A Monte Carlo spectropolarimetric model for the high-soft state of neutron star low-mass X-ray binaries

¹ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via P. Gobetti 101, I-40129 Bologna, Italy
² INAF – Osservatorio Astronomico di Cagliari, Via della Scienza 5, I-09047 Selargius (CA), Italy

^⋆ Corresponding author: ruben.farinelli@inaf.it

Received: 21 February 2025
Accepted: 4 August 2025

Abstract

Context. After a few years of X-ray polarimetric astrophysics provided by the IXPE space observatory, the knowledge of the physics and geometry of weakly magnetized neutron star X-ray binaries is newly increasing both in quantity and quality terms. Energy resolved polarimetry is thus an undoubtedly powerful tool to investigate the accretion properties of this class of objects. While numerical spectropolarimetric codes of black hole X-ray binaries, whose results can be compared with observations, are available to the research, similar tools for neutron star systems still must be developed.

Aims. This work is part of an ambitious project aimed at providing the scientific community with a series of models for spectropolarimetric studies of binary systems containing neutron stars in different spectral states.

Methods. We developed a new Monte Carlo radiative transfer ray-tracing code in which both quantum mechanics and general relativity effects are fully taken into account and accurate counting statistics for the spectra are obtained by parallel computing on multi-core clusters. The model was built in the framework of an accretion disk plus boundary layer scenario specifically focused on the high-soft state of neutron star low-mass X-ray binaries. We applied the code to build a tabular spectroscopic model for the XSPEC package and fit the NuSTAR and IXPE spectra of the Z source Cyg X-2 and the bright atoll source GX 9+9.

Results. The new model successfully fits the source spectra and predicts a dependence with energy of the polarization degree and polarization angle consistent with observations. Further updates will be implemented in the future to study the challenging cases of polarimetric behavior suggesting accretion geometries different from those with azimuthal symmetry.