ChandraDeep 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 regionat (l,b) ~ (28.5, 0.0), where no discrete X-ray source had been reportedpreviously. We have detected 274 new point X-ray sources (4 sigma confidence)as well as strong Galactic diffuse emission within two partially overlappingACIS-I fields (~250 arcmin^2in total). Sum of all the detected point sourcefluxes 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 pointsources, the total observed X-ray flux cannot be explained. Therefore, weconclude that X-ray emission from the Galactic plane has truly diffuse origin.Only 26 point sources were detected both in the soft and hard bands, indicatingthat there are two distinct classes of the X-ray sources distinguished by thespectral hardness ratio. Surface number density of the hard sources is onlyslightly higher than that measured at the high Galactic latitude regions,indicating that majority of the hard sources are background AGNs. Following upthe Chandra observation, we have performed a near-infrared (NIR) survey withSOFI at ESO/NTT. Almost all the soft X-ray sources have been identified in NIRand their spectral types are consistent with main-sequence stars, suggestingmost of them are nearby X-ray active stars. On the other hand, only 22 % of thehard sources had NIR counterparts, which are presumably Galactic. From X-rayand NIR spectral study, they are most likely to be quiescent cataclysmicvariables. We have also carried out a precise spectral study of the Galacticdiffuse X-ray emission excluding the point sources.Comment: Accepted to Ap