An XMM-Newton Survey of the Soft X-ray Background. II. An All-Sky Catalog of Diffuse O VII and O VIII Emission Intensities

TL;DR

This paper provides an all-sky catalog of diffuse O VII and O VIII X-ray emission intensities from XMM-Newton data, analyzing their variation with solar activity, sky position, and models of solar wind charge exchange, revealing complex spatial and temporal patterns.

Contribution

It presents the first comprehensive all-sky catalog of diffuse O VII and O VIII intensities, including temporal variation analysis and comparison with SWCX models, highlighting discrepancies and complexities.

Findings

O VII and O VIII intensities vary with solar cycle and ecliptic latitude.

Observed intensities account for 40-50% of the non-AGN 3/4 keV X-ray background.

Intensity variations are inconsistent with simple SWCX or halo models.

Abstract

We present an all-sky catalog of diffuse O VII and O VIII line intensities, extracted from archival XMM observations. The O VII and O VIII intensities are typically ~2-11 and <~3 ph/cm^2/s/sr (LU), respectively, although much brighter intensities were also recorded. Our data set includes 217 directions observed multiple times by XMM. 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 <~2 LU between repeat observations, although several intensity enhancements of >10 LU were observed. We compared our measurements with SWCX models. 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 (as expected) and at low ecliptic latitudes (not as expected). The geocoronal SWCX intensity is expected to depend on the solar wind proton flux 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. The oxygen lines account for ~40-50% of the 3/4 keV X-ray background that is not due to unresolved AGN, in good agreement with a previous measurement. However, this fraction is not easily explained by a combination of SWCX emission and emission from hot plasma in the halo. The line 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. (Abridged)