\begin{table}%t1
\caption{\label{tab:data}High resolution X-ray observations of NGC 5548.  }                     
\par
%\centerline
 {                         
\begin{tabular}{lcc}        
\hline\hline                  
Year (month) & Instrument & Reference$^a$\\     
\hline 
 1999 (Dec.) &LETGS & 1 \\
 2000 (Feb.) &HETGS & 1 + 2  \\
 2000 (Dec.) &RGS & 6 \\
 2001 (Jul.) &RGS & 3 + 6 \\
 2002 (Jan.) &LETGS + HETGS & 4 \\   
 2005 (Apr.) &LETGS & 5 \\
 2007 (Aug.) &LETGS & 6 \\ 
\hline
\end{tabular}}
\medskip
\par
$^a$ References for observations: (1) \citet{Kaastra02b}; (2) \citet{Yaqoob01}; 
(3) \citet{Steenbrugge03}; (4) \citet{Steenbrugge05}; (5) D08; (6) Present work. 
\end{table}