\begin{table}%t1
\caption{Source and background observations.}
\label{obs_bkg}
\small%\centerline
{
\begin{tabular}{c c c}
\hline
\hline
&& \\[-8pt]
Obj name &  $N_{\rm H}$ & SXR bkg $^a$  \\ 
 & ($10^{22}$ cm$^{-2}$)&(cts s$^{-1}$ arcmin$^{-2}$)\\
\hline
&& \\[-8pt]
\object{A3558} &  0.0389 & $(301 \pm 10) \times 10^{-6}$\\
PHL 5200 & 0.0526 & $(111 \pm 8) \times 10^{-6}$ \\
PKS 0312-770 & 0.0797 & $(122 \pm 6) \times 10^{-6}$\\
\hline
\end{tabular}}
\medskip
$^a$ Average X-ray background  count rate in the 3/4 keV band, derived from the \emph{ROSAT} all-sky survey diffuse background maps.
\end{table}