SUZAKUOBSERVATIONS OF THE DIFFUSE X-RAY EMISSION ACROSS THE FERMI BUBBLES' EDGES

Abstract

We present Suzaku X-ray observations along two edge regions of the Fermi Bubbles, with eight 20 ks pointings across the northern part of the North Polar Spur (NPS) surrounding the north bubble and six across the southernmost edge of the south bubble. After removing compact X-ray features, diffuse X-ray emission is clearly detected and is well reproduced by a three-component spectral model consisting of unabsorbed thermal emission (temperature kT 0.1 keV) from the Local Bubble, 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 northeast bubble, with no accompanying temperature change. However, such a jump in the EM is not clearly seen in the south bubble data. While it is unclear whether the NPS originates from a nearby supernova remnant or is related to previous activity within or around the Galactic center, our Suzaku observations provide evidence that suggests the latter scenario. In the latter framework, the presence of a large amount of neutral matter absorbing the X-ray emission as well as the existence of the kT 0.3 keV gas can be naturally interpreted as a weak shock driven by the bubbles' expansion in the surrounding medium, with velocity v exp ∼ 300 km s −1 (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-thermal X-ray emission component associated with the bubbles and demonstrate that, in agreement with the aforementioned findings, the non-thermal pressure and energy estimated from a one-zone leptonic model of its broadband spectrum, are in rough equilibrium with that of the surrounding thermal plasma.

A possible origin of gamma rays from the Fermi Bubbles

“…The ROSAT X-rays map at 1.5 keV shows limb brightening [4], which agrees also with the recent SUZAKU measurements at 0.3 keV [13]. The Xrays emission is most likely thermal bremsstrahlung, although a synchrotron component from very high-energy electrons cannot be neglected.…”

Multiwavelength features of Fermi bubbles as signatures of a Galactic wind

“…enhancement of temperature and density across the FB edge as observed by Kataoka et al (2013). Though we assume that the injection region is spherical symmetric, a departure from this assumption does not change the qualitative/quantitative picture much.…”

“…The speed required to reach ∼ 10 kpc in 1 Myr is ∼ 10 4 km s −1 , achievable by relativistic jets slowed down by the hot circumgalactic medium (CGM). The power required for inflating young FBs is much higher and the outer shock is much stronger (Guo & Mathews 2012;Zubovas & Nayakshin 2012), and X-ray emissivity and temperature much higher than what is observed (Kataoka et al 2013). The SNe-driven models of FBs consider them to be long lived ( 10 Myr), and thus the injected power is smaller.…”

Studying the Interstellar Medium and the Inner Region of NPS/Loop 1 With Shadow Observations Toward Mbm36

“…The N excess is possibly due, instead, either to local enrichment that occurred at a previous stage or to some non-local source associated with the hypershell model. Kataoka et al (2013) performed a large set of Suzaku observations crossing the northern and southern edges of the Fermi Bubbles, therefore probing several fields within the NPS. They found strong X-ray absorption, which was hardly explained with a local origin, but rather suggested that the NPS emission comes from gas associated with strong past activity of the Galactic Center.…”