[Paper Review] A Significant Detection of X-ray Polarization in Sco X-1 with PolarLight and Constraints on the Corona Geometry

Xiangyun Long, Hua Feng+24 more|arXiv (Cornell University)|Jan 1, 2022

Astrophysical Phenomena and ObservationsPhysics and Astronomy30 references52 citations

TL;DR

This study presents the first significant detection of X-ray polarization in Sco X-1 using the PolarLight instrument, revealing a 5σ detection at 4–8 keV with a polarization fraction of 0.043 ± 0.008 and a polarization angle of 52.6° ± 5.4° during high-flux states. The results favor an optically thin corona located in the transition layer between the accretion disk and neutron star surface, disfavoring extended disk corona models and confirming alignment with the system's jet axis.

ABSTRACT

We report the detection of X-ray polarization in the neutron-star low-mass X-ray binary Scorpius (Sco) X-1 with PolarLight. The result is energy-dependent, with a nondetection in 3-4 keV but a 4s detection in 4-8 keV; it is also flux-dependent in the 4-8 keV band, with a nondetection when the source displays low fluxes but a 5s detection during high fluxes, in which case we obtain a polarization fraction of 0.043 0.008 and a polarization angle of 52. 6 5. 4. This confirms a previous marginal detection with OSO-8 in the 1970s and marks Sco X-1 as the second astrophysical source with a significant polarization measurement in the keV band. The measured polarization angle is in line with the jet orientation of the source on the sky plane (54 ), which is supposedly the symmetry axis of the system. Combining previous spectral analysis, our measurements suggest that an optically thin corona is located in the transition layer under the highest accretion rates, and disfavor the extended accretion disk corona model.

Motivation & Objective

• To measure X-ray polarization in Sco X-1 with high sensitivity using the PolarLight instrument.
• To determine the geometry and location of the X-ray corona in this high-accretion-rate neutron star LMXB.
• To test competing corona models—specifically, the extended accretion disk corona versus the transition layer corona—using polarization data.
• To resolve long-standing ambiguities in spectral modeling by linking polarization measurements to physical corona structure.
• To provide a benchmark for future X-ray polarimetry missions by validating polarization analysis techniques in a bright, variable source.

Proposed method

• PolarLight performed four observing campaigns on Sco X-1 from 2019 to 2021, accumulating 884 ks of exposure time.
• Polarization was measured via the impact point method, using the spatial distribution of energy deposition in the detector to infer photoelectron emission angles.
• Stokes parameters were calculated using a Bayesian inference framework to estimate intrinsic polarization fraction (PF) and angle (PA), minimizing bias in low-significance measurements.
• Data were split into high- and low-flux intervals based on count rate, with separate polarization analysis per flux state to assess flux dependence.
• Energy-dependent analysis was performed in 3–4 keV and 4–8 keV bands to assess spectral energy dependence of polarization.
• Systematic effects were evaluated by folding data with the instrument’s roll pattern and comparing with laboratory calibrations, confirming robustness against instrumental response.

Experimental results

Research questions

• RQ1 Is X-ray polarization detectable in Sco X-1 at the 4–8 keV energy band with current polarimetric sensitivity?
• RQ2 Does the polarization fraction and angle vary with source flux, and what does this imply about the emission mechanism?
• RQ3 Does the measured polarization angle align with the known jet orientation on the sky, and what does this suggest about the corona’s geometry?
• RQ4 Can the polarization data distinguish between an extended accretion disk corona and a transition layer corona?
• RQ5 What constraints do the polarization measurements place on the optical depth and spatial structure of the corona?

Key findings

• A 5σ detection of X-ray polarization was achieved in the 4–8 keV energy band during high-flux states, with a polarization fraction of 0.043 ± 0.008.
• No significant polarization was detected in the 3–4 keV band, indicating energy-dependent emission properties.
• The polarization angle was measured as 52.6° ± 5.4°, consistent with the 54° orientation of the radio jet, supporting alignment with the system’s symmetric axis.
• The flux-dependent detection—non-detection at low flux, significant at high flux—indicates that polarization is linked to the corona’s emission state during high-accretion activity.
• The results disfavor the extended accretion disk corona model and instead support an optically thin corona located in the transition layer near the neutron star surface.
• The polarization signature is consistent with single-scattering in a low optical depth corona, with the dominant scattering geometry aligned with the system axis.