Abstract
The Fermi bubbles were possibly created by large injections of energy into the Galactic Center (GC), either by an active galactic nucleus (AGN) or by nuclear starburst more than ~10 Myr ago. However, the origin of the diffuse gamma-ray emission associated with Loop I, a radio continuum loop spanning across 100° on the sky, is still being debated. The northern-most part of Loop I, known as the North Polar Spur (NPS), is the brightest arm and is even clearly visible in the ROSAT X-ray sky map. In this paper, we present a comprehensive review on the X-ray observations of the Fermi bubbles and their possible association with the NPS and Loop I structures. Using uniform analysis of archival Suzaku and Swift data, we show that X-ray plasma with kT~0.3 keV and low metal abundance (Z~0.2 Z◉) is ubiquitous in both the bubbles and Loop I and is naturally interpreted as weakly shock-heated Galactic halo gas. However, the observed asymmetry of the X-ray-emitting gas above and below the GC has still not been resolved; it cannot be fully explained by the inclination of the axis of the Fermi bubbles to the Galactic disk normal. We argue that the NPS and Loop I may be asymmetric remnants of a large explosion that occurred before the event that created the Fermi bubbles, and that the soft gamma-ray emission from Loop I may be due to either π0 decay of accelerated protons or electron bremsstrahlung.
Highlights
A supermassive black hole, the mass of which ranges from 105 to 1010 M, can be found in the center of almost all spiral and elliptical galaxies (e.g., [1,2])
Throughout this paper, we argue that the North Polar Spur (NPS)/Loop I is a giant structure in the Galactic halo and is possibly related to the Fermi bubbles rather than being a local supernova remnants remnants (SNRs)
As for the relationship between the NPS/Loop I and the Fermi bubbles seen in X-ray skies, we showed that (1) the morphology of the NPS seen in the ROSAT all-sky map aligns well with the northeast boundary of the bubble
Summary
A supermassive black hole, the mass of which ranges from 105 to 1010 M , can be found in the center of almost all spiral and elliptical galaxies (e.g., [1,2]). The recently discovered linearly polarized giant radio lobes emanating from the GC closely correspond to the Fermi bubbles [31] With these new findings, the NPS/Loop I structure has reentered the spotlight, because the estimated energy (~1055–56 erg) and timescale (~10 Myr) needed to create such structures are consistent with those required to create the Fermi bubbles. The NPS/Loop I structure has reentered the spotlight, because the estimated energy (~1055–56 erg) and timescale (~10 Myr) needed to create such structures are consistent with those required to create the Fermi bubbles In this context, the bright X-ray enhancement associated with the NPS, clearly seen in the ROSAT all-sky map [32], may be Galactic halo gas swept up and weakly heated via shock expansion [33]. Loop I, and why only Loop I is exceptionally bright in the radio and X-ray skies? In this paper, we try to answer all these questions by summarizing the current knowledge contained in the archival X-ray data on the Fermi bubbles and the NPS/Loop I structure
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