\begin{table}%t3
%\centering
\par
\caption {\label{tab_SNRs}Characteristics of the six SNRs used to compute their 511~keV annihilation lightcurves. }
\begin{tabular}{ccccccc}
\hline\hline
\noalign{\smallskip}
\celltspace  & Cas A & Tycho & Kepler & SN1006 & SN1987A & G1.9+0.3 \cellbspace \\
\hline
\celltspace Age (yr) & 338 & 437 & 405 & 1003 & 22 & 100\tablefootmark{a} \cellbspace \\
\celltspace Distance (kpc) & 3.4\tablefootmark{b} & 2.4\tablefootmark{c} & 6\tablefootmark{d} & 2.2\tablefootmark{e} & 50 & 8.5\tablefootmark{a} \cellbspace \\
\celltspace Type & SNIIb\tablefootmark{f} & SNIa & SNIa & SNIa & SNII & SNIa\tablefootmark{a} \cellbspace \\
\celltspace Ejecta mass ($M_{\odot}$) & 2.2\tablefootmark{g} & \textit{1.4} & \textit{1.4} & \textit{1.4} & 14\tablefootmark{h} & \textit{1.4} \cellbspace \\
\celltspace Explosion energy (10$^{51}$~erg) & 2.0--4.0\tablefootmark{i} & \textit{1} & \textit{1} & \textit{1} & 1.1 $\pm$ 0.3\tablefootmark{h} & \textit{1} \cellbspace \\
\celltspace $^{56}$Ni yield ($M_{\odot}$) & 0.07--0.15 \tablefootmark{f} & \textit{0.6} & \textit{0.6} & \textit{0.6} & 0.07 & \textit{0.6} \cellbspace \\
\celltspace $^{44}$Ti yield ($M_{\odot}$) & $1.6\times 10^{-4}$\tablefootmark{j} &  $<$$ 2\times 10^{-4}$\tablefootmark{k} & $<$$ 3\times 10^{-5}$\tablefootmark{l} & $<$$ 3\times 10^{-5}$\tablefootmark{l} & $2\times 10^{-4}$ & $<$$ 3\times 10^{-5}$\tablefootmark{l} \cellbspace \\
\celltspace CSM density (cm$^{-3}$) & 3.2\tablefootmark{m} & 0.3\tablefootmark{m} & 0.5\tablefootmark{d}  & 0.1\tablefootmark{n} & 0.02\tablefootmark{o} & 0.04\tablefootmark{a}  \cellbspace \\
\hline
\end{tabular}
\tablefoot{Numbers in italic are typical values assumed when no established measurements were found in the literature.}
\tablebib {\tablefoottext{a}{\citet{Reynolds:2008};} 
        \tablefoottext{b}{\citet{Reed:1995};} 
        \tablefoottext{c}{\citet{Cassam-Chenai:2007};} 
        \tablefoottext{d}{\citet{Vink:2008};} 
        \tablefoottext{e}{\citet{Winkler:2003};} 
        \tablefoottext{f}{\citet{Krause:2008};} 
        \tablefoottext{g}{\citet{Willingale:2003};} 
        \tablefoottext{h}{\citet{Blinnikov:2000};} 
        \tablefoottext{i}{\citet{Laming:2006};} 
        \tablefoottext{j}{\citet{Renaud:2006};} 
        \tablefoottext{k}{\citet{Renaud:2009};} 
        \tablefoottext{l}{\citet{Renaud:2006a};} 
        \tablefoottext{m}{\citet{Truelove:1999};} 
        \tablefoottext{n}{\citet{Raymond:2007};} 
        \tablefoottext{o}{\citet{Sugerman:2005}.}}
\end{table}