Interpretation of the center-filled emission from the supernova remnant W44

We report the results of spectral and morphological studies of X-ray data from the supernova remnant (SNR) W44. Spectral analysis of archival data from the Einstein Observatory, ROSAT, and Ginga, covering a total energy range from 0.3 to 8 keV, indicates that the SNR can be described well by a nonequilibrium ionization (NEI) model with temperature similar to 0.9 keV and ionization timescale of order 6000 cm(-3) yr. All elemental abundances are found to be within about a factor of 2 of their cosmic values, with iron possibly appearing to show significant depletion. No clear evidence for emission from supernova ejecta can be inferred from the observed metal abundances. The column density toward the SNR is high-around 10(22) atoms cm(-2)-as expected given the location of the remnant in the Galactic plane. In addition to the spectral analysis, we have investigated two different evolutionary scenarios to explain the centrally brightened X-ray morphology of the remnant: (1) a model involving the slow thermal evaporation of clouds engulfed by the supernova blast wave as it propagates though a clumpy interstellar medium (ISM), and (2) a hydrodynamical simulation of a blast wave propagating through a homogeneous ISM, including the effects of radiative cooling. Both models can have their respective parameters tuned to reproduce approximately the morphology of the SNR. The mean temperature of the hot plasma in W44 as determined by our NEI X-ray analysis provides the essential key to discriminating between these scenarios. Based on the size (using the well-established distance of 3 kpc) and temperature of W44, the dynamical evolution predicted by the White & Long model gives an age for the SNR of merely 6500 yr. We argue that because this age is inconsistent with the characteristic age (P/2(P) over dot similar to 20,000 yr) of PSR 1853 + 01, the radio pulsar believed to be associated with W44, this model does not provide the explanation for the center-filled morphology. We favor the radiative-phase shock model since it can reproduce both the morphology and the age of W44, assuming reasonable values for the initial explosion energy,in the range 0.7-0.9 x 10(51) ergs, and an ambient ISM density of between 3 and 4 cm(-3).