Preamble

Vol. 66 No. 4

July 2025

Acta Astronomica Sinica Vol. 66 No. 4 Jul., 2025
doi: 10.15940/j.cnki.0001-5245.2025.04.011

Insight-HXMT's Study of Low-Magnetic-Field Neutron Star Low-Mass X-ray Binaries

(Institute of Astrophysics, School of Physics and Technology, Central China Normal University, Wuhan 430079)

Abstract

Low-magnetic-field neutron star low-mass X-ray binaries (NS-LMXBs) generally refer to systems where the magnetic field strength of the primary star (neutron star) is below 10^10 G. In these systems, the neutron star accretes matter from its companion via Roche lobe overflow through an accretion disk. During this process, a substantial amount of gravitational energy is released and converted into X-ray radiation near the neutron star surface. Their X-ray emission typically exhibits rapid temporal variability on short timescales, accompanied by spectral evolution. Consequently, these objects are of great significance for fundamental physics, particularly in testing general relativity and strong gravitational field effects, as well as investigating the equation of state of ultra-dense matter. This review primarily summarizes observational studies of weakly magnetized (10^8–10^10 G) neutron star low-mass X-ray binaries conducted since the launch of the Insight-HXMT satellite, focusing on advances in kilohertz quasi-periodic oscillations, the origin of high-energy hard X-ray tails, and the evolution of disk-corona geometry with accretion state.

Keywords: X-ray: binaries, compact object: accretion, stars: neutron star
Classification: P142; Document code: A

1 Introduction

Weakly magnetized neutron star low-mass X-ray binaries (NS-LMXBs) generally refer to low-mass X-ray binary systems where the primary star (neutron star) has a magnetic field strength in the range of 10^8–10^10 G. The companion stars are typically main-sequence stars, white dwarfs, or red giants with masses below one solar mass, and their orbital periods are generally less than one day. Since the discovery of the first weak-field NS-LMXB system, Sco X–1, in 1962, more than 200 such sources have been observed in the Milky Way. In these systems, the neutron star accretes matter from its companion through an accretion disk, releasing enormous gravitational energy that is converted into X-ray radiation near the neutron star. Since the magnetospheric radius is positively correlated with magnetic field strength and inversely correlated with accretion rate, the inner edge of the accretion disk can approach the neutron star surface when the magnetic field is weak and the accretion rate is sufficiently high. In short, the accretion process in weak-field NS-LMXBs is directly modulated by both the neutron star's magnetic field and the accretion rate, making these systems ideal laboratories for studying extreme physical processes near neutron stars under weak magnetic field conditions.

Based on their distinct patterns in color-color diagrams (CCD) and hardness-intensity diagrams (HID), weak-field NS-LMXBs can be divided into two categories: Z sources and Atoll sources [1]. Atoll sources are either persistent or transient, with luminosities generally below 0.1 times the Eddington limit, displaying a "C"-shaped track in CCDs. Z sources are typically persistent, with luminosities at or above 0.1–1 times the Eddington limit, showing a "Z"-shaped track in CCDs. To date, six Z sources have been identified, including the first weak-field NS-LMXB system in the Milky Way, Sco X–1.

Both Atoll and Z sources display three main spectral states, which roughly correspond to the hard, intermediate, and soft states observed in black hole systems [2]. In Atoll sources (see Figure 1 [FIGURE:1], left panel), these states are called the Island State (IS), Lower Banana (LB), and Upper Banana (UB). In Z sources (see Figure 1, right panel), they are the Horizontal Branch (HB), Normal Branch (NB), and Flaring Branch (FB). Morphological transitions between Z and Atoll patterns observed in special sources such as XTE J1701–462 and IGR 17480–2446 indicate that the primary distinction between these two classes is the average mass accretion rate (ṁ): Z sources have significantly higher average accretion rates than Atoll sources. Although ṁ generally increases along the track from hard states (IS, HB) to soft states (UB, FB), the evolution characteristics of ṁ across different branches of Z/Atoll sources remain controversial. For a more comprehensive review of weak-field neutron star low-mass X-ray binaries, see the latest review article [3].

