\begin{table}%T2
%\centering
\small
\caption{Orbital epochs by pulse timing analysis.}\label{t2}
\begin{tabular}{c c c c} \hline\hline
Experiment & Orbit number & Orbital epoch (MJD) & Reference \\ \hline
OSO 8 & --1128 &      43015.800 $\pm$ 0.1  &  Becker et~al. (\cite{Becker77}) \\
Tenma & --457  &     45517.660 $\pm$ 0.050 &  Makishima et~al. (\cite{Makishima87}) \\
Ginga &  0    &    47221.474 $\pm$ 0.020  &  Corbet et~al. (\cite{Corbet93})\\
BATSE &  370  &    48600.979 $\pm$ 0.027  &  Rubin et~al. (\cite{Rubin97})\\
BATSE &  478  &    49003.629 $\pm$ 0.022  &  Rubin et~al. (\cite{Rubin97})\\
BATSE &  584  &    49398.855 $\pm$ 0.029  &  Rubin et~al. (\cite{Rubin97})\\
BATSE &  691  &    49797.781 $\pm$ 0.022  &  Rubin et~al. (\cite{Rubin97})\\
RXTE  &  866  &    50450.206 $\pm$ 0.014  &  Clark (\cite{Clark00}) \\
RXTE  & 1511  &     52855.0421 $\pm$ 0.025& present work\\ \hline
\end{tabular}
\end{table}