MOS1} & \multicolumn{2}{c}{EPIC-MOS2}\\
\multicolumn{1}{c}{dd/mm/yy} & \multicolumn{1}{c}{Frame} & \multicolumn{1}{c}{Filter} & \multicolumn{1}{c}{Exp} & \multicolumn{1}{c}{Frame} & \multicolumn{1}{c}{Filter} & \multicolumn{1}{c}{Exp}\\
\hline  
25/05/2000 & FU & Md & 18000 & PW & Md & 24001 \\
01/11/2000 & PW & Md & 37251 & PW & Md & 37250 \\      
13/11/2000 & PW & Md & 46950 & PW & Md & 46951 \\         
14/11/2000 & PW & Md & 41948 & PW & Md & 41948 \\ 
08/05/2001 & PW & Tn & 38408 & PW & Tn & 38409 \\
04/05/2002 & FU & Tn & 38898 & FW & Tn & 39163 \\ 
05/05/2002 & FW & Tk & 19422 & FW & Tn & 19419 \\
04/11/2002 & FU & Tn & 23407 & FU & Tn & 23409 \\
04/11/2002 & FU & Tn & 23416 & FU & Tn & 23410 \\
14/11/2002 & FW & Tn & 71200 & FW & Tn & 71200 \\
14/11/2002 & FW & Tn & 11172 & FW & Tn & 11174 \\ 
15/11/2002 & FW & Tn & 11175 & FW & Tn & 11174 \\ 
01/12/2002 & FW & Tn & 70873 & FW & Tn & 70873 \\
02/12/2002 & FW & Tn & 11173 & FW & Tn & 11172 \\
02/12/2002 & FW & Tn & 11171 & FW & Tn & 11175 \\
01/06/2003 & FU & Md & 40542 & PW & Md & 40923 \\
02/06/2003 & FU & Tk & 8912  & PW & Tk & 8911  \\
07/06/2003 & FU & Tk & 8000  & PW & Tk & 8000  \\ 
14/11/2003 & PW & Tn & 48668 & FU & Tn & 48413 \\
10/12/2003 & PW & Md & 25848 & PW & Tk & 25851 \\
06/05/2004 & PW & Md & 61532 & PW & Md & 61534 \\ 
07/11/2005 & RF & Tn & 9765  & RF & Tn & 9771  \\
07/11/2005 & RF & Md & 9465  & RF & Md & 9470  \\   
09/11/2005 & PW & Tk & 59809 & PW & Tk & 59802 \\ 
\hline
\end{tabular}}
\medskip
 (a) FRAME: PW = partial window, FU = fast uncompressed, FW = full window, RF~= refresh frame. \\
 (b) FILTER: Tn = thin, Md = Medium, Tk = thick.
\end{table}