\begin{table}%t2
  \caption{\label{tab2}Adopted values for the free parameters.}
  \small%\centering
\par
  \begin{tabular}{ll}
  \hline\hline
   Parameter and unit  &  Value \\
  \hline\noalign{\smallskip}
$B$: magnetic field [G] & 1 \\
$L_{\rm norm}$: injection luminosity normalization [erg~s$^{-1}$] & $1.3\times 10^{35}$ \\
$\eta$: acceleration efficiency parameter & 0.01\\
$p$: electron energy distribution power-law index & 2 \\  
  \noalign{\smallskip}\hline
  \end{tabular}
\end{table}