Cosmic-ray electrons and the magnetic field of the North Polar Spur

TL;DR

This study constrains the cosmic-ray electron and magnetic field properties of the North Polar Spur using multiwavelength data, revealing insights into shock acceleration and the relic's age in the Galactic halo.

Contribution

It provides the first precise independent measurements of cosmic-ray electron energy density, spectral index, magnetic field strength, and shock acceleration efficiency in the NPS.

Findings

CRe energy density is (3-6)×10^{-14} erg/cm^3

Magnetic field strength is 3 μG

Shock acceleration efficiency is 1-2%

Abstract

The recent confirmation of the bipolarity of the eROSITA bubbles suggests that the well-known North Polar Spur (NPS)/Loop I probably is a 10 kpc sized relic in the Galactic halo and not a small local structure near the Sun. By virtue of multiwavelength observations of the NPS, unprecedentedly precise parameter constraints on the cosmic-ray electrons (CRes) and magnetic field in the post-shock halo medium can be provided. The parameters of the CRes and the magnetic field can be derived independently by modeling the gamma-ray and the radio data of the NPS via inverse Compton scattering and synchrotron emission, respectively. Our main results are (1) that the energy density of the CRe is (3-6)$\times 10^{-14}$ erg cm$^{-3}$, and the spectral index is $p\simeq 2.0\pm 0.1, $ below the cooling break energy of about 5 GeV; (2) that the magnetic field strength is 3 $\mu$G; and (3) that the shock acceleration efficiency of the CRe is (1-2)%. Given the Mach number of 1.5, the high acceleration efficiency and flat spectrum of the CRe suggest that preexisting relativistic electrons may be reaccelerated in the NPS. Alternatively, these CRes could be accelerated by an evolving shock in the early epoch when its Mach number is high, and efficiently diffuse throughout the post-shock halo gas. In addition, the cooling break energy suggests that the cooling timescale is $10^7$ yr, which agrees with the age of the eROSITA bubbles.