2006ApJ...647L.135V

We have searched the OGLE-II archive for candidate counterparts of X-ray sources detected in two low-extinction windows included in our Galactic bulge Chandra-HST survey. We find that a significant number–that is, in excess of the expected level of random associations–can be matched with probable M giants. Their X-ray properties can be understood if these sources are symbiotic binaries in which the X-rays are typically, either directly or indirectly, the result of a white dwarf accreting from the wind of a cool giant. Optical and near-infrared properties of selected sources are consistent with a symbiotic nature, although none of the spectra collected for eight out of 13 candidate counterparts show the high-ionization nebular emission lines observed for many symbiotics. The hard X-ray emission for several sources (power-law photon indices -1.5≲Γ≲1.5) suggests that our sample includes systems similar to the symbiotics recently detected with INTEGRAL and Swift.