Peculiar lapse of periodic eclipsing event at low-mass X-ray binary GRS 1747−312 during Suzaku observation in 2009

Abstract

Abstract GRS 1747−312 is a neutron star low-mass X-ray binary in the globular cluster Terzan 6, located at a distance of 9.5 kpc from the Earth. During its outbursts, periodic eclipses were known to occur. Observations for the outbursts were performed with Chandra in 2004 and Swift in 2013. XMM-Newton observed its quiescent state in 2004. In addition, when Suzaku observed it in 2009 as a part of Galactic center mapping observations, GRS 1747−312 was found to be in a low-luminosity state with Lx ∼ 1.2 × 1035 erg s−1. All of the observations except for XMM-Newton included the time of the eclipses predicted. We analyzed archival data of these observations. During the Chandra and Swift observations, we found clear flux decreases at the expected time of the eclipses. During the Suzaku observation, however, there were no clear signs for the predicted eclipses. The lapse of the predicted eclipses during the Suzaku observation can be explained by a contaminant source quite close to GRS 1747−312. When GRS 1747−312 is in the quiescent state, we observe X-rays from the contaminant source rather than from GRS 1747−312. However, we have no clear evidence for the contaminant source in our data. The lapse might also be explained by thick material (NH > 1024 cm−2) between the neutron star and the companion star, though the origin of the thick material is not clear.