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We report preliminary results of a 24 ks ROSAT PSPC observation of the supernova remnant 3C391 (G31.9+0.0). The PSPC image reveals centrally concentrated X-ray emission inside the radio shell, while Einstein IPC observation (Wang \& Seward 1984, ApJ) was not able to determine whether the SNR is shell-like or center-filled. The northwestern part has a strong radio shell and weak central X-ray emission. In contrast, the southeastern part has strong central X-ray emission and weak radio emission. The X-ray image is largely circular and slightly extended outside the radio shell in southwestern direction.
The PSPC spectra alone give similarly good fits using one-temperature thermal model with N$_H$ = 1.9 $\times 10^{22}$ cm$^{-2}$ and kT = 1.1 keV and with N$_H$ = 1.5 $\times 10^{22}$ cm$^{-2}$ and kT = 2.30 keV. This bimodality in PSPC fit contours has been noted elsewhere (Bregman \& Pildis 1992, ApJL; Rho et al. 1994, ApJ). However, the fit combining PSPC and IPC yields a best fit of N$_H$ = 1.8 $^{+1.0}_{-0.2} \times 10^{22}$ cm$^{-2}$ and kT = 1.13$^{+0.7}_{-0.6}$ keV, which rules out the higher temperature fit.
There are large spectral variations between the northwestern and southeastern parts of the remnant: either the northwestern part has higher N$_H$ or the southeastern part has lower temperature. But the PSPC data can not determine one or the other. When assuming constant temperature across the remnant, the difference in N$_H$ is large enough to state that the low X-ray surface brightness in the northwestern part is due to the absorption. When assuming constant N$_H$, the northwestern emission shows higher temperature and pressure than the southeastern part. The interpretation of the southeastern radio and X-ray emission representing a ``breakout'' from the northwestern main shell, will be discussed.
