Probing the All-sky Distribution of Soft X-Ray Emission Associated with the Milky Way

Abstract We present an all-sky analysis of soft X-ray emission associated with the Milky Way using HaloSat and ROSAT data, extending our previous study of five Galactic-plane regions to 330 fields across the sky. We performed uniform spectral fitting with a model including an unabsorbed Local Hot Bubble and two additional absorbed thermal components, representing emission from warm–hot and hot plasma, while accounting for solar wind charge exchange and the cosmic X-ray background. The warm–hot component is often interpreted as emission from the circumgalactic medium, whereas the origin of the hot component remains uncertain. Individual-field fits generally favored the inclusion of the hot component and the field-to-field temperature distributions had medians of kT = 0.19 and 0.74 keV, with 16th–84th percentile ranges of 0.17–0.21 and 0.61–0.90 keV, respectively. The corresponding 16th–84th percentile EM ranges were (1.0–8.2) × 10 −2 and (0.5–4.1) × 10 −3 cm −6 pc. Stacked spectra from fields within 60 ∘ ≤ l ≤ 180 ∘ supported the individual-field results. After excluding bright X-ray sources and large-scale Galactic structures, we modeled the EM distributions of 178 fields using a hybrid spatial framework comprising stellar emission and disk-like and halo-like diffuse-plasma components. The warm–hot emission was described mainly by the disk-like term, whereas the hot emission was described mainly by stellar emission with an additional halo-like term. These results support a widespread two-temperature description and suggest a composite origin for the hot emission.