XMM-Newton Measurement of the Galactic Halo X-ray Emission using a
  Compact Shadowing Cloud
  Observations of interstellar clouds that cast shadows in the soft X-ray
background can be used to separate the background Galactic halo emission from
the local emission due to solar wind charge exchange (SWCX) and/or the Local
Bubble (LB). We present an XMM-Newton observation of a shadowing cloud,
G225.60-66.40, that is sufficiently compact that the on- and off-shadow spectra
can be extracted from a single field of view (unlike previous shadowing
observations of the halo with CCD-resolution spectrometers, which consisted of
separate on- and off-shadow pointings). We analyzed the spectra using a variety
of foreground models: one representing LB emission, and two representing SWCX
emission. We found that the resulting halo model parameters (temperature $T_h
\approx 2 \times 10^6$ K, emission measure $E_h \approx 4 \times 10^{-3}$
cm$^{-6}$ pc) were not sensitive to the foreground model used. This is likely
due to the relative faintness of the foreground emission in this observation.
However, the data do favor the existence of a foreground. The halo parameters
derived from this observation are in good agreement with those from previous
shadowing observations, and from an XMM-Newton survey of the Galactic halo
emission. This supports the conclusion that the latter results are not subject
to systematic errors, and can confidently be used to test models of the halo
emission.
