# Relativistic X-Ray Reflection and Photoionised Absorption in the Neutron-Star Low-Mass X-ray Binary GX 13+1

**Authors:** Enzo A. Saavedra, Federico García, Federico A. Fogantini, Mariano Méndez, Jorge A. Combi, Pedro L. Luque-Escamilla, Josep Marí  
**arXiv:** [2304.03130](https://arxiv.org/abs/2304.03130)  
**Date:** 2023-04-06  
**Summary page:** https://www.summarizepaper.com/en/paper/2304.03130v1/

## Key points

- The study analyzed a neutron-star low-mass X-ray binary Z-source GX 13+1 using NuSTAR
- The source transitioned from the normal branch to the flaring branch during the observation
- Spectra in each branch were fitted with a model consisting of an accretion disc, Comptonized blackbody, relativistic reflection (relxillNS), and photo-ionized absorption (warmabs)
- Evidence of relativistic reflection was found in both the Fe K line profile and Compton hump present in the 10-25 keV energy range
- The inner disc radius was constrained to be $R_{\rm in} \lesssim 9.6~r_g$, which allowed for further constraint on magnetic field strength to $B \lesssim 1.8 \times 10^8$ G
- Evidence for hot wind leading to photo-ionized absorption of Fe and Ni was found with a Ni overabundance of $\sim$6 times solar
- The distance between the ionizing source and slab of ionized absorbing material is $\sim4-40\times10^5$ km
- The width of boundary layer extends $\sim$3 km above surface level of neutron star yielding a neutron star radius $R_{\rm NS}\lesssim16$ km
- High electron densities in accretion disk ($n_e\sim10^{22} - 10^{23}$ cm$^{−3}$) consistent with Shakura–Sunyaev disc models were indicated by spectral fits

## Summary

In this study, the authors analyzed a dedicated observation of the neutron-star low-mass X-ray binary Z-source GX 13+1 using NuSTAR to investigate its timing and spectral properties. They found that during the observation, the source transitioned from the normal branch to the flaring branch based on a color-color diagram. The authors fitted spectra in each branch with a model consisting of an accretion disc, Comptonized blackbody, relativistic reflection (relxillNS), and photo-ionized absorption (warmabs). Using NuSTAR's large effective area and good energy resolution at high energies, they found evidence of relativistic reflection in both the Fe K line profile and Compton hump present in the 10-25 keV energy range. The inner disc radius was constrained to be $R_{\rm in} \lesssim 9.6~r_g$, which allowed for further constraint on magnetic field strength to $B \lesssim 1.8 \times 10^8$ G. Additionally, evidence for hot wind leading to photo-ionized absorption of Fe and Ni was found with a Ni overabundance of $\sim$6 times solar. From spectral fits, it was determined that the distance between the ionizing source and slab of ionized absorbing material is $\sim4-40\times10^5$ km. Furthermore, it was discovered that the width of boundary layer extends $\sim$3 km above surface level of neutron star yielding a neutron star radius $R_{\rm NS}\lesssim16$ km. The scenario inferred from spectral modeling becomes self-consistent only for high electron densities in accretion disk ($n_e\sim10^{22} - 10^{23}$ cm$^{−3}$) as expected for Shakura–Sunyaev disc significantly above densities provided by relxillNS models. Overall, these findings have significant implications for our understanding of physical conditions in GX 13+1 as they provide constraints on magnetic field strength and neutron star radius while also indicating high electron densities in accretion disk consistent with Shakura–Sunyaev disc models.