XMM-Newton observations of the Galactic Centre Region - I: The distribution of low-luminosity X-ray sources

TL;DR

This study uses XMM-Newton data to analyze the distribution and origin of low-luminosity X-ray sources in the Galactic Centre, revealing that unresolved sources, likely magnetic CVs, dominate the hard X-ray emission.

Contribution

It provides a detailed decomposition of the GC X-ray emission into three components and constrains the population of magnetic CVs as primary sources.

Findings

Unresolved sources account for most of the 6.7 and 6.9 keV line emission.

X-ray surface brightness decreases faster than mass models predict.

Magnetic CVs are likely the main contributors to the unresolved hard X-ray emission.

Abstract

We exploit XMM-Newton archival data in a study of the extended X-ray emission emanating from the Galactic Centre (GC) region. EPIC-pn and EPIC-MOS observations, with a total exposure approaching 0.5 and 1 Ms respectively, were used to create mosaiced images of a 100 pc x 100 pc region centred on Sgr A* in four bands covering the 2-10 keV energy range. We have also constructed a set of narrow-band images corresponding to the neutral iron fluorescence line at 6.4 keV and the K-shell lines at 6.7 keV and 6.9 keV from helium-like and hydrogenic iron. We use a combination of spatial and spectral information to decompose the GC emission into three distinct components. These comprise: the emission from hard X-ray emitting unresolved point sources; the reflected continuum and fluorescent line emission from dense molecular material; and the soft diffuse emission from thermal plasma in the temperature range, kT ~ 0.8-1.5 keV. We show that the unresolved-source component accounts for the bulk of the 6.7-keV and 6.9-keV line emission. We fit the observed X-ray surface brightness distribution with an empirical 2-d model, which we then compare with a 3-d mass-model prediction for the old stellar population in the GC. The X-ray surface brightness falls-off more rapidly with angular offset from Sgr A* than predicted. One interpretation is that the 2-10 keV X-ray emissivity increases from 5 x 10^27 erg s^-1 Msun^-1 at 20' up to almost twice this value at 2'. Alternatively, some refinement of the mass model may be required. The unresolved hard X-ray emitting source population, on the basis of spectral comparisons, is most likely dominated by magnetic CVs. We use the X-ray observations to set constraints on the number density of such sources. Our analysis does not support the conjecture that a significant fraction of the hard X-ray emission from the GC originates in very-hot diffuse thermal plasma.