Evolved SNRs

The majority of X-ray studies of Galactic remnants have concentrated on the early stages of evolution when the remnants are bright and shell-like with angular extent that can be covered in a modest number of observations. Evolved remnants are considerably larger than their younger counterparts; objects that fit in the field of view of most imaging instruments are distant, and therefore subject to absorption by intervening galactic material. If most of the emission is very soft, we get a distorted impression of them. Nearby evolved remnants, while not absorbed, subtend large solid angle and have low surface brightness, making observations challenging. ROSAT, with its soft response plus the unlimited field of view provided by the All-Sky Survey and the 2-degree field of view of the PSPC when used in pointed mode, was the first observatory to facilitate systematic study of cooler, evolved remnants. In contrast to the study of younger remnants, which has been revolutionized by XMM-Newton and Chandra, only limited advances have been made since ROSAT.

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