AN XMM-NEWTON SURVEY OF THE SOFT X-RAY BACKGROUND. I. THE O VII AND O VIII LINES BETWEEN l = 120° AND l = 240°

Abstract

We present measurements of the soft X-ray background (SXRB) O vii and O viii intensity between l = 120° and l = 240°, 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. In an attempt to minimize contamination from near-Earth solar wind charge exchange (SWCX) emission, we remove times of high solar wind proton flux from the data. Without this filtering we are able to extract measurements from 586 XMM-Newton observations. With this filtering, ∼1/2 of the observations are rendered unusable, and we are able to extract measurements from 303 observations. The oxygen intensities are typically ∼0.5–10 photons cm −2 s −1 sr −1 (line units, L.U.) for O vii and ∼0–5 L.U. for O viii . The proton flux filtering does not systematically reduce the oxygen intensities measured from a given observation. However, the filtering does preferentially remove the observations with higher oxygen intensities. Our data set 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 two other observations of the same direction, although most SWCX enhancements are ≲4 L.U. for O vii and ≲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 × 10 6 K, in good agreement with previous studies.