Comparing Suzaku and XMM-Newton Observations of the Soft X-ray Background: Evidence for Solar Wind Charge Exchange Emission

Abstract

We present an analysis of a pair of Suzaku spectra of the soft X-ray background (SXRB), obtained from pointings on and off a nearby shadowing filament in the southern Galactic hemisphere. Because of the different Galactic column densities in the two pointing directions, the observed emission from the Galactic halo has a different shape in the two spectra. We make use of this difference when modeling the spectra to separate the absorbed halo emission from the unabsorbed foreground emission from the Local Bubble (LB). The temperatures and emission measures we obtain are significantly different from those determined from an earlier analysis of XMM-Newton spectra from the same pointing directions. We attribute this difference to the presence of previously unrecognized solar wind charge exchange (SWCX) contamination in the XMM-Newton spectra, possibly due to a localized enhancement in the solar wind moving across the line of sight. Contemporaneous solar wind data from ACE show nothing unusual during the course of the XMM-Newton observations. Our results therefore suggest that simply examining contemporaneous solar wind data might be inadequate for determining if a spectrum of the SXRB is contaminated by SWCX emission. If our Suzaku spectra are not badly contaminated by SWCX emission, our best-fitting LB model gives a temperature of log T = 5.98 +0.03/-0.04 and a pressure of p/k = 13,100-16,100 cm^-3 K. These values are lower than those obtained from other recent observations of the LB, suggesting the LB may not be isothermal and may not be in pressure equilibrium. Our halo modeling, meanwhile, suggests that neon may be enhanced relative to oxygen and iron, possibly because oxygen and iron are partly in dust.