\begin{table}%t1
\par
  \caption{\label{tab_results}Analysis results: $N_{\rm on}$ and $N_{\rm off}$ are the event numbers in the ON and 
   OFF regions, $\alpha$ is the normalisation factor. The results are reported for the
   standard $\theta^2$ cut (column labelled {\em Standard}), for the cuts reproducing the
    conditions of the CANGAROO detection (column {\em CANGAROO}) and for the analysis of the radio arc (column 
    {\em Radio arc}).}
   %\centering
\par
          \begin{tabular}{c c c c}
         \hline\hline
                                     &{\em Standard} &{\em CANGAROO}   &{\em Radio arc}\\
                  \hline
          $N_{\rm on}$   &352            &112              &4746\\
          $N_{\rm off}$  &2243           &512              &13~688\\
          $\alpha$          &0.15620        &0.19258          &0.34592\\
          Excess            &$1.6\pm20.2$   &$13.4\pm11.1$    &$11.0\pm79.9$\\
          Significance      &$0.1\sigma$  &$1.2\sigma$    &$0.1\sigma$\\
                \hline
      \end{tabular}
 \end{table}