XMM-Newton Measurement of the Galactic Halo X-ray Emission using a Compact Shadowing Cloud

TL;DR

This study uses XMM-Newton observations of a compact interstellar cloud to measure the Galactic halo's X-ray emission, effectively separating it from local foreground emissions and confirming previous findings.

Contribution

It demonstrates a novel single-field shadowing method with a compact cloud, reducing systematic uncertainties in measuring the Galactic halo X-ray emission.

Findings

Halo temperature ~2 million K

Halo emission measure ~4 x 10^-3 cm^-6 pc

Results consistent with previous shadowing studies

Abstract

Observations of interstellar clouds that cast shadows in the soft X-ray background can be used to separate the background Galactic halo emission from the local emission due to solar wind charge exchange (SWCX) and/or the Local Bubble (LB). We present an XMM-Newton observation of a shadowing cloud, G225.60-66.40, that is sufficiently compact that the on- and off-shadow spectra can be extracted from a single field of view (unlike previous shadowing observations of the halo with CCD-resolution spectrometers, which consisted of separate on- and off-shadow pointings). We analyzed the spectra using a variety of foreground models: one representing LB emission, and two representing SWCX emission. We found that the resulting halo model parameters (temperature $T_h \approx 2 \times 10^6$ K, emission measure $E_h \approx 4 \times 10^{-3}$ cm$^{-6}$ pc) were not sensitive to the foreground model used. This is likely due to the relative faintness of the foreground emission in this observation. However, the data do favor the existence of a foreground. The halo parameters derived from this observation are in good agreement with those from previous shadowing observations, and from an XMM-Newton survey of the Galactic halo emission. This supports the conclusion that the latter results are not subject to systematic errors, and can confidently be used to test models of the halo emission.