Spectral properties of the neutron star low-mass X-ray binaries 4U 1636–53, XTE J1739–285, and MAXI J1816–195

¹ Department of Physics, Xiangtan University, Xiangtan, Hunan 411105, China
² Key Laboratory of Stars and Interstellar Medium, Xiangtan University, Xiangtan, Hunan 411105, China
³ Yunnan Observatories, Chinese Academy of Sciences (CAS), Kunming 650216, PR China
⁴ Key Laboratory for the Structure and Evolution of Celestial Objects, CAS, Kunming 650216, PR China
⁵ Instituto Argentino de Radioastronomía (CCT La Plata, CONICET; CICPBA; UNLP), C.C.5, (1894) Villa Elisa, Argentina

^⋆ Corresponding author; lvming@xtu.edu.cn

Received: 21 June 2024
Accepted: 15 January 2025

Abstract

We investigated simultaneous NICER plus NuSTAR observations of the three neutron star low-mass X-ray binaries 4U 1636–53, XTE J1739–285, and MAXI J1816–195 using the latest reflection models. The seed photons in our models that are fed into the corona originated from either the neutron star (NS) or the accretion disk. We found that for the sources in the hard spectral state, more than ∼50% of the NS photons enter the corona when the NS provides seed photons, while only ∼3–5% of the disk photons enter the corona when the seed photons come from the disk. This finding, together with the derived small height of the corona, favors a lamp-post geometry or a boundary layer scenario in which the corona is close to the central NS. Additionally, we found that the source of the seed photons has a strong influence on the significance of the NS radiation, especially for the soft spectral state. This result may help us to explain why the NS radiation in MAXI J1816–195 was weak in previous work. More importantly, for the first time, we explored the properties of the corona in the NS systems with the compactness (l − θ) diagram. We found that coronae in NS systems all lie on the left side of the pair-production forbidden region, away from the predicted pair-production lines. This finding indicates that either the corona in these NS systems is not pair dominated, possibly due to the additional cooling from NS photons, or the corona is composed of both thermal and nonthermal electrons.