An XMM-Newton Survey of the Soft X-ray Background. III. The Galactic Halo X-ray Emission

TL;DR

This study measures the X-ray emission from the Galactic halo across 110 sight lines, revealing a patchy distribution of hot gas with uniform temperature but varying brightness, providing constraints on halo heating models.

Contribution

It offers a comprehensive survey of the Galactic halo's X-ray emission, highlighting the spatial distribution and properties of hot gas, and challenges the disk-like halo model assumptions.

Findings

Hot gas detected on ~80% of sight lines.

Temperature median of 2.22 million K with uniformity.

Emission measure varies over an order of magnitude.

Abstract

We present measurements of the Galactic halo's X-ray emission for 110 XMM-Newton sight lines, selected to minimize contamination from solar wind charge exchange emission. We detect emission from few million degree gas on ~4/5 of our sight lines. The temperature is fairly uniform (median = 2.22e6 K, interquartile range = 0.63e6 K), while the emission measure and intrinsic 0.5--2.0 keV surface brightness vary by over an order of magnitude (~(0.4-7)e-3 cm^-6 pc and ~(0.5-7)e-12 erg cm^-2 s^-1 deg^-2, respectively, with median detections of 1.9e-3 cm^-6 pc and 1.5e-12 erg cm^-2 s^-1 deg^-2, respectively). The high-latitude sky contains a patchy distribution of few million degree gas. This gas exhibits a general increase in emission measure toward the inner Galaxy in the southern Galactic hemisphere. However, there is no tendency for our observed emission measures to decrease with increasing Galactic latitude, contrary to what is expected for a disk-like halo morphology. The measured temperatures, brightnesses, and spatial distributions of the gas can be used to place constraints on models for the dominant heating sources of the halo. We provide some discussion of such heating sources, but defer comparisons between the observations and detailed models to a later paper.