\begin{table}%T3
\par
\caption{\label{tab3}Spectral index of radio halos.}
\small
%\centerline
{
\begin{tabular}{lcccc}
\hline\hline
Name     & $\alpha$1     & $\alpha$2 & Temperature & Reference \\
         &               &           & ~keV        &           \\
\hline 
Straight & $>$3 points                 &    &  &           \\
A1914    & $\alpha_{0.26}^{1.4}$ = 1.88 &    &  7.9 & 1        \\
A2256    & $\alpha_{0.22}^{1.4}$ = 1.61 &    &  6.6 & 7       \\
1E 0657  & $\alpha_{0.84}^{5.9}$ = 1.3  &    &  10.6 & 10       \\
\noalign{\smallskip}\hline
Steepening& $>$3 points                &           \\
A1656    & $\alpha_{0.31}^{1.4}$ = 1.16 &$\alpha_{1.4}^{4.8}$ = 2.28& 8.4&11 \\
\noalign{\smallskip}
\hline
Straight & 3 points                &   &         &  \\
A521     & $\alpha_{0.24}^{1.4}$ = 1.8 &    &  5.9 & *           \\
RXC J2003& $\alpha_{0.2}^{1.4}$ = 1.3 &    & -- & 4       \\
\noalign{\smallskip}\hline
Steepening& 3 points                &         &   \\
A754     & $\alpha_{0.07}^{0.3}$ = 1.1 &$\alpha_{0.3}^{1.4}$ = 1.5 & 9.5 & 1 \\
A2163    & $\alpha_{0.3}^{1.3}$  = 1.1 &$\alpha_{1.3}^{1.6}$ = 1.5 &13.3 & 5 \\
A2319    & $\alpha_{0.4}^{0.6}$  = 0.9 &$\alpha_{0.6}^{1.4}$ = 2.2 & 8.8 &8 \\
A3562    & $\alpha_{0.3}^{0.8}$  = 1.3 &$\alpha_{0.8}^{1.4}$ = 2.1 & 5.2 &9 \\
\noalign{\smallskip}
\hline
          & 2 points               &   & \\
A545     & $\alpha_{1.3}^{1.6} > 1.4$ &   & 5.5 & 1       \\
A665     & $\alpha_{0.3}^{1.4}$ = 1.0 &    & 9.0 & 3       \\
A697     & $\alpha_{0.3}^{1.4}$ = 1.2  &    & 10.2& *       \\
A1300    & $\alpha_{0.3}^{1.4}$ = 1.8 &     & 9.2 & 4      \\
A2218    & $\alpha_{1.4}^{5.0}$ = 1.6 &     & 7.1 & 3      \\
A2219    & $\alpha_{0.3}^{1.4}$ = 0.9  &    & 12.4 & 2       \\
A2255    & $\alpha_{0.3}^{1.4}$ = 1.7 &     & 6.9 & 6      \\
A2744    & $\alpha_{0.3}^{1.4}$ = 1.0 &     & 10.1 & 2      \\
\noalign{\smallskip}\hline
\end{tabular}}
\smallskip
  Notes: Temperature is the average cluster 
temperature in~keV from literature data. \\
  References to spectral index information: 
* = this work; 1 = \citet{bac03}; 
2 = \citet{orr07};  
3 = \citet{gf00};
4 = \citet{ven09}; 
5 = \citet{fer01}; 6 = \citet{gov05}; 
7 = \citet{bre08}; 
8 = \citet{fer97}; 
9 = \citet{ven03};
10 = \citet{lia00}; 11 = \citet{thi03}. 
\end{table}