\begin{table}%t6
\caption{\label{tab6}Measued spectral indices from known PWN.}
%\centerline
 {\small \begin{tabular}{c c c}
\hline\hline
Pulsar (Area)& Wavelength range & Spectral index  \\
\hline\noalign{\smallskip}
Crab (Wisp 1) & $U \rightarrow K_ {\rm s}$ & $-0.58\pm0.08^a$ \\
\vspace{1mm}
Crab (Wisp 2) & $U \rightarrow  K_ {\rm s}$ & $-0.63\pm0.07$\\
\vspace{1mm}
Crab (Wisp 3) & $U \rightarrow K_ {\rm s}$ & $-0.53\pm0.08$\\
\vspace{1mm}
Crab (Anvil Wisp) & $U \rightarrow K_ {\rm s}$ & $-0.62\pm0.10$\\
\vspace{1mm}
Crab (Interwisp 1)& $U \rightarrow K_ {\rm s}$ & $-0.61\pm0.08$\\
\vspace{1mm}
Crab (Interwisp 2)& $U \rightarrow K_ {\rm s}$ & $-0.50\pm0.10$\\
\vspace{1mm}
Crab (Interwisp 3)& $U \rightarrow K_ {\rm s}$ & $-0.49\pm0.10$\\
\vspace{1mm}
Crab (Knot)& $I \rightarrow  K_ {\rm s}$ & $-0.63\pm0.02$\\
\vspace{1mm}
Vela (Inner arc)$^c$ & X-rays$\rightarrow$IR &  $-0.5 \rightarrow -0.3$ \\
\vspace{1mm}
3C58 (Optical region)$^b$ & X-rays &  $-0.88\pm0.08$\\
\vspace{1mm}
3C58 & Optical$\rightarrow$IR &  $-1.2\rightarrow$-0.5$^d$\\
\vspace{1mm}
PSR B0540-69.3 (Area 1)$^d$ & $U\rightarrow I$& $\rm -1.09^{+0.30}_{-0.33}$ \\
\vspace{1mm}
PSR B0540-69.3 (Area 2) & $U\rightarrow I$& $\rm -1.58^{+0.42}_{-0.43}$ \\
\vspace{1mm}
PSR B0540-69.3 (Area 3) & $U\rightarrow I$& $\rm -1.28^{+0.41}_{-0.43}$ \\
\vspace{1mm}
PSR B0540-69.3 (Area 4) & $U\rightarrow I$& $\rm -1.49^{+0.59}_{-0.67}$ \\
\vspace{1mm}
PSR B0540-69.3 (Area 5) & $U\rightarrow I$& $\rm -0.92^{+0.33}_{-0.37}$ \\
\vspace{1mm}
PSR B0540-69.3 (Area 6) & $U\rightarrow I$& $\rm -0.27^{+0.90}_{-0.91}$ \\
\hline
\end{tabular}}
\medskip
$^a$  1$\sigma$ error in the uncertainties of the spectral indices of the Crab PWN; $^b$ from Shibanov et~al. (\cite{shi03}); $^c$ from Shibanov et~al. (\cite{shi08}); $^d$~see Shibanov et~al. (\cite{shi08}) for details; $^e$~see Serafimovich et~al. (\cite{ser04}) for orientation.
\end{table}