Letter to the Editor

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

¹ Department of Astronomy, School of Physics and Technology, Wuhan University, Wuhan 430072, PR China
e-mail: gbmou@whu.edu.cn
² Department of Physics and Institute of Theoretical Physics, The Chinese University of Hong Kong, Shatin, Hong Kong, SAR, China
³ School of Computer Science, Wuhan University, Wuhan 430072, PR China
⁴ Institute of Astronomy, The University of Tokyo, Mitaka, Tokyo 181-0015, Japan

Received: 8 November 2022
Accepted: 15 July 2023

Abstract

Aims. 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.

Methods. 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.

Results. Our main results are (1) that the energy density of the CRe is (3–6)×10⁻¹⁴ erg cm⁻³, and the spectral index below the cooling break energy of about 5 GeV is p ≃ 2.0 ± 0.1; (2) that the magnetic field strength is 3 μG; 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 the 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 10⁷ yr, which agrees with the age of the eROSITA bubbles.