\begin{table}%t2
  \caption{\label{tab2}The definition of the four X-ray colours.}
%\centerline
 {\begin{tabular}{lccc}
\hline\hline
\noalign{\smallskip}
HR & S & H & $\chi^2$/d.o.f. \\
& keV & keV & \\
\hline
HR1 & 0.4--1 & 1--2.5 & 42.2/45 \\
HR2 & 1--2.5 & 2.5--5 & 323/45 \\
HR3 & 2.5--5 & 5--10 & 115/45 \\
HR4 & 0.4--1 & 5--10 & 117/45 \\
\hline
\end{tabular}}
\tablefoot {Each colour indicator (HR) is defined by $\rm (H-S)/(H+S)$, where H and
    S are count rates measured in the two energy bands given
    above. $\chi^2$ values for a constant hypothesis are also
    included.}
\end{table}