\begin{table}%t6
 %\centering
\par
 \caption{\label{tab:chandraobs}The observation log of the {\it Chandra} data sets.}
% \begin{minipage}{90mm} 
%  \begin{tabular}{@{}lllllll@{}} 
  \begin{tabular}{lllllll} 
\hline \hline
ObsID & Exposure &  $T_{\rm start}$ & $T_{\rm end}$ & $\phi_{\rm start}$ & $\phi_{\rm end}$ & Count rate\\ 
 & ks  & JD & JD & & & cts/s  \\
\hline 
5395  & 36.43           & 2~453~674.4519389  & 2~453~674.8897456 & 0.0096      & 0.0461 & 0.086 \\ 
6138  & 29.54 & 2~453~675.0851294  & 2~453~675.4490259 & 0.0624      & 0.0927 & 0.089\\ 
7194  & 38.17 & 2~453~677.1208892  & 2~453~677.5798238 & 0.2320      & 0.2702 & 0.082 \\ 
7195  & 27.15 & 2~453~677.7494105  & 2~453~678.0855105 & 0.2844      & 0.3124 & 0.075\\ 
\hline 
\end{tabular} 
\end{table}