\begin{table}%t1
%\centering
\par
\caption {\label{tab_obs}Upper limits on the 511~keV flux in units of 10$^{-5}$~ph~cm$^{-2}$~s$^{-1}$, in three different energy bands and for various source sizes for SN1006 and Tycho. Also listed are the INTEGRAL/SPI exposure for each object in cm$^{2}$~s.}
\begin{tabular}{ccccccc}
\hline\hline
\noalign{\smallskip}
\celltspace {\it SNR} & SPI exposure & Sky model & 510--512~keV & 508.5--513.5~keV & 506--516~keV \cellbspace \\
\hline
\celltspace Cassiopeia A & $\sim$$ 3 \times 10^{8}$ & point-source & 2.8 & 3.9 & 4.7 \cellbspace \\
\hline
\celltspace Tycho & $\sim$$ 3 \times 10^{8}$ & point-source & 2.6 & 3.8 & 4.6 \cellbspace \\
\celltspace            &  & 3$^{\circ}$ Gaussian &  & 5.4 &  \cellbspace \\
\celltspace            &  & 6$^{\circ}$ Gaussian &  & 7.5 &  \cellbspace \\
\hline
\celltspace Kepler & $\sim $$8 \times 10^{8}$ & point-source & 1.8 & 2.5 & 3.0 \cellbspace \\
\hline
\celltspace SN1006 & $\sim $$3 \times 10^{8}$ & point-source & 2.8 & 4.0 & 4.8 \cellbspace \\
\celltspace               &  & 3$^{\circ}$ Gaussian &  & 5.1 &  \cellbspace \\
\celltspace               &  & 6$^{\circ}$ Gaussian &  & 6.4 &  \cellbspace \\
\celltspace               &  & 9$^{\circ}$ Gaussian &  & 7.2 &  \cellbspace \\
\celltspace               &  & 12$^{\circ}$ Gaussian &  & 7.9 &  \cellbspace \\
\celltspace               &  & 15$^{\circ}$ Gaussian &  & 8.5 &  \cellbspace \\
\hline
\celltspace SN1987A & $\sim $$8 \times 10^{7}$ & point-source & 3.8 & 5.4 & 6.6 \cellbspace \\
\hline
\celltspace G1.9+0.3 & $\sim$$ 9 \times 10^{8}$ & point-source & 1.8 & 2.6 & 3.1 \cellbspace \\
\hline
\end{tabular}
\end{table}