X-ray polarimetry

X-ray polarimetry is a branch of high-energy astrophysics that studies the polarisation properties of X-ray radiation emitted by astrophysical sources. By measuring the orientation and degree of polarisation of X-rays, astronomers can infer details about magnetic fields, emission mechanisms, and the geometry of energetic cosmic environments that cannot be obtained from spectroscopy, timing, or imaging alone.

Background

Electromagnetic radiation, including X-rays, is described by three main properties: intensity, spectrum, and polarisation. While intensity and spectrum have been extensively studied in astronomy, polarisation measurements at X-ray energies have historically been limited due to technological challenges.
Theoretical models predict that X-rays from compact objects such as neutron stars, black holes, and pulsar wind nebulae should exhibit measurable polarisation. However, X-ray polarimetry requires highly sensitive detectors capable of resolving the subtle polarisation signatures of faint sources.
The first detection of X-ray polarisation was achieved in the 1970s with the OSO-8 satellite, which measured polarisation from the Crab Nebula. Since then, advances in detector technology have enabled renewed progress, particularly in the 21st century.

Principles of X-ray Polarimetry

Polarisation refers to the orientation of the electric field vector of electromagnetic radiation. X-ray polarimetry involves determining two main parameters:

• Degree of Polarisation: The fraction of radiation that is polarised relative to the total intensity.
• Polarisation Angle: The orientation of the electric field vector relative to a reference axis.

Measurement techniques rely on physical processes sensitive to polarisation, such as:

• Thomson/Compton Scattering: The scattering probability depends on the polarisation orientation.
• Photoelectric Effect: The emission direction of photoelectrons correlates with the incoming photon’s polarisation.
• Bragg Diffraction: Polarisation affects reflection from crystals at specific angles.

Astrophysical Applications

X-ray polarimetry provides unique insights into high-energy astrophysical environments:

• Black Holes: Polarisation measurements can probe the geometry of accretion discs, relativistic jets, and the effects of strong gravitational fields near event horizons.
• Neutron Stars and Pulsars: Polarisation reveals magnetic field structure, emission mechanisms, and quantum electrodynamics (QED) effects such as vacuum birefringence in extremely strong magnetic fields.
• Supernova Remnants: Polarisation data clarify the role of shock acceleration and magnetic turbulence in producing high-energy emission.
• Active Galactic Nuclei (AGN): Polarisation helps to study the alignment of relativistic jets and the scattering environments around supermassive black holes.
• Galaxy Clusters and Hot Gas: Polarimetry assists in constraining scattering and magnetic field structures on large scales.

Instrumentation

Recent advances in X-ray polarimetry are largely driven by new detector technologies, particularly gas pixel detectors (GPDs) that exploit the photoelectric effect.
Key missions and instruments include:

• OSO-8 (1975–78): First detection of X-ray polarisation (Crab Nebula).
• IXPE (Imaging X-ray Polarimetry Explorer, launched 2021): A NASA–ASI mission dedicated to imaging X-ray polarimetry, capable of mapping polarisation across extended sources.
• POLAR and POLAR-2: Instruments designed for polarisation studies of gamma-ray bursts.
• Future Missions: Concepts such as eXTP (enhanced X-ray Timing and Polarimetry mission) aim to expand polarimetric sensitivity.

Challenges

• Low Signal Levels: X-ray sources often produce limited photon counts, requiring long observation times.
• Background Noise: Accurate polarimetry demands careful separation of signal from instrumental and cosmic background noise.
• Instrument Complexity: Polarimeters must balance sensitivity, energy range, and imaging capabilities.

Significance and Future Prospects

X-ray polarimetry adds a fourth dimension—polarisation—to the toolkit of X-ray astronomy, alongside spectroscopy, timing, and imaging. Its significance lies in its ability to probe otherwise inaccessible physical processes, such as:

• Testing theories of general relativity near compact objects.
• Investigating QED effects under extreme magnetic fields.
• Differentiating between competing emission models in high-energy astrophysics.