Dominance of Magnetic Cataclysmic Variables in the Resolved Galactic Ridge X-ray Emission of the Limiting Window

TL;DR

This study investigates the sources of the Galactic Ridge X-ray Emission, finding that magnetic cataclysmic variables dominate the resolved hard X-ray flux, challenging previous assumptions about the contribution of active binaries.

Contribution

The paper provides an independent analysis showing magnetic cataclysmic variables are the primary contributors to the resolved hard X-ray flux in the Galactic Ridge, emphasizing their dominant role over other source types.

Findings

70-80% of the 6.5-7.1 keV flux is resolved.

Magnetic cataclysmic variables dominate the hard X-ray sources.

Active binaries contribute less than 20% to the flux.

Abstract

The diffuse appearance of the Galactic Ridge X-ray Emission (GRXE) has been puzzling since its discovery due to lack of compelling theories for sustainable hot diffuse X-ray emission in the Galactic plane. Recently (Revnivtsev et al. 2009; R09) claimed that about 90% of the 6.5-7.1 keV X-ray flux from a small section of a low extinction region at 1.4 degree south of the Galactic Center has been resolved to discrete sources with 2-10 keV L_X > 4x10^{-16} erg s cm^-2, using ultra-deep (1 Ms) Chandra ACIS-I observations. They also concluded thatcoronally active stars such as active binaries (ABs) contribute about 60% of the resolved flux. However, our recent discovery of a large population of magnetic cataclysmic variables (MCVs) in the same region suggests their significant role in the resolved hard X-ray flux. In addition, deep X-ray surveys of other several Galactic Bulge fields over the past decade have indicated that MCVs are likely the major contributor in the hard X-ray emission above 2-3 keV. To solve this mystery, we have conducted an independent indepth analysis of discrete X-ray sources in the low extinction region. The total fraction of the 6.5-7.1 keV flux we can confidently claim as resolved is 70-80%, which largely agrees with R09 but leaves some room for diffuse components. However, despite the various attempts, we consistently find that the resolved hard X-ray flux above 3 keV is dominated by relatively bright, hard X-ray sources such as MCVs, whereas the contribution from relatively faint, soft sources such as ABs is below 20%. We describe in detail our analysis procedure in order to elucidate possible origins of the discrepancy.