\begin{table}%t4
\caption{Rotation measure.}
\label{rmtab}
\small%\centerline
{
\begin{tabular}{c c c c c c} 
\hline\hline
Source   & Centroid Dist. & $\langle {\rm RM} \rangle$         &   $\sigma_{\rm RM}$  & $\arrowvert {\rm RM_{max}}\arrowvert$ & $N$ \\
         &  (kpc)         & (rad/m$^{2}$)  &   (rad/m$^{2}$)  & (rad/m$^{2}$)       & (Beams)\\
\hline 
J1712.4+6401  & 315  & $-$81        &   79 & 300& 29  \\
J1713.5+6402  & 450  &    67        &   59 & 236& 41  \\ 
J1713.3+6347  & 1570 &    36        &   42 & 149& 6   \\ \hline 
\end{tabular}}
\medskip
Column~1: source; Col.~2: projected distance from the X-ray centroid; Col.~3: mean of the RM~distribution; Col.~4: rms of the RM~distribution. Column~5: maximum absolute value of the RM~distribution;  Col.~6: number of independent beams in the RM~images.
\end{table}