[Paper Review] Broadband Polarimetry with the Square Kilometre Array: A Unique Astrophysical Probe

B. M. Gaensler, I. Agudo+16 more|arXiv (Cornell University)|Jan 4, 2015

Stellar, planetary, and galactic studies3 references4 citations

TL;DR

This paper proposes broadband spectropolarimetric surveys with the Square Kilometre Array (SKA) to overcome degeneracies in Faraday rotation measurements and unlock detailed insights into cosmic magnetic fields, ionised gas, and galaxy evolution. By leveraging wide bandwidths (350–1500 MHz) across SKA1 and SKA2, the method enables high-fidelity reconstruction of Faraday depth structures, detecting ~700,000 polarised sources and revealing magnetic field geometry, turbulence, and entrainment in radio sources up to redshift z ~ 3–4.

ABSTRACT

Faraday rotation of polarised background sources is a unique probe of astrophysical magnetic fields in a diverse range of foreground objects. However, to understand the properties of the polarised sources themselves and of depolarising phenomena along the line of sight, we need to complement Faraday rotation data with polarisation observations over very broad bandwidths. Just as it is impossible to properly image a complex source with limited u-v coverage, we can only meaningfully understand the magneto-ionic properties of polarised sources if we have excellent coverage in $λ^2$-space. We here propose a set of broadband polarisation surveys with the Square Kilometre Array, which will provide a singular set of scientific insights on the ways in which galaxies and their environments have evolved over cosmic time.

Motivation & Objective

• To overcome degeneracies in Faraday rotation measurements caused by limited bandwidths in existing polarimetric surveys.
• To enable detailed characterization of magnetic fields, ionized gas, and turbulence in galaxies and their environments using broadband polarimetry.
• To develop a comprehensive survey strategy for SKA1 and SKA2 that maximizes scientific yield from spectropolarimetric data.
• To identify and model complex polarisation structures—such as multiple RM components and depolarisation effects—using full λ²-space coverage.
• To lay the foundation for early science programs with SKA1-SUR that will inform and calibrate full SKA2 surveys.

Proposed method

• Conducting spectropolarimetric surveys across 350–1500 MHz to achieve full coverage in λ²-space, essential for resolving complex Faraday depth structures.
• Using RM synthesis and full-Stokes parameter analysis (Q, U, I, Π) to disentangle multiple Faraday components and detect depolarisation effects.
• Applying broadband data to detect non-linear θ(λ²) relationships and variable fractional polarisation (Π) as indicators of internal magnetic complexity.
• Leveraging high sensitivity and wide bandwidth to detect polarised sources down to 5–10 mJy flux density, with ~75% detection efficiency for radio galaxies at z ~ 0.5–3.
• Designing an early science program with SKA1-SUR (50% sensitivity) covering 700–1200 MHz to achieve ~7 μJy beam⁻¹ sensitivity at 10″ resolution.
• Combining data from SKA1 and SKA2 to extend observations to z ~ 3–4, probing magnetic field evolution during early galaxy formation.

Experimental results

Research questions

• RQ1 How can broadband spectropolarimetry resolve degeneracies in Faraday rotation measurements that arise from narrowband observations?
• RQ2 What physical conditions—such as turbulence, ionisation fraction, and magnetic field geometry—can be inferred from full λ²-space polarisation data?
• RQ3 How do depolarisation mechanisms and multiple RM components affect polarisation angle and fractional polarisation across wide bandwidths?
• RQ4 What is the yield of detectable polarised sources at flux levels ≥5–10 mJy across the SKA1 and SKA2 frequency bands?
• RQ5 How can early science programs with SKA1-SUR inform the design and execution of full SKA2 broadband polarimetry surveys?

Key findings

• The proposed SKA1 and SKA2 surveys will detect approximately 700,000 polarised sources at ≥20σ significance, corresponding to total intensity fluxes >5–10 mJy.
• Broadband coverage (350–1500 MHz) enables the detection of complex Faraday structures, including multiple RM components and depolarisation effects, which are missed in narrowband surveys.
• An early science program with SKA1-SUR (50% sensitivity) can achieve ~7 μJy beam⁻¹ sensitivity at 10″ resolution over 700–1200 MHz, identifying ~10,000 polarised radio lobes with signatures of thermal entrainment.
• The method allows detection of magnetic field evolution and dynamo activity up to redshift z ~ 3–4, extending the reach of current surveys like POSSUM and WODAN.
• Full-Stokes spectropolarimetry across wide bandwidths reveals non-linear θ(λ²) and variable Π, providing robust diagnostics of magnetic field complexity and ionised gas turbulence.
• The survey will enable the study of supermassive black hole feedback, galaxy evolution, and magnetic field structure across cosmic time with unprecedented statistical power.