Suzaku Observations of Unidentified X-Ray Sources toward the Galactic Bulge

Abstract

Abstract We present Suzaku observations of two unidentified sources towards the Galactic bulge, 1RXS J165256.3$-$264503 and 1RXS J174459.5$-$172640, both of which were discovered with the ROSAT All-Sky Survey. We obtained wide-band spectra of these sources above 2 keV for the first time. The X-ray spectrum of 1RXS J165256.3$-$264503 consists of two absorbed thin thermal plasma components with temperatures of 0.7 and 2.4 keV. An RS CVn star, HD 152178, was found within a 90% error circle of the source position. Due to this position coincidence, and an X-ray spectrum analogous to those of other RS CVn binaries, we concluded that 1RXS J165256.3$-$264503 is an RS CVn-type active binary. The X-ray spectrum of 1RXS J174459.5$-$172640 was dominated by continuum emission extending up to $\sim$ 20 keV, which can be explained by an absorbed cutoff power-law or a combination of absorbed multicolor disk-blackbody and Comptonized emission. We inferred that the source is a new Galactic-bulge low-mass X-ray binary (LMXB) with a luminosity of 2 $\times$ 10$^{35}$ erg s$^{-1}$, based on its X-ray spectrum and lightcurve. Hence, 1RXS J174459.5$-$172640 may be an intriguing example of an LMXB in an intermediate state in which X-ray emission mechanisms can be readily explored.