NICER observation list: Target ID 357501

We detected periodic X-ray eclipse from a low-mass X-ray binary 1RXS J175721.2-304405 with NICER. It shows a step-function like time variability observed with the X-ray satellite ASCA. NICER monitored it from September 2018 to February 2019. We made the light curve with NICER and ASCA light curves superposed to investigate the time variability, and then 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. Future follow-up observation with NICER will find the evidence of a periodic eclipse. we propose one month NICER monitoring (1 ksec x 29 visits).