An Analysis of the North Polar Spur Using HaloSat

Abstract

We present HaloSat X-ray observations of the entirety of the bright X-ray emitting feature known as the North Polar Spur (NPS). The large field of view of HaloSat enabled coverage of the entire bright NPS in only 14 fields, which were each observed for ≈30,000 s. We find that the NPS fields are distinct in both brightness and spectral shape from the surrounding halo fields. We fit the NPS as two thermal components in ionization equilibrium with temperatures kT keV cool » 0.087 and kT keV hot » 0.28 . We note a temperature gradient in the NPS hot component with an inner arc temperature warmer than the outer arc. The emission measures we find for the cool component of the NPS is a factor of 3–5 greater than that of the hot component, which suggests that the bulk of the NPS material is in the ≈0.1 keV component. We evaluate distance estimates of 0.4 and 8.0 kpc for the NPS. Our findings suggest a preference for a distant NPS with an energy of ≈ 6 × 10⁵⁴ erg, an age of ≈ 10 Myr, and pressures consistent with a 10μG magnetic field associated with the Fermi bubbles. The electron density ≈10 × 10⁻³ cm⁻³ is consistent with estimates for the shock region surrounding a Galactic-scale event.