A re-analysis of the NuSTAR and XMM-Newton broad-band spectrum of Serpens X-1

Context. High-resolution X-ray spectra of neutron star low-mass X-ray binaries (LMXBs) in the energy range 6.4-6.97 keV are often characterized by the presence of Kα transition features of iron at different ionization stages. Since these lines are thought to originate by reflection of the primary Comptonization spectrum over the accretion disk, the study of these features allows us to investigate the structure of the accretion flow close to the central source. Thus, the study of these features gives us important physical information on the system parameters and geometry. Ser X-1 is a well studied LMXB that clearly shows a broad iron line. Several attempts to fit this feature as a smeared reflection feature have been performed on XMM-Newton, suzaku, NuSTAR, and, more recently, on Chandra data, finding different results for the inner radius of the disk and other reflection or smearing parameters. High-quality broad-band NuSTAR data on Ser X-1 have recently been published. Using relativistically smeared self-consistent reflection models, a value of R_in close to 1.0 R_ISCO (corresponding to 6 R_g, where R_g is the Gravitational radius, defined as usual R_g = GM/c²) was found, as well as a low inclination angle of less than 10°.
Aims: The aim of this paper is to probe to what extent the choice of reflection and continuum models (and uncertainties therein) can affect the conclusions about the disk parameters inferred from the reflection component. To this aim, we re-analyze all the available public NuSTAR and XMM-Newton observations, which have the best sensitivity at the iron line energy, of Ser X-1. Ser X-1 is a well studied source, its spectrum has been observed by several instruments, and is therefore one of the best sources for this study.
Methods: We used slightly different continuum and reflection models with respect to those adopted in the literature for this source. In particular, we fit the iron line and other reflection features with self-consistent reflection models, such as reflionx (with a power-law illuminating continuum modified with a high-energy cutoff to mimic the shape of the incident Comptonization spectrum) and rfxconv. With these models, we fit NuSTAR and XMM-Newton spectra yielding consistent spectral results.
Results: Our results are in line with those already found by Miller et al. (2013) but are less extreme. In particular, we find the inner disk radius to be 13 R_g with an inclination angle with respect to the line of sight of 27°. We conclude that, while the choice of the reflection model has little impact on the disk parameters, as soon as a self-consistent model is used, the choice of the continuum model can be important in the precise determination of the disk parameters from the reflection component. Hence broad-band X-ray spectra are highly preferable to constrain the continuum and disk parameters.