Suzaku observations of the diffuse x-ray emission across the fermi bubbles' edges

We present SuzakuX-ray observations along two edge regions of the Fermi Bubbl es, with eight≃ 20 ksec pointings across the northern part of the North Polar Spur (N PS) surrounding the north bubble and six across the southernmost edge of the south bubble. After removing co mpact X-ray features, diffuse X-ray emission is clearly detected and is well reproduced by a three-componen t sp ctral model consisting of unabsorbed thermal emission (temperature kT ≃ 0.1 keV) from the Local Bubble (LB), absorbed kT ≃ 0.3 keV thermal emission related to the NPS and/or Galactic Halo (GH), and a power-law component at a level consistent with the cosmic X-ray background. The emission measure (EM) of the 0. 3 keV plasma decreases by ≃ 50% toward the inner regions of the north-east bubble, with no accompan ying temperature change. However, such a jump in the EM is not clearly seen in the south bubble data. While it is unclear if the NPS originates from a nearby supernova remnant or is related to previous activity wi hin/around the Galactic Center, our Suzaku observations provide evidence suggestive of the latter sce nario. In the latter framework, the presence of a large amount of neutral matter absorbing the X-ray emission as wel l as the existence of the kT ≃ 0.3 keV gas can be naturally interpreted as a weak shock driven by the bubble s’ expansion in the surrounding medium, with velocityvexp ∼ 300 km s (corresponding to shock Mach number M ≃ 1.5), compressing the GH gas to form the NPS feature. We also derived an upper limit for any non-th ermal X-ray emission component associated with the bubbles and demonstrate, that in agreement with the findings above, the non-thermal pressure and energy estimated from a one-zone leptonic model of its broad -band spectrum, are in rough equilibrium with that of the surrounding thermal plasma. Subject headings: acceleration of particles — cosmic rays — Galaxy: center — Ga laxy: halo — X-rays: ISM