XMM-Newton and Suzaku X-ray Shadowing Measurements of the Solar Wind
  Charge Exchange, Local Bubble, and Galactic Halo Emission
  We present results from a sample of XMM-Newton and Suzaku observations of
interstellar clouds that cast shadows in the soft X-ray background (SXRB) - the
first uniform analysis of such a sample from these missions. By fitting to the
on- and off-shadow spectra, we separated the foreground and Galactic halo
components of the SXRB. We tested different foreground models - two solar wind
charge exchange (SWCX) models and a Local Bubble (LB) model. We also examined
different abundance tables. We found that Anders & Grevesse (1989) abundances,
commonly used in previous SXRB studies, may result in overestimated foreground
brightnesses and halo temperatures. We also found that assuming a single solar
wind ionization temperature for a SWCX model can lead to unreliable results. We
compared our measurements of the foreground emission with predictions of the
SWCX emission from a smooth solar wind, finding only partial agreement. Using
available observation-specific SWCX predictions and various plausible
assumptions, we placed an upper limit on the LB's OVII intensity of ~0.8
photons/cm^2/s/sr (90% confidence). Comparing the halo results obtained with
SWCX and LB foreground models implies that, if the foreground is dominated by
SWCX and is brighter than ~1.5e-12 erg/cm^2/s/deg^2 (0.4-1.0 keV), then using
an LB foreground model may bias the halo temperature upward and the 0.5-2.0 keV
surface brightness downward by ~(0.2-0.3)e6 K and ~(1-2)e-12 erg/cm^2/s/deg^2,
respectively. Similarly, comparing results from different observatories implies
that there may be uncertainties in the halo temperature and surface brightness
of up to ~0.2e6 K and ~25%, respectively, in addition to the statistical
uncertainties. These uncertainties or biases may limit the ability of X-ray
measurements to discriminate between Galactic halo models.
