Spectro-Polarimetric Study of Accretion Geometry Evolution in the Neutron Star X-ray Binary XTE J1701-462

Core Concepts

The polarization degree of the X-ray emission from the neutron star X-ray binary XTE J1701-462 decreases as the source evolves along the Z-track, indicating a shrinking of the Comptonizing corona.

Abstract

This study presents a comprehensive spectro-polarimetric analysis of the neutron star low-mass X-ray binary XTE J1701-462 during its 2022 outburst. The key findings are:

1. Significant X-ray polarization is detected on the horizontal branch (HB) of the Z-track, with a polarization degree (PD) of ~5%. The PD decreases to ~3.8% as the source moves along the HB and drops to ≤1% on the normal branch (NB).

2. Simultaneous spectral analysis using Insight-HXMT and NuSTAR data reveals that the evolution of the PD is closely linked to changes in the flux and covering factor of the Comptonized component along the Z-track.

3. Under the "eastern model" scenario, the decreasing PD can be explained by a reduction in the size or coverage of the Comptonizing corona around the neutron star as the source evolves from the HB to the NB.

4. Under the "western model" scenario, the decreasing PD can be attributed to a thinning of an extended slab/wedge corona that covers most of the inner accretion disk.

5. In both scenarios, the extremely low PD and near-zero covering factor of the Comptonized component on the NB indicate a significant evolution in the accretion flow, with the Comptonizing corona nearly disappearing.

The correlated spectro-polarimetric study provides new insights into the accretion geometry and its evolution in the neutron star X-ray binary XTE J1701-462.

Stats

The polarization degree decreases from 4.98 ± 0.44% on the horizontal branch (BOX1) to 3.79 ± 0.54% on the horizontal branch (BOX2), and drops to 0.9 ± 0.3% on the normal branch (BOX3).
The flux ratio of the Comptonized component increases from 71% on BOX1 to 84% on BOX3 in the "eastern model", while it decreases from 65% on BOX1 to 21% on BOX3 in the "western model".
The covering factor of the Comptonized component decreases from 0.7 on BOX1 to 0.39 on BOX2, and approaches zero on BOX3 in both models.

Quotes

"The polarization degree highly depends on the geometrical configurations of accretion flow and the spectral states of the source."
"The co-evolution of polarization and the Comptonized component flux further corroborates the Comptonized origin of the observed polarization in XTE J1701-462."

Key Insights Distilled From

Correlated spectro-polarimetric study along the Z track in XTE J1701-462 puts constraints on its coronal geometry

by Wei Yu, Qing... at arxiv.org 10-01-2024

https://arxiv.org/pdf/2401.02658.pdf

Deeper Inquiries

How would the accretion geometry and corona properties change if the source exhibited different Z-track behaviors, such as transitioning between Cyg-like and Sco-like Z-tracks?

The accretion geometry and corona properties in neutron star low-mass X-ray binaries (NS-LMXBs) are significantly influenced by the source's Z-track behavior. In Cyg-like Z-sources, which exhibit a prominent horizontal branch (HB) and a weak flaring branch (FB), the accretion flow is typically characterized by a thicker corona and a more substantial contribution from the Comptonized emission. This is due to the increased mass accretion rate, which enhances the thermal and Comptonized components of the X-ray spectrum. The polarization degree (PD) is expected to be higher in Cyg-like sources due to the more effective scattering of soft photons in a well-defined corona.
Conversely, in Sco-like Z-sources, where the flaring branch is more pronounced, the accretion geometry may involve a thinner corona and a more complex interaction between the accretion disk and the neutron star surface. The transition from a Cyg-like to a Sco-like behavior could lead to a decrease in the PD, as observed in XTE J1701–462, where the PD dropped significantly when moving from the HB to the normal branch (NB). This suggests that the corona's covering factor diminishes, leading to less effective Compton scattering and a reduction in the polarization signal. Therefore, the transition between these Z-track behaviors would likely result in observable changes in the spectral properties, such as variations in the flux ratios of thermal and Comptonized components, as well as shifts in the polarization characteristics, reflecting the underlying changes in the accretion geometry.

