X-Ray Searches for Emission from the WHIM in the Galactic Halo and the
  Intergalactic Medium
  At least 50% of the baryons in the local universe are undetected and
predicted to be in a hot dilute phase (1E5-1E7 K) in low and moderate
overdensity environments. We searched for the predicted diffuse faint emission
through shadowing observations whereby cool foreground gas absorbs more distant
diffuse emission. Observations were obtained with Chandra and XMM-Newton. Using
the cold gas in two galaxies, NGC 891 and NGC 5907, shadows were not detected
and a newer observation of NGC 891 fails to confirm a previously reported X-ray
shadow. Our upper limits lie above model predictions. For Local Group studies,
we used a cloud in the Magellanic Stream and a compact high velocity cloud to
search for a shadow. Instead of a shadow, the X-ray emission was brighter
towards the Magellanic Stream cloud and there is a less significant brightness
enhancement toward the other cloud also. The brightness enhancement toward the
Magellanic Stream cloud is probably due to an interaction with a hot ambient
medium that surrounds the Milky Way. We suggest that this interaction drives a
shock into the cloud, heating the gas to X-ray emitting temperatures.
