A Chandra deep X-ray exposure on the Galactic plane and near infrared identification

Using the Chandra ACIS-I instruments, we have carried out a deep X-ray observation on the Galactic plane region at (l, b) ≈ (28.°5, 0.°0), where no discrete X-ray sources have been known previously. We have detected, as well as strong diffuse emission, 274 new point X-ray sources (4 σ confidence) within two partially overlapping fields (∼250 arcmin² in total) down to the flux limit ∼ 3 × 10^-15 erg s ^-1 cm^-2 (2 - 10 keV) and ∼ 7 × 7 × 10 ^-16 erg s^-1 cm^-2 (0.5 - 2 keV). We clearly resolved point sources and the Galactic diffuse emission, and found that ∼ 90 % of the flux observed in our field of view originates from diffuse emission. Many point sources are detected either in the soft X-ray band (below 2 keV) or in the hard band (above 2 keV), and only a small number of sources are detected in both energy bands. On the other hand, most soft X-ray sources are considered to be nearby X-ray active stars. We have carried out a follow-up near-infrared (NIR) observation using SOFI at ESO/NTT. Most of the soft X-ray sources were identified, whereas only a small number of hard X-ray sources had counterparts in NIR. Using both X-ray and NIR information, we can efficiently classify the point X-ray sources detected in the Galactic plane. We conclude that most of the hard X-ray sources are background Active Galactic Nuclei seen through the Milky Way, whereas majority of the soft X-ray sources are nearby X-ray active stars.