\begin{table}%t3
\caption{\label{tab:parameters}Important parameters of NGC 5548.}             
\par
%\centerline
{                          
\begin{tabular}{cccc}      
\hline\hline                 
Parameter & Description & Value &Reference$^a$\\   
\hline                       
$L$ & Luminosity (1--1000 Ryd) & $6.6 \times 10^{36}$~W & 1 \\
log $\xi$ & Ionization parameter & 1.2 & 1
 \\
$a$  & Parameter defined in Eq.~\ref{trec} &  $1.64 \times  10^{17}$~m$^{-3}$~s  & 1
 \\   
$M_{{\rm BH}}$  & Black hole mass &  $6.54\times10^{7}~M_{\odot}$ & 2
\\
$Y$ & Emission measure &   $4.3 \times 10^{70}$~m$^{-3}$ & 1 \\ 
\hline
\end{tabular}}
 \medskip
\par
$^a$ References for parameter values: (1) Present Work; (2) \citet{Bentz07}.  
\end{table}