Mapping the Overionized Plasma in W44 with XMM-Newton

Mixed morphology supernova remnants combine a radio shell with a central region that emits primarily thermal X-rays. Observations show that these SNR are typically found near or in molecular clouds and, since they usually contain compact objects, arise from high-mass precursors. However, our theoretical understanding of these remnants lags far behind our observational data. There are at least four distinct models for their appearance, usually explaining observations from one or at most a few of the remnants, but there is no general solution. A recent breakthrough showed that a significant fraction of the thermal X-rays are from a non-equilibrium recombining plasma, a surprising result since SNR are expected to generate ionizing, not recombining, plasmas. Using a recently obtained 300 ksec XMM-Newton observation of the canonical mixed-morphology supernova remnant (MM SNR) W44, we localize and map the over-ionized plasma via its characteristic radiative recombination continuum and enhanced emission lines to show the origin of this wholly-unexpected component.