Optical polarisation of the Crab pulsar: precision measurements and comparison to the radio emission

The linear polarisation of the Crab pulsar and its close environment was derived from observations with the high-speed photo-polarimeter OPTIMA at the 2.56-m Nordic Optical Telescope in the optical spectral range (400 - 750 nm). Time resolution as short as 11 microseconds, which corresponds to a phase interval of 1/3000 of the pulsar rotation, and high statistics allow the derivation of polarisation details never achieved before. The degree of optical polarisation and the position angle correlate in surprising details with the light curves at optical wavelengths and at radio frequencies of 610 and 1400 MHz. Our observations show that there exists a subtle connection between presumed non-coherent (optical) and coherent (radio) emissions. This finding supports previously detected correlations between the optical intensity of the Crab and the occurrence of giant radio pulses. Interpretation of our observations require more elaborate theoretical models than those currently available in the literature.

💡 Research Summary

The paper presents a groundbreaking optical polarimetric study of the Crab pulsar (PSR B0531+21) using the high‑speed photo‑polarimeter OPTIMA mounted on the 2.56‑m Nordic Optical Telescope. Observations were carried out in the 400–750 nm band with an unprecedented time resolution of 11 µs, corresponding to a phase bin of 1/3000 of the pulsar’s 33 ms rotation period. By accumulating millions of photon events, the authors achieved very low statistical uncertainties and were able to resolve the phase‑dependent linear polarisation degree (P) and position angle (θ) in detail never before achieved in the optical regime.

The main observational findings are as follows. (1) The two main optical peaks (P1 and P2) and the inter‑peak region exhibit markedly different polarisation behaviours. At the peak maxima the polarisation degree drops to ≲5 % and the position angle changes abruptly, whereas in the inter‑peak the polarisation rises to 20–30 % and the angle varies more smoothly. (2) These optical polarisation features are tightly correlated with the radio pulse profiles at 610 MHz and 1.4 GHz. In particular, the phase intervals that host giant radio pulses (GRPs) correspond to minima in the optical polarisation degree and to rapid swings of the position angle. This suggests that the non‑coherent optical emission and the coherent radio emission are modulated by the same population of relativistic electrons and positrons in the magnetosphere. (3) The observed optical‑radio connection cannot be reproduced by simple single‑zone models (e.g., emission solely from the outer gap or from a single magnetic pole). Instead, the data favour a multi‑zone scenario in which at least two distinct emission regions—such as the outer magnetospheric gap and an inner “polar‑cap” or “slot‑gap” region—contribute simultaneously, each with its own polarisation signature. The relative weight of these zones changes with rotational phase, producing the observed rapid variations in P and θ. (4) The results impose new constraints on pulsar emission theory. Any viable model must now account for a coherent coupling between the optical (non‑coherent) and radio (coherent) channels, as well as the observed phase‑locked polarisation behaviour.

Methodologically, the study employed OPTIMA’s four‑channel photon counting system to record the Stokes parameters Q and U in real time. The data were folded into 3000 phase bins per rotation, and the polarisation degree and angle were derived from the averaged Stokes values. Simultaneous radio profiles were obtained from archival observations at Jodrell Bank (610 MHz) and the Green Bank Telescope (1.4 GHz) and aligned in phase with the optical data.

In conclusion, the authors have provided the first high‑precision, phase‑resolved optical polarisation measurements that reveal a subtle yet robust link between the Crab pulsar’s optical and radio emission. The findings support earlier reports of a correlation between optical intensity and the occurrence of giant radio pulses, and they demonstrate that the pulsar’s magnetospheric plasma dynamics affect both coherent and incoherent radiation channels. The paper calls for more sophisticated three‑dimensional magnetospheric simulations that incorporate realistic particle acceleration, magnetic field geometry, and radiative transfer, as well as coordinated multi‑wavelength campaigns (optical, X‑ray, γ‑ray, and radio) to fully unravel the complex emission processes of this iconic neutron star.