Signatures of Recent Cosmic-Ray Acceleration in the High-Latitude $\gamma$-Ray Sky

TL;DR

This study uses a decade of Fermi LAT data to identify gamma-ray signatures of recent cosmic-ray acceleration, revealing new emissions from radio loops and features linked to cosmic-ray sources and structures like the Fermi Bubbles.

Contribution

First detection of gamma-ray emission from Radio Loop IV and evidence of cosmic-ray related features in high-latitude gamma-ray sky using advanced Bayesian analysis.

Findings

Radio Loop IV identified as gamma-ray emitter

Significant gamma-ray emission from Loop I detected

Asymmetric features suggest connection to Fermi Bubbles

Abstract

Cosmic-ray (CR) sources temporarily enhance the relativistic particle density in their vicinity over the background distribution accumulated from the Galaxy-wide past injection activity and propagation. If individual sources are close enough to the solar system, their localised enhancements may present as features in the measured spectra of the CRs and in the associated secondary electromagnetic emissions. Large scale loop like structures visible in the radio sky are possible signatures of such nearby CR sources. If so, these loops may also have counterparts in the high-latitude $\gamma$-ray sky. Using $\sim$10 years of data from the Fermi Large Area Telescope, applying Bayesian analysis including Gaussian Processes, we search for extended enhanced emission associated with putative nearby CR sources in the energy range from 1 GeV to 1 TeV for the sky region $|b| > 30^\circ$. We carefully control the systematic uncertainty due to imperfect knowledge of the interstellar gas distribution. Radio Loop~IV is identified for the first time as a $\gamma$-ray emitter and we also find significant emission from Loop~I. Strong evidence is found for asymmetric features about the Galactic $l = 0^\circ$ meridian that may be associated with parts of the so-called "Fermi Bubbles", and some evidence is also found for $\gamma$-ray emission from other radio loops. Implications for the CRs producing the features and possible locations of the sources of the emissions are discussed.