X-Ray and Gamma-Ray Observations of the Fermi Bubbles and NPS/Loop I Structures

Galaxies

Yoshiaki Sofue¹  Tomonori Totani¹  Jun Kataoka²  Masahiro Akita²  Shinya Nakashima³  Yoshiyuki Inoue³ 

[1] Department of Astronomy, The University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan;Institute for Science and Engineering, Waseda University, 3-4-1, Okubo, Shinjuku, Tokyo 169-8555, Japan;RIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan;

关键词: Fermi bubbles;    North Polar Spur;    Loop I;    Galactic halo;    X-rays;   

DOI  :  10.3390/galaxies6010027

来源: DOAJ

【 摘 要 】

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.

【 授权许可】

Unknown