An XMM-Newton survey of the soft X-ray background. II. An all-sky catalog of diffuse O VII and O VIII emission intensities

Abstract

We present an all-sky catalog of diffuse O VII and O VIII line intensities, extracted from archival XMM-Newton observations. This catalog supersedes our previous catalog, which covered the sky between l = 120° and l = 240°. We attempted to reduce the contamination from near-Earth solar wind charge exchange (SWCX) emission by excluding times of high solar wind proton flux from the data. Without this filtering, we were able to extract measurements from 1868 observations. With this filtering, nearly half of the observations became unusable, and only 1003 observations yielded measurements. The O VII and O VIII intensities are typically ∼2-11 and ≲3photonscm -2s -1sr -1 (line unit, L.U.), respectively, although much brighter intensities were also recorded. Our data set includes 217 directions that have been observed multiple times by XMM-Newton. The time variation of the intensities from such directions may be used to constrain SWCX models. The O VII and O VIII intensities typically vary by ≲5 and ≲2L.U. between repeat observations, although several intensity enhancements of >10L.U. were observed. We compared our measurements with models of the heliospheric and geocoronal SWCX. The heliospheric SWCX intensity is expected to vary with ecliptic latitude and solar cycle. We found that the observed oxygen intensities generally decrease from solar maximum to solar minimum, both at high ecliptic latitudes (which is as expected) and at low ecliptic latitudes (which is not as expected). The geocoronal SWCX intensity is expected to depend on the solar wind proton flux incident on the Earth and on the sightline's path through the magnetosheath. The intensity variations seen in directions that have been observed multiple times are in poor agreement with the predictions of a geocoronal SWCX model. We found that the oxygen lines account for ∼40%-50% of the 3/4keV X-ray background that is not due to unresolved active galactic nuclei, in good agreement with a previous measurement. However, we found that this fraction is not easily explainable by a combination of SWCX emission and emission from hot plasma in the halo. We also examined the correlations between the oxygen intensities and Galactic longitude and latitude. We found that the intensities tend to increase with longitude toward the inner Galaxy, possibly due to an increase in the supernova rate in that direction or the presence of a halo of accreted material centered on the Galactic center. The variation of intensity with Galactic latitude differs in different octants of the sky, and cannot be explained by a single simple plane-parallel or constant-intensity halo model. © 2012. The American Astronomical Society. All rights reserved.