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We have observed thermal X-ray emission from the resolved shell component of the composite Galactic SNR G326.3$-$1.8 with the ROSAT PSPC. The data are best fit by a single component thermal line spectrum with temperature $kT=0.56 \pm 0.04$ keV, hydrogen column density $N_H=8.9 \pm 0.3 \times 10^{21}$ cm$^{-2}$, and unabsorbed X-ray flux $F_{X 0} = 3.9 \pm 0.5 \times 10^{-10}$ erg cm$^{-2}$ sec$^{-1}$ (0.1--2.4 keV). The standard Sedov analysis with our only assumption being an assumed initial kinetic energy $\epsilon_0 = 10^{51}$ erg, gives a radius $R \simeq 20$ pc, distance $D \simeq 3.7$ kpc, age $t \simeq 1.0 \times 10^4$ yr, X-ray luminosity $L_X \simeq 6.1 \times 10^{35}$ erg sec$^{-1}$ (0.1--2.4 keV), and an ambient ISM density $n_0 \simeq 0.1$ cm$^{-3}$.
The derived distance ($3.7 \pm 0.2$ kpc) falls within the range of a variety of previous but poor and uncertain distance estimates, and is consistent with the only reliable lower limit of $D \ge 1.5$ kpc. Evidence exists in the literature from both optical and radio studies that would place the SNR significantly further than this lower limit. Higher quality radio HI absorption measurements are warranted to confirm our distance determination. Since $D$ scales only weakly with $\epsilon_0$ ($D \propto \epsilon_0^{2/5}$), this result, along with other recent ROSAT studies of SNRs, implies that improved distance estimates may be established for the large number of extended shell-type SNRs with very poor distance estimates which fall within ROSAT's all sky survey.
In order to quantify how well existing ROSAT data can be used to infer distances to other shell-type SNRs, we are currently analyzing archival data for those few SNRs whose distances have been previously well determined from other means. We will present the results of our ongoing analysis and comparisons. Preliminary results imply that this technique could have important implications regarding the resolution of one of the most outstanding problems in Galactic astrophysics, namely the poorly determined distances to the great majority of identified Galactic SNRs.
