The Crab optical and ultraviolet polarimetry

Polarisation measurements of pulsars and of their pulsar wind nebulae (PWNe) are uniquely able to provide deep insights into the highly magnetised relativistic environment of young, rotation-powered isolated neutron stars (INSs). Besides the radio band, optical observations are primarily suited to providing such insights. The first INS for which optical polarisation observations were performed is the Crab pulsar which is also the brightest one (V=16.5). For this reason, the Crab pulsar is also the only INS for which repeated, phase-resolved polarisation measurements have been performed through the years. Moreover, it is the only case, together with the much fainter and distant PSR B0540-69 in the Large Magellanic Cloud (LMC), of an optical pulsar embedded in an optical PWN. Thus, the Crab is a perfect test case to study the optical polarisation properties of pulsars and of their PWNe. In this paper, we review the polarisation properties of the Crab pulsar and of its PWN in the optical and ultraviolet domains, we summarise the state of the art of the polarisation observations of other INSs, and we outline perspectives for INS polarisation studies with present and future generations of optical telescopes

💡 Research Summary

The paper provides a comprehensive review of optical and ultraviolet (UV) polarimetric observations of the Crab pulsar and its surrounding pulsar wind nebula (PWN), positioning the Crab as the benchmark for studying the polarisation properties of rotation‑powered isolated neutron stars (INSs). The authors begin by emphasizing that polarisation measurements uniquely probe the geometry of magnetic fields and relativistic particle flows in neutron star magnetospheres, offering insights that cannot be obtained from intensity‑only data. While radio polarimetry has a long history, optical/UV polarimetry is especially valuable because it samples the transition region of the synchrotron spectrum where the electron energy distribution changes, allowing a direct test of competing emission models (e.g., curvature radiation, synchrotron from outer gaps, slot‑gap models).

The review then chronicles the historical development of Crab polarimetry. The first optical polarisation detection in the early 1970s reported a modest linear polarisation of ~5 %. Subsequent high‑time‑resolution observations in the 1990s and 2000s, using instruments such as the OPTIMA photometer and the Hubble Space Telescope (HST) polarimeters, enabled phase‑resolved measurements. These data reveal that the main pulse exhibits a linear polarisation degree (PD) of ~9 % while the interpulse shows ~5 %, with the polarisation angle (PA) rotating sharply across the pulse profile. The rapid PA swing is interpreted as evidence for multiple emission zones within the magnetosphere rather than a single, simple geometry. UV polarimetry, obtained with HST/STIS and FOS, shows a slightly higher PD (~12 %) and a PA that is broadly consistent with the optical values but with subtle phase offsets, suggesting that higher‑energy electrons radiate in regions of stronger magnetic field curvature.

The paper also summarises the polarimetric characteristics of the Crab’s PWN. High‑resolution optical polarimetric imaging reveals a highly ordered magnetic field in the torus and jet structures, with linear polarisation up to ~30 % and distinct PA orientations for each component. This indicates that the nebular synchrotron emission is dominated by large‑scale, well‑aligned magnetic fields, and that the nebular polarisation is largely decoupled from the pulsar’s phase‑dependent polarisation, reflecting the global dynamics of the relativistic wind and its interaction with the surrounding supernova ejecta.

Beyond the Crab, the authors review the sparse polarimetric data for other young INSs such as PSR B0540‑69 (in the Large Magellanic Cloud), the Vela pulsar, and Geminga. Although these objects are fainter and more distant, limited measurements suggest linear PDs in the 5–10 % range, hinting that the Crab’s polarimetric behaviour may be representative of a broader class of rotation‑powered pulsars. However, the lack of phase‑resolved data for these sources limits the ability to discriminate between emission models.

Looking forward, the paper outlines the transformative potential of next‑generation facilities. Extremely Large Telescopes (ELTs) such as the European ELT, the Thirty‑Meter Telescope, and the Giant Magellan Telescope, equipped with high‑efficiency polarimeters and adaptive optics, will push the sensitivity limits to V ≈ 28 mag, enabling phase‑resolved polarimetry of many more pulsars. The authors also highlight emerging detector technologies—Microwave Kinetic Inductance Detectors (MKIDs) and Superconducting Nanowire Single‑Photon Detectors (SNSPDs)—which offer microsecond timing and photon‑counting capabilities, crucial for resolving rapid PA swings within a single rotation. Multi‑wavelength polarimetry, extending from the near‑infrared through the UV, will allow simultaneous mapping of the electron energy distribution and magnetic field geometry across the synchrotron spectrum.

In conclusion, the Crab pulsar remains the archetype for optical/UV polarimetric studies of INSs. Its bright, well‑characterised emission provides a testbed for refining magnetospheric emission models, while the highly polarised nebular structures offer a laboratory for studying relativistic wind dynamics. The synthesis of existing data presented in this review, together with the anticipated capabilities of ELTs and advanced detectors, promises a new era in which the magnetic topology and particle acceleration mechanisms of neutron stars can be probed with unprecedented precision.