  We examine X-rays from radiatively cooled shocks, focusing on how their
thin-shell instability reduces X-ray emission. For 2D simulations of collision
between equal expanding winds, we carry out a parameter study of such
instability as a function of the ratio of radiative vs. adiabatic-expansion
cooling lengths. In the adiabatic regime, the extended cooling layer suppresses
instability, leading to planar shock compression with X-ray luminosity that
follows closely the expected (Lx ~ M^2) quadratic scaling with mass-loss rate M
. In the strongly radiative limit, the X-ray emission now follows an expected
linear scaling with mass loss (Lx ~ M), but the instability deforms the shock
compression into extended shear layers with oblique shocks along fingers of
cooled, dense material. The spatial dispersion of shock thermalization limits
strong X-ray emission to the tips and troughs of the fingers, and so reduces
the X-ray emission (here by about a factor 1/50) below what is expected from
analytic radiative-shock models without unstable structure. Between these two
limits, X-ray emission can switch between a high-state associated with extended
shock compression, and a low-state characterized by extensive shear. Further
study is needed to clarify the origin of this "shear mixing reduction factor"
in X-ray emission, and its dependence on parameters like the shock Mach number.
