Suzaku Observations of the Local and Distant Hot ISM

Suzaku observed the molecular cloud MBM 12 and a blank field less than $3^{\circ}$ away to separate the local and distant components of the diffuse soft X-ray background. Towards MBM 12, a local ($D \lesssim 275 \,\mathrm{pc}$) O VII emission line was clearly detected with an intensity of $3.5 \;\mathrm{photons} \;\mathrm{cm}^{-2} \,\mathrm{s}^{-1} \,\mathrm{sr}^{-1}$ (or line units, LU), and the O VIII flux was ${<0.34 \;\mathrm{LU}}$. The origin of this O VII emission could be hot gas in the Local Hot Bubble (LHB), charge exchange within the heliosphere with oxygen ions from the solar wind (SWCX), or both. If entirely from the LHB, the emission could be explained by a region with emission measure of $0.0075 \,\mathrm{cm}^{-6} \,\mathrm{pc}$ and a temperature of $1.2 \times 10^6 \,\mathrm{K}$. However, this temperature and emission measure implies $1/4 \;\mathrm{keV}$ emission in excess of observations. There is no evidence in the X-ray light curve or solar wind data for a significant contribution from geocoronal SWCX, although interplanetary SWCX is still possible. In any case, the observed O VII flux represents an upper limit to both the LHB emission and interplanetary SWCX in this direction. The blank field was observed immediately afterwards. The net off-cloud O VII and O VIII intensities were (respectively) $2.34 \pm 0.33$ and $0.77 \pm 0.16 \;\mathrm{LU}$, after subtracting the on-cloud foreground emission. If this more distant O VII and O VIII emission is from a thermal plasma in collisional equilibrium beyond the Galactic disk, we infer it has a temperature of $(2.1 \pm 0.1) \times 10^6 \,\mathrm{K}$ with an emission measure of $(4 \pm 0.6) \times 10^{-3} \,\mathrm{cm}^{-6} \,\mathrm{pc}$.