The X-ray radiation from weak-field NS-LMXBs can typically be described by a soft thermal component and a hard Comptonized component. The soft component may be either single-temperature blackbody radiation from the neutron star surface or boundary layer, or multi-temperature blackbody radiation from the accretion disk. The hard component arises from inverse Compton scattering of soft photons in a hot plasma corona. Based on different choices for these two components, two classic models for neutron star X-ray binaries have been developed, known as the "Eastern model" and the "Western model." In the Eastern model, the soft component is described by a multi-temperature blackbody disk, while the hard Comptonized component originates from weak Comptonization at the neutron star surface or boundary layer. In the Western model, the soft component is a single-temperature blackbody from the neutron star surface or boundary layer, while the hard Comptonized component comes from a slab-like corona covering the accretion disk. Consequently, X-ray continuum spectral analysis is crucial for understanding the physical properties and spatial geometry of accretion flows near neutron stars.

The X-ray continuum spectral characteristics of weak-field NS-LMXBs change significantly with the source's position in the CCD/HID, accompanied by the evolution of timing features [4]. In power spectra, in addition to common low-frequency quasi-periodic oscillations (QPOs) and broadband noise components, the discovery of kilohertz QPOs has made a transformative contribution to the study of weak-field NS-LMXBs. These kilohertz QPOs generally appear in pairs in power spectra, with characteristic frequencies ranging from several hundred hertz to kilohertz, representing the fastest temporal variations observed from celestial objects to date (on microsecond timescales, see Figure 2 [FIGURE:2] left panel). These extremely short timescales suggest that kilohertz QPOs likely originate from radiation regions very close to the neutron star surface, providing the most direct observational evidence for studying the dynamics of accretion flows around neutron stars. To date, paired kilohertz QPOs have been observed in more than 30 NS-LMXBs. Since the upper frequency of these kilohertz QPO pairs is very close to the Keplerian frequency at the neutron star surface, they can be directly used to constrain the mass and radius of neutron stars (see Figure 2 right panel, and review article [6] for details).

Over the past few decades, important progress has been made in studying weak-field NS-LMXBs thanks to a series of successful X-ray telescope missions. For example, the broad energy band coverage of BeppoSAX and RXTE (Rossi X-ray Timing Explorer) helped us better understand the accretion physics near weakly magnetized neutron stars. RXTE's high time resolution and large effective area enabled the discovery of kilohertz QPOs, providing new approaches to constrain the equation of state of neutron star matter. Chandra and XMM-Newton's high spectral resolution in the soft X-ray band allowed better analysis of emission lines and absorption edges in X-ray continua, further constraining neutron star accretion physics in strong gravitational fields. However, several controversies remain in the study of weak-field NS-LMXBs, such as the evolution of accretion rates along different branches of Z sources, the origin of hard X-ray tails, the physical nature of kilohertz QPOs, and the evolution of disk-corona geometry at different accretion rates. The successful launch of China's first X-ray telescope, Insight-HXMT (Insight Hard X-ray Modulation Telescope), provides an opportunity to investigate these issues in depth. As the first X-ray telescope combining wide energy band coverage (1–250 keV), high time resolution, and large effective area, Insight-HXMT provides unprecedented high-quality observational data [7]. In the following sections, we highlight representative research achievements published in recent years using Insight-HXMT data.

2 Tracing the Origin of Kilohertz QPOs

The debate over the origin of kilohertz QPOs has persisted for nearly 30 years. Currently, the most widely discussed models focus on the dynamical nature of kilohertz QPOs, including the beat-frequency model, relativistic precession model, and relativistic resonance model. These dynamical models generally assume that kilohertz QPOs arise from some form of oscillation in the accretion disk or corona surface (for more detailed introductions, see review article [6]). In contrast, theoretical studies on the radiation mechanism of kilohertz QPOs are relatively scarce, currently focusing on the production location of kilohertz QPOs and their X-ray radiation characteristics. Over the past 30 years, by studying the evolution of kilohertz QPO characteristic parameters (frequency, time lag, amplitude) with spectral state and energy, the origin of kilohertz QPOs has been constrained to regions outside the accretion disk, namely the neutron star surface or corona region. However, their specific physical origin and radiation mechanism remain unclear and require further investigation.

