Interpretation of the Center - Filled Emission from the Supernova Remnant W 44

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 ∼0.9 keV and ionization timescale of order 6000 cm years. All elemental abundances are found to be within about a factor of two 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 atoms cm – 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) (White & Long 1991, hereafter WL), 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 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge MA 02138 Department of Physics, Columbia University, 550 W 120 Street, New York, NY 10027 Department of Physics and Astronomy, Rutgers University, P.O. Box 849, Piscataway, NJ 08855-0849 Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai, 400 005 India Department of Physics, Kyoto University, Kitashirakawa, Oiwake-cho, Sakyo-ku, Kyoto 606, Japan Max-Planck-Institute für Extraterrestrische Physik, W-8046 Garching bei München, Germany