Galactic halo bubble magnetic fields and UHECR deflections

TL;DR

This study models the magnetic fields in the Galactic halo bubble to understand their impact on ultra high energy cosmic ray deflections, using synchrotron emission simulations and observational data to constrain the magnetic field parameters.

Contribution

It introduces a simple analytical model for the Galactic halo bubble magnetic field and assesses its effects on UHECR deflections and flux suppression.

Findings

High flux suppression (down to 2%) is possible for UHECRs passing through the halo bubble.

Deflection angles can be as large as 80 degrees or more due to the magnetic fields.

The halo bubble magnetic field can significantly influence UHECR propagation from local sources.

Abstract

We consider the synchrotron emission from electrons out in the Galactic halo bubble region where the Fermi bubble structures reside. Utilising a simple analytical expression for the non-thermal electron distribution and a toy magnetic field model, we simulate polarised synchrotron emission maps at a frequency of 30~GHz. Comparing these maps with observational data, we obtain constraints on the parameters of our toy Galactic halo bubble magnetic field model. Utilising this parameter value range for the toy magnetic field model, we determine the corresponding range of arrival directions and suppression factors of ultra high energy cosmic rays (UHECRs) from potential local source locations. We find that high levels of flux suppression (down to 2\%) and large deflection angles ($\geq 80^{\circ}$) are possible for source locations whose line-of-sight pass through the Galactic halo bubble region. We conclude that the magnetic field out in the Galactic halo bubble region can strongly dominate the level of deflection UHECRs experience whilst propagating from local sources to Earth.