An XMM-Newton Survey of the Soft X-ray Background. I. The O VII and O VIII Lines Between l=120 and l=240

TL;DR

This survey of the soft X-ray background using XMM-Newton data measures O VII and O VIII emissions across multiple directions, analyzes variability, and assesses the Galactic halo's properties while accounting for solar wind charge exchange effects.

Contribution

First comprehensive survey of SXRB O VII and O VIII lines over a wide sky region using archival XMM-Newton data, including variability analysis and halo characterization.

Findings

O VII and O VIII intensities typically 0.5-10 and 0-5 L.U.

Detected SWCX enhancements up to 25 L.U. in some observations.

Galactic halo temperature estimated at 2.0-2.5 MK.

Abstract

We present measurements of the soft X-ray background (SXRB) O VII and O VIII intensity between $l = 120\degr$ and $l = 240\degr$, the first results of a survey of the SXRB using archival XMM-Newton observations. We do not restrict ourselves to blank-sky observations, but instead use as many observations as possible, removing bright or extended sources by hand if necessary. The oxygen intensities are typically ~0.5-10 photons/cm^2/s/sr (line units, L.U.) for O VII and ~0-5 L.U. for O VIII. Our dataset includes 69 directions with multiple observations, whose oxygen intensity variations can be used to constrain SWCX models. One observation exhibits an O VII enhancement of ~25 L.U. over 2 other observations of the same direction, although most SWCX enhancements are $\la 4$ L.U. for O VII and $\la 2$ L.U. for O VIII. We find no clear tendency for the O VII centroid to shift toward the forbidden line energy in observations with bright SWCX enhancements. There is also no universal association between enhanced SWCX emission and increased solar wind flux or the closeness of the sightline to the sub-solar region of the magnetosheath. After removing observations likely to be contaminated by heliospheric SWCX emission, we use our results to examine the Galactic halo. There is some scatter in the halo intensity about the predictions of a simple plane-parallel model, indicating a patchiness to the halo emission. The O VII/O VIII intensity ratio implies a halo temperature of ~2.0-2.5 MK, in good agreement with previous studies. (Abridged)