\begin{table}%t2
\caption{\label{table_spectrum}
Parameters of the power law fit to the differential spectrum for the
different darkness periods of HESS\ observations on
PSR~B1259$-$63. For the periods May and June 2004, no spectrum could
be derived due to insufficient statistics. Shown are the photon index
$\Gamma$ and the flux normalisation $F_{\rm 0}$ (with statistical
error only), the~$\chi^2$ per number of degrees of freedom
$\chi^2/{\rm ndf}$ and the $\chi^2$ probability~$P_{\chi^2}$ for
the power law fit of the spectrum, and the corresponding mean
threshold energy $E_{\rm th}$ (rounded to 10~GeV).}
%\centerline
{{
\begin{tabular}{cccccc}
\hline\hline
Period
 & $\Gamma$
 & $F_{\rm 0}$
 & $\chi^2/$
 & $P_{\chi^2}$
 & $E_{\rm th}$
\\
2004
 &
 & ${\rm [TeV}^{-1}~~{\rm cm}^{-2}~~{\rm s}^{-1}]$
 & ${\rm ndf}$
 &
 & $[{\rm GeV}]$
\\
\hline
February
 & $2.8\pm0.3$ & $2.1\pm0.3\times10^{-12}$ & $1.8/4$ & $0.77$ & 370\\
March
 & $3.0\pm0.6$ & $1.0\pm0.2\times10^{-12}$ & $1.8/4$ & $0.76$ & 420\\
April
 & $2.8\pm0.4$ & $2.0\pm0.3\times10^{-12}$ & $0.4/3$ & $0.93$ & 350\\
\hline
Overall$^*$
 & $2.7\pm0.2$ & $1.3\pm0.1\times10^{-12}$ & $2.3/5$ & $0.81$ & 380\\
\hline
\end{tabular}
}}
\smallskip
{{\footnotesize $^*$ Includes all data from February to May 2004.}} 
\end{table}