AN X-RAY AND OPTICAL STUDY OF THE SUPERNOVA REMNANT W44

We report the results of a 8000 s observation of the supernova remnant W44 using the ROSAT PSPC. The image shows the same centrally peaked morphology observed by the Einstein IPC and contrasts with the shell-like radio morphology. The eastern limb shows a lack of X-ray emission within the radio shell, probably due to the interaction between the SNR and a molecular cloud. No counterpart to the pulsar 1853+01 in W44 has been detected, with L(X) < 1.3 x 10(32) ergs s-1 in the 0.2-2.4 keV band. The spectral analysis of the central part of W44, combining EXOSAT ME and Einstein SSS data, shows that the shocked plasma has not reached ionization equilibrium. The best nonequilibrium fit to PSPC, ME, and SSS spectra gives eta = 10(51) ergs cm-6, T(s) = 10(7) K with T(e) = T(i), suggesting conditions are approaching ionization equilibrium. There is no evidence of enhanced abundances of Mg, Si, S, or Fe. The variation of temperature and column density was obtained region by region using the PSPC and Einstein IPC. The temperature is largely uniform over the remnant, but strong column density variations are found to be consistent with molecular clouds in the line of sight. An evaporation model with a two-phase interstellar medium structure of clumps and interclump gas (White & Long 1991) can explain the X-ray centrally peaked morphology of W44. The clumps remaining behind a SN shock provide a reservoir of material, and evaporate to increase the density of X-ray emitting gas in the interior of a SNR. The uniform temperature distribution of W44 strongly supports the predictions of this model. In addition, mosaiced Halpha and [S II] images of W44, taken using the PFUEI camera on the Palomar 60'' telescope, reveal the first discovery of optical filaments (both Halpha and (S II]) in the northwestern and southeastern portion of the remnant, within the X-ray emitting region. The optical filaments and the X-ray image showing locally brighter emission and clumps along the filaments suggest both are produced by the interaction between the supernova shock front and regions of enhanced ambient density.