Studying kilohertz QPOs requires satellites with both high time resolution and large effective area. Due to the limited effective area of the previous generation timing telescope RXTE in the hard X-ray band, early studies of kilohertz QPOs were mainly concentrated below 20 keV. Insight-HXMT is the first satellite since RXTE to detect kilohertz QPOs and has, for the first time, improved the detection capability for kilohertz QPOs in the 20–60 keV energy band to above 3σ, demonstrating Insight-HXMT's capability for kilohertz QPO detection in the hard X-ray band. For example, in the Z source Sco X–1, Jia et al. [8] simultaneously detected low-frequency QPOs and kilohertz QPOs in the 20–60 keV band for the first time (see Figure 3 [FIGURE:3] left panel). This discovery provides important observational evidence for constraining the origin of kilohertz QPOs, indicating that such high-energy QPOs cannot originate from lower-energy radiation regions such as the accretion disk or neutron star surface, but are more likely to come from higher-energy hard Comptonized radiation regions.

The basic physical picture of weak-field NS-LMXBs involves a boundary layer (TL) between the accretion disk and the neutron star, where matter is accreted onto the neutron star surface primarily through this boundary layer and the accretion disk. Jia et al. [9] constrained the origin of kilohertz QPOs to the inner region of the boundary layer by fitting the evolution of the root-mean-square (rms) intensity of kilohertz QPOs with photon energy (see Figure 3 right panel). They found that the rms spectrum of kilohertz QPOs is dominated by Compton scattering of photons from the inner layer of the boundary layer (dashed line in the figure). Simultaneously, they discovered that the rms spectral index of kilohertz QPOs is smaller (harder spectrum) than that of the continuum spectrum, suggesting that kilohertz QPOs likely originate from clumpy motions in the inner region of the boundary layer.

3 Unveiling the Origin of High-Energy Hard X-ray Tails

Broadband spectral analysis of weak-field NS-LMXBs reveals that in soft states, their continuum spectra often exhibit a hard X-ray tail dominated by a power-law component around 20–30 keV, with a photon index typically between 1.9 and 3.3. The flux of this hard component decreases with increasing accretion rate. Generally, hard X-ray tails are more commonly observed in Atoll sources, with only a few cases detected in Z sources. The observational properties of hard X-ray tails evolve significantly with the source's position in the CCD, particularly evident in Z sources where the radiation intensity is highest on the HB (photon index ~2.3) and can decrease to 1/20 of that value on the NB.

The origin of hard X-ray tails in neutron star systems has long been debated. Current mainstream models include: (1) Non-thermal Comptonization by relativistic electrons from local outflows or coronal regions, or bulk motion of accreting matter near the neutron star surface, where the corona is a hybrid of thermal and non-thermal electrons. Note that in high accretion rate states, radiation pressure from the neutron star surface slows down the bulk motion of surrounding accreting material, suppressing bulk Comptonization (see latest review article [3] for details). (2) Synchrotron radiation from relativistic electrons escaping from jets. In this model, since jets originate from and are powered by hot electron coronae, the radio flux from jets correlates positively with X-ray radiation flux [10]. This correlation has been observed not only in NS-LMXBs but also widely in black hole binary systems, providing additional constraints for studying the physical origin of hard X-ray tails.

Studying hard X-ray tails requires satellites with sufficiently large effective area in the hard X-ray band, making Insight-HXMT's observational capabilities particularly suitable. Currently, Insight-HXMT has observed high-energy hard X-ray tails in the Z sources Sco X–1, GX 17+2, and GX 349+2. Ding et al. [11] analyzed Insight-HXMT observations of the hard X-ray tail in Sco X–1 and found that its spectral characteristics evolve significantly along the Z track, with flux decreasing markedly from the HB through the NB to the FB branch (see Figure 4 [FIGURE:4]). Through further analysis of the broadband spectral features in the 2–240 keV energy range, Ding et al. [11] found that the hard X-ray tails on the HB and NB branches can be well explained by the Thermal and bulk Comptonization of a seed blackbody-like spectrum (COMPTB) model, suggesting that these high-energy photons are likely produced by inverse Compton scattering between soft photons from the neutron star surface and boundary layer and hot electrons flowing toward the neutron star.

