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

TL;DR

This paper reviews X-ray and gamma-ray observations of the Fermi bubbles and Loop I, suggesting they are remnants of past Galactic explosions with plasma heated by shocks, and discusses their asymmetric features.

Contribution

It provides a uniform analysis of Suzaku and Swift data, revealing plasma characteristics and proposing a new interpretation of the origin and asymmetry of these structures.

Findings

X-ray plasma with kT ~ 0.3 keV is common in Fermi bubbles and Loop I.

Asymmetry in X-ray emission cannot be explained by bubble inclination alone.

Loop I's gamma-ray emission may be from pi^0 decay or electron bremsstrahlung.

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 deg 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_solar) 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 pi^0 decay of accelerated protons or electron bremsstrahlung.