Identifications of Four INTEGRAL Sources in the Galactic Plane via Chandra Localizations

Hard X-ray imaging of the Galactic plane by the INTEGRAL satellite is uncovering large numbers of 20-100 keV ``IGR'' sources. We present results from Chandra, INTEGRAL, optical, and IR observations of four IGR sources: three sources in the Norma region of the Galaxy (IGR J16195-4945, IGR J16207-5129, and IGR J16167-4957) and one that is closer to the Galactic center (IGR J17195-4100). In all four cases, one relatively bright Chandra source is seen in the INTEGRAL error circle, and these are likely to be the soft X-ray counterparts of the IGR sources. They have hard 0.3-10 keV spectra with power-law photon indices of Γ=0.5-1.1. While many previously studied IGR sources show high column densities (N_H~10²³-10²⁴ cm^-2), only IGR J16195-4945 has a column density that could be as high as 10²³ cm^-2. Using optical and IR sky survey catalogs and our own photometry, we have obtained identifications for all four sources. The J-band magnitudes are in the range 14.9-10.4, and we have used the optical/IR spectral energy distributions (SEDs) to constrain the nature of the sources. Blackbody components with temperature lower limits of >9400 K for IGR J16195-4945 and >18,000 K for IGR J16207-5129 indicate that these are very likely high-mass X-ray binaries (HMXBs). However, for IGR J16167-4957 and IGR J17195-4100, low extinction and the SEDs indicate later spectral types for the putative companions, suggesting that these are not HMXBs.