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

TL;DR

This study presents simultaneous NICER and NuSTAR observations of Serpens X-1, analyzing broadband X-ray spectra to confirm reflection features and constrain the accretion disk's inner radius, while investigating the elusive Fe L complex.

Contribution

First simultaneous broadband X-ray spectral analysis of Serpens X-1 with NICER and NuSTAR, confirming reflection features are independent of modeling choices and constraining disk geometry.

Findings

Inner disk radius consistent with last stable orbit ($R_{ISCO}$)

Reflection features detected regardless of model choice

Fe L complex not definitively detected, at most 2% feature

Abstract

We present the first contemporaneous NICER and NuSTAR analysis of the low-mass X-ray binary Serpens X-1 obtained in June 2023, performing broadband X-ray spectral analysis modeling of the reprocessed emission with RELXILLNS from $0.4-30$ 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 ($R_{\rm in} \leq 1.2$~$R_{ISCO}$) and a low inclination of $i\leq 8.3 ^{\circ}$. Additionally, we investigate the presence of the low energy ($\sim$ 1 keV) Fe L complex in the data from NICER and the Reflection Grating Spectrometer (RGS) 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 dataset. However, we discuss plausible conditions and systems that would increase the likelihood of detecting this feature in the future.