The Supernova Remnant G284.3$-$1.8 and Its Relation to the Gamma-ray Binary 1FGL J1018.6$-$5856

G284.3$-$1.8 is a supernova remnant with a radio shell and thermal X-ray emission. Located near its center is the gamma-ray binary 1FGL J1018.6$-$5856, although the physical association between the two systems is not clear yet. Our X-ray spectroscopy with Suzaku reveals that G284.3$-$1.8 and 1FGL J1018.6$-$5856 have compatible absorption column densities of $N_{\mathrm{H}}=6\text{–}7\times {10}^{21}{\mathrm{c}\mathrm{m}}^{-2}$, indicating that the two systems have similar distances. The actual distance is determined as $3\mathrm{k}\mathrm{p}\mathrm{c}$ using ${}^{12}\mathrm{C}\mathrm{O}$ ($J=1\text{–}0$) data obtained with NANTEN. The X-ray spectrum of G284.3$-$1.8 shows a strong K-shell emission line of Mg, confirming that the earlier claim that the SNR is one of the few Mg-rich SNRs. Comparing recent stellar models taking into account the “shell merger” processes, we find that the obtained Mg-to-Ne mass ratio of $M_{\mathrm{M}\mathrm{g}}\mathrm{/}{M}_{\mathrm{N}\mathrm{e}}=0.73_{-0.03}^{+0.07}$ and Si-to-Mg mass ratio of $M_{\mathrm{S}\mathrm{i}}\mathrm{/}{M}_{\mathrm{M}\mathrm{g}}=0.44\pm 0.03$ suggest a supernova explosion that would have left a neutron star. The characteristics of 1FGL J1018.6$-$5856, on the other hand, are better explained with a model in which its compact object is neutron star. The present results, therefore, would suggest a possible scenario where G284.3$-$1.8 and 1FGL J1018.6$-$5856 are both remnants of a common supernova explosion although further observational tests are necessary.