X-ray and gamma-ray observations of the fermi bubbles and NPS/Loop I structures

Jun Kataoka, Yoshiaki Sofue, Yoshiyuki Inoue, Masahiro Akita, Shinya Nakashima, Tomonori Totani

    Research output: Contribution to journalReview article

    8 Citations (Scopus)

    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 associationwith theNPS 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.

    Original languageEnglish
    Article number27
    JournalGalaxies
    Volume6
    Issue number1
    DOIs
    Publication statusPublished - 2018 Feb 26

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    Keywords

    • Fermi bubbles
    • Galactic halo
    • Loop I
    • North Polar Spur
    • X-rays

    ASJC Scopus subject areas

    • Astronomy and Astrophysics

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