  We present a joint analysis of optical emission-line and X-ray observations
of the archetypical Galactic mixed-morphology supernova remnant (MMSNR) W28
(G6.4-0.1). MMSNRs comprise a class of sources whose shell-like radio
morphology contrasts with a filled center in X-rays; the origin of these
contrasting morphologies remains uncertain. Our CTIO images reveal enhanced [S
II] emission relative to H-alpha along the northern and eastern rims of W28.
Hydroxyl (OH) masers are detected along these same rims, supporting prior
studies suggesting that W28 is interacting with molecular clouds at these
locations, as observed for several other MMSNRs. Our ROSAT HRI mosaic of W28
provides almost complete coverage of the SNR. The X-ray and radio emission is
generally anti-correlated, except for the luminous northeastern rim, which is
prominent in both bands. Our Chandra observation sampled the X-ray-luminous
central diffuse emission. Spectra extracted from the bright central peak and
from nearby annular regions are best fit with two over-ionized recombining
plasma models. We also find that while the X-ray emission from the central peak
is dominated by swept-up material, that from the surrounding regions shows
evidence for oxygen-rich ejecta, suggesting that W28 was produced by a massive
progenitor. We also analyze the X-ray properties of two X-ray sources (CXOU
J175857.55-233400.3 and 3XMM J180058.5-232735) projected into the interior of
W28 and conclude that neither is a neutron star associated with the SNR. The
former is likely to be a foreground cataclysmic variable or a quiescent
low-mass X-ray-binary while the latter is likely to be a coronally-active main
sequence star.