4 Tracking the Evolution of Disk-Corona Geometry

X-ray continuum spectral analysis plays a crucial role in understanding the physical properties and spatial geometry of accretion flows near compact objects. However, due to strong degeneracies commonly present in continuum spectral fitting of weak-field NS-LMXB systems, spectral analysis alone is often insufficient to describe the complete picture of disk-corona geometry. Building upon spectral analysis, timing or polarization analysis can provide an additional dimension that helps break degeneracies in spectral models to some extent. For example, combining spectral analysis with Fourier-resolved spectroscopy can investigate the properties of different spectral components at characteristic frequencies, thereby constraining disk-corona geometry. Additionally, since the polarization of photons produced by Compton scattering is highly sensitive to the geometry of scattering material, different geometric configurations and viewing angles result in different observed polarization degrees (PD) and polarization angles (PA). Therefore, combining spectral analysis with polarization analysis enables more direct investigation of the geometric properties of radiation regions.

Through joint observations with Insight-HXMT and the Imaging X-ray Polarimetry Explorer (IXPE), three different dimensions of information—spectral, timing, and polarization—can be obtained simultaneously for target sources, providing better constraints on neutron star accretion geometry. Seed photons become polarized after undergoing inverse Compton scattering in the corona, which is considered one of the primary sources of photon polarization. In this scenario, PD depends strongly on the geometric properties of the accretion flow and the source's spectral state. For a slab-like corona, the seed photons for inverse Compton scattering mainly originate from the accretion disk, whereas for a spherical shell-like corona, seed photons primarily come from the neutron star surface. For the same source, different accretion geometries produce different evolution characteristics of polarization degree and angle with energy. Therefore, studying the evolution of polarization properties with energy and spectral state can constrain the accretion geometry.

Recently, Yu et al. [12] used joint observations from Insight-HXMT, NuSTAR, and IXPE to study the evolution of disk-corona geometry along the Z track in XTE J1701–462. They found that the PD decreases significantly from the HB to the NB and even disappears (see Figure 5 [FIGURE:5]), while the Comptonized flux also decreases substantially, indicating that the coronal scale shrinks significantly from the HB to the NB and may even disappear completely on the NB. Their results also show that the polarization of this source mainly originates from reflection components, ruling out the possibility of a spherical corona geometry. These findings demonstrate that joint polarization-spectral analysis can effectively break degeneracies in spectral fitting and provide meaningful constraints on the accretion geometry of neutron star systems.

5 Summary

Research results over the past six years have demonstrated that Insight-HXMT has made significant progress in studying the origin of hard X-ray tails, the physical nature of kilohertz QPOs, and the evolution of disk-corona geometry in weak-field NS-LMXBs, confirming its observational advantages in wide energy band coverage, high time resolution, and high statistical quality. Combined observations with other satellites have also proven that multi-messenger, multi-dimensional analysis approaches will be the primary and effective method for studying weak-field NS-LMXBs. In the future, Insight-HXMT will continue its observational strategy for weak-field NS-LMXBs, expanding the observational sample of kilohertz QPOs and exploring their energy upper limits. By combining spectral and polarization evolution, it will further constrain the disk-corona geometry of neutron stars. Through combined timing and spectral evolution studies, it will investigate the hard X-ray tails on the FB branch.

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Highlight Sciences on Low Magnetic Field Neutron Stars X-ray Binaries with Insight-HXMT

BU Qing-cui
(Institute of Astrophysics, Central China Normal University, Wuhan 430079)

ABSTRACT
Low magnetic field neutron stars X-ray binaries are the systems where the magnetic field strength of the primary star (neutron star) is less than 10^10 Gauss. In these systems, the neutron star undergoes Roche lobe overflow, whereby matter is accreted from its companion star through an accretion disk. During the accretion process, a substantial quantity of gravitational energy is released and transformed into X-ray radiation in the vicinity of the neutron star. The X-ray emissions from these systems typically exhibit rapid temporal variability with short time scales, along with changes in spectral characteristics. These celestial objects are of great significance in fundamental physics, particularly in testing general relativity and strong gravitational field effects, as well as studying the equation of state of ultra-dense matter. This review presents a summary of observational research on weakly magnetic field neutron stars in low-mass X-ray binaries, with a focus on recent advancements in the study of kilohertz quasi-periodic oscillations, hard X-ray tails and the evolution of the accretion disk-corona geometry along the accretion state, since the launch of the Insight-HXMT (Insight Hard X-ray Modulation Telescope) satellite.

Key words: X-ray: binaries, compact object: accretion, stars: neutron star