NICER observation list: Target ID 557101

Cir X-1 is the youngest known binary system in our Galaxy, hosting a neutron star. A NICER AO2 observation was carried out to cover the entire Cir X-1 orbit with 99 regular snapshots. Three iron emission/absorption lines were observed near the periastron (phase=0) at 6.4 keV, 6.7 keV and 7.0 keV. In particular, the highly ionized Fe lines at 6.7 keV and 7.0 keV switched from emission to absorption between the orbital phases 0.93 and 0.96. We hope to clarify these line production mechanisms by studying orbit-by-orbit variation/stability of the emission/absorption line strengths and the transition phase. We concentrate on the orbital phase-period when the emission lines switch to the absorption lines (0.85-1.05) and propose to monitor this phase-period over the AO4 duration.