Detection of Reflection Features in the Neutron Star Low-mass X-Ray Binary Serpens X-1 with NICER

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 R ISCO and Fe K is at R ISCO (errors quoted at 90%). This corresponds to a position of km and km for a canonical NS mass ( ) 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.