ROSAT OBSERVATIONS OF THE COMPOSITE SUPERNOVA REMNANT G326.3-1.8

We have observed X-ray emission from the radio-defined composite (shel l plus filled-center plerion) Galactic supernova remnant (SNR) G326.3- 1.8 with the ROSAT position sensitive proportional counter (PSPC). The data are best fitted by a single-component, thermal line spectrum wit h temperature kT = 0.56 +/- 0.04 keV, hydrogen column density N(H) = 8 .9 +/- 0.3 x 10(21) cm-2, and unabsorbed X-ray flux F(x0) = 3.9 +/- 0. 5 x 10(-10) ergs cm-2 s-1 (0.1-2.4 keV). The standard Sedov analysis w ith an assumed initial kinetic energy epsilon0 = 10(51) ergs gives a r adius R congruent-to 20 pc, distance D congruent-to 3.7 kpc, age t con gruent-to 1.0 x 10(4) yr, X-ray luminosity L(x) congruent-to 6.1 x 10( 35) ergs s-1 (0.1-2.4 keV), and an ambient interstellar medium (ISM) d ensity n0 congruent-to 0.1 CM-3. The derived distance falls within the range of a variety of previous but poor and uncertain distance estima tes and is consistent with the only reliable lower limit of D greater- than-or-equal-to 1.5 kpc; Evidence exists in the literature from both optical and radio studies that would place the SNR significantly furth er than this lower limit. Higher quality radio H I absorption measurem ents are warranted to confirm our distance determination. Since D scal es only weakly with co (D is-proportional-to epsilon(2/5)0), this resu lt, along with other recent ROSAT studies of SNRs, implies that improv ed distance estimates may be established for the large number of exten ded shell-type SNRs with very poor distance estimates which fall withi n ROSAT's all-sky survey. No X-ray analog to the plerionic radio emiss ion appears on our ROSAT image, but our exposure is not sufficient to establish a stringent limit on any filled-center nonthermal X-ray emis sion. An indistinguishable nonthermal component with spectral index -0 .5 less-than-or-equal-to alpha less-than-or-equal-to -4 (S is-proporti onal-to v(+alpha)) could be present with a luminosity comparable to or exceeding that of the detected thermal component, particularly in lig ht of the significant absorption that our results indicate must exist toward this distant source. A deeper X-ray exposure is required to set more meaningful limits on any plerionic X-ray emission.