  Existing stellar X-ray surveys suggest major problems in our understanding of
the evolution of stellar magnetic activity in solar and late-type stars,
reaching conflicting conclusions over the rate of decay of X-ray activity and
the spectral types responsible. We are confronting these discrepancies with a
new model of the expected stellar X-ray luminosity distribution, combining a
Galactic population synthesis model with current theories for rotational spin-
down and the rotation - activity relation for the stellar magnetic dynamo. Here
we test our model using new observations of the stellar content of the Chandra
COSMOS survey, for which 60 new stellar X-ray sources are identified from the
thin disk and Galactic halo populations. Our model is in approximate agreement
with the observed X-ray luminosity distribution and the distribution of
spectral types responsible. However, slight differences in the form of the
X-ray luminosity distribution exist that may hint at problems in our
understanding of stellar X-ray emission.
