\begin{table}%t2
\caption{\label{tab_gain}Summary of the releases of the XRT post-launch gain files.}
\small%\centerline
 {
\begin{tabular}{cclc}
\hline \hline
Release  & Release  & Main improvements in the gain & Text section \\
number$^\dagger$  & date  &  & number\\
\hline
05  & 2005 April 5  &  Introduction of a temperature-dependent gain file & 3.2.3  \\
\hline
06  &  2005 October 28  &  Update of the gain coefficients over time & -- \\
\hline
07$^*$  & 2007 July 9  &  Update of the CTI values over time in WT \& PC
mode & 3.2.2\\
    &    &  Update of the gain coefficients in WT \& PC
    modes after the substrate & 4.3\\
    &    & voltage ($V_{\rm ss}$) change from 0~V to 6~V on 2007 August 30 &\\
\hline
08 & 2008 June 25 & Introduction of an offset of $C_3=17.6$ eV to restore the
WT energy scale & 3.2.4\\
\hline
\end{tabular}}
\medskip
$^\dagger$ The v003-v004 gain files correspond to the pre-launch gain files. \\
$^*$ The gain file has now added a suffix ``s0'' or ``s6'' to indicate the appropriate substrate voltage~($V_{\rm ss}$).  The ``s0'' files are valid from the start of the mission until when the substrate voltage was raised to $V_{\rm ss}=6$~V on 2007~August~30.
\end{table}