Simultaneous NICER and NuSTAR Observations of the Neutron Star Low-mass X-Ray Binary Serpens X-1

We present the first contemporaneous NICER and NuSTAR analysis of the low-mass X-ray binary Serpens X-1 obtained in 2023 June, performing broadband X-ray spectral analysis modeling of the reprocessed emission with relxillNS from 0.4 to 30 keV. We test various continuum and background estimation models to ensure that our results do not hinge on the choice of model used, and found that the detection of reflection features is independent of the choice of both continuum and background model. The position of the inner accretion disk is consistent with the last stable circular orbit (Rin ≤ 1.2 RISCO) and a low inclination of i ≤ 8.3°. Additionally, we investigate the presence of the low-energy (∼1 keV) Fe L complex in the data from NICER and the Reflection Grating Spectrometer on XMM-Newton that was previously reported in the literature. We find that the line is at most a 2% feature relative to the reprocessed continuum, and are unable to claim a definitive detection for the current data set. However, we discuss plausible conditions and systems that would increase the likelihood of detecting this feature in the future.