The Long Eclipse in the Low-mass X-ray Binary near the Galactic Center

We detected rapid flux drops from a low-mass X-ray binary 1RXS J175721.2-304405 with the X-ray detector NICER. The drops are consistent with the X-ray dip detected by the ASCA satellite about 22 years ago and can be identified as an ingress of a periodic eclipse. 1RXS J175721.2-304405 is located about three degrees away from the Galactic Center. It shows a step-function like time variability observed with the X-ray sattelite ASCA. NICER observed it from September 2018 to February 2019, discretely. We overlaied the ASCA light curve on the NICER one to investigate the time-scale and dependence of the variability. As a result, we preliminary identified a periodicity of about 14.5 (or 7.3) days. Since the long orbital period suggests a giant companion with a neutron star or a blackhole, 1RXS J175721.2-304405 is likely a new and rare eclipsing low-mass X-ray binary with a giant companion.