Dominance of magnetic cataclysmic variables in the resolved Galactic ridge X-ray emission of the limiting window

Abstract

The diffuse appearance of the Galactic ridge X-ray emission has been puzzling since its discovery due to the lack of compelling theories for sustainable hot diffuse X-ray emission in the Galactic plane. Recently, Revnivtsev et al. claimed that ∼90 per cent of the 6.5-7.1 keV X-ray flux from a small section of a low-extinction region at 1. • 4 south of the Galactic Centre has been resolved to discrete sources with L X, 2−10 keV 4 × 10 −16 erg s −1 cm −2 , using ultradeep (1 Ms) observations made by the Chandra X-ray Observatory. They also concluded that coronally active stars such as active binaries (ABs) contribute ∼60 per cent 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 in-depth 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 per cent, which largely agrees with Revnivtsev et al., 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 per cent. We describe in detail our analysis procedure in order to elucidate possible origins of the discrepancy.

Scale heights and equivalent widths of the iron K-shell lines in the Galactic diffuse X-ray emission

“…Thus the CBF can be regarded as almost a pure GBXE region. In the CBF, Revnivtsev et al (2009) andHong (2012) reported that ∼70-80% flux (6.5-7.1 keV band) in the central region was resolved into point sources. However it is very surprising that the profiles of the 6.5-7.1 keV flux as a function of 2-10 keV luminosity by Hong (2012) is ∼2 times larger than that of Revnivtsev et al (2009) in the most important luminosity range of 10 31 -10 32 erg s −1 .…”

“…Furtheremore, about 20% of the faintest point sources are unique in each point source lists. Hong (2012) argued following his luminosity function that the major component is mCVs in contrast to major point source origin scenarios. Morihana et al (2013), on the other hand, reported that ∼50% (2-8 keV band) of the full CBF field was resolved into point sources.…”

NuSTAR HARD X-RAY SURVEY OF THE GALACTIC CENTER REGION. II. X-RAY POINT SOURCES

“…These techniques rely on the correlation between the wavelet kernels and the local count distribution of X-ray images. As researchers lower the detection thresholds of these techniques in hopes of finding fainter sources, it becomes essential to independently validate faint sources detected near the thresholds (e.g., M09; Hong 2012). An independent validation also alleviates a somewhat unavoidable subjectivity inherent in threshold setting (Townsley et al 2011).…”

“…Active binaries (ABs) have been suggested to make a significant contribution (Revnivtsev et al 2009(Revnivtsev et al , 2011, although this has been disputed (Hong 2012). Perez et al (2015) recently discovered apparent diffuse hard X-ray emission (20-40 keV) in the central 2′ region around SgrA * using NuSTAR observations.…”

Deep Chandra Survey of the Small Magellanic Cloud. II. Timing Analysis of X-Ray Pulsars

“…For the analysis of both the individual observations and the stacked data we employed the latest version of the X-ray analysis pipeline developed for the Chandra Multi-wavelength Plane survey (ChaMPlane; Grindlay et al 2005;Hong 2012). The ChaMPlane pipeline is independent of the analysis pipeline used in Antoniou et al (2017, in preparation), but the former was chosen for timing analysis because of its proven maturity of periodicity search and subsequent analysis tools (e.g., Hong et al 2009;Laycock et al 2010;Hong et al 2012).…”