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Presence of the apparently extended hard (2-10 keV) X-ray emission along theGalactic plane has been known since the early 1980s. With a deep X-ray exposureusing the Chandra X-ray Observatory of a slightly off-plane region in theGalactic bulge, most of the extended emission was resolved into faint discreteX-ray sources in the Fe K band (Revnivtsev et al.,2009). The major constituentsof these sources have long been considered to be X-ray active stars andmagnetic cataclysmic variables (CVs). However, recent works including our NIRimaging and spectroscopic studies (Morihana et al.,2013, 2016) argue that otherpopulations should be more dominant. To investigate this further, we conducteda much deeper NIR imaging observation at the center of the Chandra's exposurefield. We have used the MOIRCS on the Subaru telescope, reaching the limitingmagnitude of ~18 mag in the J, H, and Ks bands in this crowded region, andidentified ~50 % of the X-ray sources with NIR candidate counterparts. Weclassified the X-ray sources into three groups (A, B, and C) based on theirpositions in the X-ray color-color diagram and characterized them based on theX-ray and NIR features. We argue that the major populations of the Group A andC sources are, respectively, CVs (binaries containing magnetic or non-magneticwhite dwarfs with high accretion rates) and X-ray active stars. The majorpopulation of the Group B sources is presumably WD binaries with low massaccretion rates. The Fe K equivalent width in the composite X-ray spectrum ofthe Group B sources is the largest among the three and comparable to that ofthe Galactic bulge X-ray emission. This leads us to speculate that there arenumerous WD binaries with low mass accretion rates, which are not recognized asCVs, but are the major contributor of the apparently extended X-ray emission.

Deep Near-infrared Imaging Observation of the Faint X-ray Point Sources Constituting the Galactic Bulge X-ray Emission