2018ApJ...858L...5L

We present Neutron Star Interior Composition Explorer (NICER) observations of the neutron star (NS) low-mass X-ray binary Serpens X-1 during the early mission phase in 2017. With the high spectral sensitivity and low-energy X-ray passband of NICER, we are able to detect the Fe L line complex in addition to the signature broad, asymmetric Fe K line. We confirm the presence of these lines by comparing the NICER data to archival observations with XMM-Newton/Reflection Grating Spectrometer (RGS) and NuSTAR. Both features originate close to the innermost stable circular orbit (ISCO). When modeling the lines with the relativistic line model RELLINE, we find that the Fe L blend requires an inner disk radius of 1.4_–0.1^+0.2R_ISCO and Fe K is at 1.03_–0.03^+0.13R_ISCO (errors quoted at 90%). This corresponds to a position of 17.3_–1.2^+2.5 km and 12.7_–0.4^+1.6 km for a canonical NS mass (M_NS=1.4 M_☉) and dimensionless spin value of a = 0. Additionally, we employ a new version of the RELXILL model tailored for NSs and determine that these features arise from a dense disk and supersolar Fe abundance.