ROSAT observations, and to a lesser extent observations using ASCA and subsequent missions, identified and placed into the evolutionary context a new class of SNRs. Most SNRs have similar radio and X-ray morphologies: either shell-like, which tend to be dominated by shock-heated, thermal emission; centrally filled, which tend to be synchrotron emission dominated pulsar wind nebulae; or composite, with a shell and a pulsar wind nebula. Einstein observations revealed that some SNRs have distinct X-ray and radio morphologies. Rho and Petre  carried out a systematic study of all X-ray emitting SNRs and found that a substantial fraction share this observational property. Using the ROSAT PSPC and non-X-ray data, they cataloged a number of additional properties. The remnants have either flat temperature profiles or temperatures gradually increasing towards the center. There is little if any evidence for a limb brightened shell in most. They tend to be found in complex regions of higher than normal ambient density, near molecular and HI clouds. Many are surrounded by complete or partial H I shells. PSPC spectra, and higher resolution ASCA spectra, suggested that the gas in these remnants is in or near collisional ionization equilibrium, indicating that the remnants are dynamically old. These remnants were termed mixed morphology (MM) remnants. Prominent examples include W44, 3C 391, MSH11-61A, W28, IC 443, and 3C 400.2.
Two models of these objects have gained acceptance. The first entails expansion of a SNR in a cloudy medium. Clouds overtaken by the passing shock front slowly evaporate in the remnant interior, and thus provide a reservoir of additional mass. The relative smoothness of the surface brightness profile sets constraints on the cloud size, while the overall brightness constrains their mass and filling factor. The second model assumes that the remnant has entered the radiative or shell forming stage. Thus the outer shock is moving too slowly to produce appreciable new X-ray emitting gas. Thermal conduction produces the smooth interior temperature and density profiles. This model accounts for the H I shell found around many MM remnants. The ROSAT data provided no strong discriminator between the two models.
Subsequent observations of these objects have helped clarify their nature. A ROSAT catalog of Magellanic Cloud SNRs revealed three MM candidates, N206, N120, and 0454-672 . Detailed Chandra observations confirm the classification of N206 (and find an embedded bow shock nebula associated with a central point source) . These are among the largest LMC remnants, and thus are likely to be among the most evolved. Spectral studies of various MM remnants using ASCA and Chandra show that they all are near coronal ionization equilibrium, and generally show evidence for a modest temperature gradient [78,147]. The Chandra spatially resolved spectral study of 3C 391 shows variation of temperature on modest spatial scales that does not seem correlated with ISM structures . Two remnants, IC 443 and W49B (considered by some to be a MM SNR), show evidence for overioniza-tion - the hydrogen-like to helium-like line ratio in abundant metals is too large to be consistent with collisional ionization equilibrium [78,79]. This can happen in the interior of an SNR only if the gas cools faster than it recombines and requires thermal conduction.
An important new insight arose from the PSPC study of the large angular diameter remnant G65.2+5.7 . This remnant is nearby and relatively unabsorbed. A mosaic produced from a collection of PSPC pointings showed that its 0.1-0.3 keV
morphology is shell-like but that at higher energy the shell disappears and the remnant takes on morphology similar to other MM remnants. Temperature measurements show a gradual increase toward the center. This remnant provides the key to understanding the MM remnants: X-ray emission from newly shocked gas is too soft to be observed from most through the absorbing interstellar medium, and thus all that is observed is the warm central emission, the temperature profile of which is smoothed by thermal conduction.
While thermal conduction drives the evolution and appearance of MM remnants, it is not the only force at work. The complex temperature and density structure of 3C 391 suggests that cloud evaporation can play a role in MM remnants . W28 also has a complex thermal structure, with a hot component not seen in other mixed morphology remnants, a strong temperature gradient across the remnant, and the presence of numerous X-ray knots and clumps . While some of these features are environmentally induced (e.g., the temperature gradient arises from a density gradient in the ambient medium), others, such as the presence of the clumps, suggest departures from an idealized conduction model.
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