The soft X-ray background with Suzaku II: Supervirial temperature bubbles?

Abstract

Observations of the hot X-ray emitting interstellar medium in the Milky Way are important for studying the stellar feedback and understanding the formation and evolution of galaxies. We present measurements of the soft X-ray background emission for 130 Suzaku observations at $75^\circ 15^\circ$. With the standard soft X-ray background model consisting of the local hot bubble and the Milky Way halo, residual structures remain at 0.7--1 keV in the spectra of some regions. Adding a collisional-ionization-equilibrium component with a temperature of $\sim$0.8 keV, much higher than the virial temperature of the Milky Way, significantly reduces the derived C-statistic for 56 out of 130 observations. The emission measure of the 0.8 keV component varies by more than an order of magnitude: Assuming the solar abundance, the median value is 3$\times 10^{-4}~ \rm{cm^{-6} pc}$ and the 16th-84th percentile range is (1--8)$\times 10^{-4}~ \rm{cm^{-6} pc}$. Regions toward the Orion-Eridanus Superbubble, a large cavity extending from the Ori OB1 association, have the highest emission measures of the 0.8 keV component. While the scatter is large, the emission measures tend to be higher toward the lower Galactic latitude. We discuss possible biases caused by the solar wind charge exchange, stars, and background groups. The 0.8 keV component is probably heated by supernovae in the Milky Way disk, possibly related to galactic fountains.