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 m orphology observed by the Einstein IPC and contrasts with the shell-li ke radio morphology. The eastern limb shows a lack of X-ray emission w ithin the radio shell, probably due to the interaction between the SNR and a molecular cloud. No counterpart to the pulsar 1853+01 in W44 ha s 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 EXOS AT ME and Einstein SSS data, shows that the shocked plasma has not rea ched 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 reg ion using the PSPC and Einstein IPC. The temperature is largely unifor m over the remnant, but strong column density variations are found to be consistent with molecular clouds in the line of sight. An evaporati on model with a two-phase interstellar medium structure of clumps and interclump gas (White & Long 1991) can explain the X-ray centrally pea ked morphology of W44. The clumps remaining behind a SN shock provide a reservoir of material, and evaporate to increase the density of X-ra y emitting gas in the interior of a SNR. The uniform temperature distr ibution of W44 strongly supports the predictions of this model. In add ition, mosaiced Halpha and [S II] images of W44, taken using the PFUEI camera on the Palomar 60'' telescope, reveal the first discovery of o ptical filaments (both Halpha and (S II]) in the northwestern and sout heastern portion of the remnant, within the X-ray emitting region. The optical filaments and the X-ray image showing locally brighter emissi on and clumps along the filaments suggest both are produced by the int eraction between the supernova shock front and regions of enhanced amb ient density.