[Paper Review] The prospects for X-ray polarimetry and its potential use for understanding neutron stars

M. C. Weisskopf, Ronald F. Elsner+5 more|arXiv (Cornell University)|Nov 15, 2006

Pulsars and Gravitational Waves Research3 references34 citations

TL;DR

This paper advocates for a dedicated, low-cost X-ray polarimetry mission to measure polarization from neutron stars, using a large-area scattering polarimeter to probe emission mechanisms, geometry, and quantum electrodynamics effects like vacuum birefringence. It proposes a pathfinder mission with ~30M funding to survey bright X-ray sources, achieving 3% minimum detectable polarization (MDP) in 0.5–30 days per source.

ABSTRACT

We review the state of the art for measuring the X-ray polarization of neutron stars. We discuss how valuable precision measurements of the degree and position angle of polarization as a function of energy and, where relevant, of pulse phase, would provide deeper insight into the details of the emission mechanisms. We then review the current state of instrumentation and its potential for obtaining relevant data. Finally, we conclude our discussion with some opinions as to future directions.

Motivation & Objective

• Address the lack of sensitive X-ray polarimetry in astrophysics despite its potential to reveal key physical processes in neutron stars.
• Overcome the historical limitations of past missions, which achieved only upper limits on polarization due to low sensitivity and short observing time.
• Enable precision measurements of degree and position angle of X-ray polarization as a function of energy and pulse phase to test theoretical models of emission mechanisms.
• Explore the potential of detecting vacuum birefringence—a quantum electrodynamics effect—via X-ray polarization in strong magnetic fields.
• Lay the foundation for a future, more complex mission with focusing optics and electron-tracking polarimeters by first demonstrating feasibility with a simple, low-cost pathfinder.

Proposed method

• Propose a small, dedicated X-ray polarimetry mission using a large-area scattering polarimeter, avoiding the cost and complexity of focusing telescopes.
• Use a simple, proven instrument design with no deployables or moving parts, minimizing technical risk and launch constraints.
• Integrate the polarimeter on a small spacecraft with relaxed pointing accuracy and stability requirements to reduce mission cost.
• Estimate instrument cost at ~5M and total mission cost at ~30M, significantly below typical NASA Small Explorer missions.
• Design a 6-month survey mission to observe a wide range of X-ray sources, with integration times tailored to achieve 3% MDP or 0.5 days, whichever is longer.
• Use the XPE polarimeter model as a reference for sensitivity and performance, with MDP values calculated for various neutron star types and states.

Experimental results

Research questions

• RQ1 What is the degree and energy-dependent polarization of X-rays from neutron stars, and how does it vary with pulse phase?
• RQ2 Can X-ray polarimetry distinguish between competing models of pulsar emission, such as the polar cap versus outer gap models?
• RQ3 Can X-ray polarization measurements provide evidence for vacuum birefringence in the strong magnetic fields of neutron stars?
• RQ4 What is the minimum detectable polarization level (MDP) required to resolve key features in the polarization profile of sources like the Crab pulsar?
• RQ5 How can a low-cost, high-sensitivity pathfinder mission enable the development of future, more advanced X-ray polarimetry missions with focusing optics?

Key findings

• The Crab Nebula was the only X-ray source to show statistically significant polarization (19.2% ± 1.0%) in past missions, confirming its synchrotron origin.
• Past missions achieved only upper limits on polarization for other sources, such as 13.5% for Cen X-3 and 60% for Her X-1, due to low sensitivity.
• The proposed pathfinder mission would achieve a 3% MDP for most sources, with integration times ranging from 0.5 to 30 days depending on source brightness and polarization expectations.
• The mission would detect polarization from the Crab pulsar in 29.6 days, achieving 3% MDP, sufficient to resolve detailed pulse-phase-dependent polarization features.
• The mission would also detect polarization from accreting pulsars like Her X-1 and Cen X-3 with MDPs of 1.9% and 1.4%, respectively, within 0.5 days.
• The pathfinder would serve as a foundation for future missions with focusing optics and electron-tracking polarimeters, enabling higher sensitivity and energy resolution.