What other observational signatures, besides polarization and spectral properties, could provide additional constraints on the accretion flow geometry in neutron star X-ray binaries?

In addition to polarization and spectral properties, several other observational signatures can provide valuable constraints on the accretion flow geometry in neutron star X-ray binaries (NS-LMXBs). These include:

Timing Analysis: The study of timing properties, such as quasi-periodic oscillations (QPOs) and pulsations, can reveal information about the dynamics of the accretion flow and the geometry of the inner regions around the neutron star. The frequencies and patterns of QPOs can indicate the presence of specific structures in the accretion disk and the corona.

X-ray Variability: The variability of X-ray emissions on different timescales can provide insights into the stability and dynamics of the accretion flow. Rapid variability may suggest a turbulent or unstable accretion process, while more stable emissions could indicate a steady-state accretion geometry.

Iron Line Emission: The presence and characteristics of iron line emissions, particularly the broad iron Kα line, can provide information about the geometry of the accretion disk and the effects of gravitational redshift and Doppler broadening. The width and shape of the iron line can indicate the velocity and distribution of material in the accretion disk.

Reflection Spectra: The study of reflection features in the X-ray spectrum can help constrain the geometry of the accretion disk and the corona. The strength and shape of reflection features can indicate the illumination of the disk by X-rays from the corona and provide insights into the disk's structure.

Multi-wavelength Observations: Observations across different wavelengths, including optical, infrared, and radio, can provide a more comprehensive view of the accretion process. For instance, optical and infrared observations can reveal the properties of the companion star and its interaction with the accretion flow, while radio observations can provide insights into jet formation and outflows.

By combining these various observational signatures with polarization and spectral analysis, researchers can develop a more complete understanding of the accretion flow geometry in NS-LMXBs.

Could the insights gained from this study on XTE J1701-462 be applied to understand the accretion processes in other types of accreting compact objects, such as black hole X-ray binaries or active galactic nuclei?

Yes, the insights gained from the study of XTE J1701–462 can indeed be applied to understand the accretion processes in other types of accreting compact objects, including black hole X-ray binaries (BH-XRBs) and active galactic nuclei (AGNs). The fundamental principles governing accretion physics, such as the role of the corona, the interaction between the accretion disk and the compact object, and the effects of Compton scattering, are similar across these systems.

Accretion Geometry: The findings regarding the evolution of the accretion geometry and the behavior of the corona in XTE J1701–462 can inform models of accretion in BH-XRBs and AGNs. For instance, the relationship between polarization degree and covering factor observed in XTE J1701–462 may also be relevant in understanding how the corona behaves in these other systems, particularly during different spectral states.

Spectral and Polarimetric Analysis: The techniques used in the spectro-polarimetric analysis of XTE J1701–462 can be adapted to study BH-XRBs and AGNs. The ability to measure polarization can provide unique insights into the geometry of the accretion flow and the nature of the emitting regions, which is particularly important in the context of AGNs where the scales are much larger.

Comparative Studies: By comparing the behavior of NS-LMXBs like XTE J1701–462 with BH-XRBs and AGNs, researchers can identify common patterns and differences in accretion processes. This comparative approach can enhance our understanding of the underlying physics governing accretion in different environments.

Model Development: The results from this study can contribute to the development of more comprehensive models of accretion that incorporate the effects of polarization and spectral evolution. Such models can be applied to a broader range of astrophysical contexts, including the study of jets and outflows in AGNs.

In summary, the insights gained from the spectro-polarimetric study of XTE J1701–462 provide a valuable framework for understanding the complex accretion processes in various types of compact objects, enhancing our overall knowledge of high-energy astrophysics.