XMM-Newton and Suzaku spectroscopic studies of unidentified X-ray sources towards the Galactic bulge: 1RXS J180556.1-343818 and 1RXS J173905.2-392615

Abstract

With XMM-Newton and Suzaku observations, for the first time, we acquired broad-band spectra of two unidentified X-ray sources towards the Galactic bulge: 1RXS J180556.1-343818 and 1RXS J173905.2-392615. The 1RXS J180556.1-343818 spectrum in the 0.3-7 keV band was explained by X-ray emission that originated from an optically-thin thermal plasma with temperatures of 0.5 and 1.8 keV. The estimated absorption column density of N_H ∼ 4 × 10²⁰ cm^-2 was significantly smaller than the Galactic H I column density towards the source. A candidate for its optical counterpart, HD 321269, was found within 4″. In terms of the X-ray properties and the positional coincidence, it is quite conceivable that 1RXS J180556.1-343818 is an active G giant. We also found a dim X-ray source that was positionally consistent with 1RXS J173905.2-392615. Assuming that the X-ray spectrum can be reproduced with an absorbed, optically-thin thermal plasma model with kT = 1.6 keV, the X-ray flux in the 0.5-8 keV band was 8.7 × 10^-14 erg s^-1 cm^-2, fainter by a factor of ∼7 than that of 1RXS J173905.2-392615 during the ROSAT observation. The follow-up observations we conducted revealed that these two sources would belong to the Galactic disk, rather than the Galactic bulge.