Near-infrared spectroscopy of faint discrete X-ray point sources constituting the Galactic ridge X-ray emission

The Galactic Ridge X-ray Emission (GRXE) is an apparently extended X-ray emission along the Galactic plane. The X-ray spectrum is characterized by a hard continuum with a strong Fe K emission feature in the 6–7 keV band. A substantial fraction (∼80%) of the GRXE in the Fe band was resolved into point sources by deep Chandra imaging observations; thus GRXE is mostly composed of dim Galactic X-ray point sources, at least in this energy band. To investigate the populations of these dim X-ray point sources, we carried out near-infrared (NIR) follow-up spectroscopic observations in two deep Chandra fields located in the Galactic plane at (l, b) = (0 $_{.}^{\circ}$ 1, −1 $_{.}^{\circ}$ 4) and (28 $_{.}^{\circ}$ 5, 0 $_{.}^{\circ}$ 0) using NTT/SofI and Subaru/MOIRCS. We obtained well-exposed NIR spectra from 65 objects and found that there are three main classes of Galactic sources based on the X-ray color and NIR spectral features: those having (A) hard X-ray spectra and NIR emission features such as H i (Brγ), He i, and He ii (2 objects), (B) soft X-ray spectra and NIR absorption features such as H i, Na i, Ca i, and CO (46 objects), and (C) hard X-ray spectra and NIR absorption features such as H i, Na i, Ca i, and CO (17 objects). From these features, we argue that class A sources are cataclysmic variables (CVs), and class B sources are late-type stars with enhanced coronal activity, which is in agreement with current knowledge. Class C sources possibly belong to a new group of objects, which has been poorly studied so far. We argue that the candidate sources for class C are the binary systems hosting white dwarfs and late-type companions with very low accretion rates. It is likely that this newly recognized class of sources contribute to a non-negligible fraction of the GRXE, especially in the Fe K band.