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 and Grevesse 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 O vii intensity of ∼0.8 $photons$ ${cm}^{-2}$ $s^{-1}$ ${sr}^{-1}$ (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 ∼$1.5\times {10}^{-12}erg{cm}^{-2}{s}^{-1}{deg}^{-2}$ (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 ∼$(0.2-0.3)\times {10}^{6}K$ and ∼$(1-2)\times {10}^{-12}erg{cm}^{-2}{s}^{-1}{deg}^{-2}$, respectively. Similarly, comparing results from different observatories implies that there may be uncertainties in the halo temperature and surface brightness of up to ∼$0.2\times {10}^{6}K$ 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.