Chandra Deep X-Ray Observation of a Typical Galactic Plane Region and Near-Infrared Identification

Using the Chandra Advanced CCD Imaging Spectrometer Imaging array (ACIS-I), we have carried out a deep hard X-ray observation of the Galactic plane region at (l,b) ≈ (28.°5,0.°0), where no discrete X-ray source had been reported previously. We have detected 274 new point X-ray sources (4 σ confidence), as well as strong Galactic diffuse emission within two partially overlapping ACIS-I fields (~250 arcmin2 in total). The point-source sensitivity was ~3 × 10-15 ergs s-1 cm-2 in the hard X-ray band (2-10 keV) and ~2 × 10-16 ergs s-1 cm-2 in the soft band (0.5-2 keV). The sum of all the detected point-source fluxes accounts for only ~10% of the total X-ray flux in the field of view. Even hypothesizing a new population of much dimmer and numerous Galactic point sources, the total observed X-ray flux cannot be explained. Therefore, we conclude that X-ray emission from the Galactic plane has a truly diffuse origin. Removing point sources brighter than ~3 × 10-15 ergs s-1 cm-2 (2-10 keV), we have determined the Galactic diffuse X-ray flux to be 6.5 × 10-11 ergs s-1 cm-2 deg-2 (2-10 keV). Only 26 point sources were detected in both the soft and hard bands, indicating that there are two distinct classes of X-ray sources distinguished by their spectral hardness ratios. The surface number density of the hard sources is only slightly higher than that measured at the high Galactic latitude regions, indicating that the majority of the hard sources are background AGNs. Following up the Chandra observation, we have performed a near-infrared (NIR) survey with SofI at ESO/NTT. Almost all the soft X-ray sources have been identified in the NIR, and their spectral types are consistent with main-sequence stars, suggesting that most of them are nearby X-ray-active stars. On the other hand, only 22% of the hard sources had NIR counterparts, which are presumably Galactic. From X-ray and NIR spectral study, they are most likely to be quiescent cataclysmic variables. Our observation suggests a population of ≳104 cataclysmic variables in the entire Galactic plane fainter than ~2 × 1033 ergs s-1. We have carried out a precise spectral study of the Galactic diffuse X-ray emission excluding the point sources. Confirming previous results, we have detected prominent emission lines from highly ionized heavy elements in the diffuse emission. In particular, the central energy of the iron emission line was determined to be 6.52 keV (90% confidence), which is significantly lower than what is expected from a plasma in thermal equilibrium. The downward shift of the iron line center energy suggests nonequilibrium ionization states of the plasma or the presence of a nonthermal process to produce 6.4 keV fluorescent lines.