Rotation Measure Variation Achieves New Insights into Millisecond Pulsar J2144-5237 Emission

The dynamic spectro-polarimetric behaviour of pulsars remains a largely uncharted area of astrophysics, yet holds considerable promise for understanding the complex physics of these rapidly rotating neutron stars. Rahul Sharan, Bhaswati Bhattacharyya, and Simon Johnston, alongside their colleagues, have now presented the first investigation into these time-varying properties, focusing on the millisecond pulsar J2144-5237. Using the Parkes UWL receiver, the team analysed the pulsar’s full Stokes parameters, revealing correlated variations in Stokes I, Q, and V, alongside more complex behaviour in Stokes U. This research is significant as it demonstrates systematic changes in the pulsar’s polarisation with rotational phase, offering a new window into the emission mechanisms, magnetic field geometry, and interstellar environment surrounding these fascinating objects.

Spectro-polarimetric Analysis of Pulsar J2144-5237’s Temporal Variations Researchers

The study pioneered a novel approach to investigating the spectro-polarimetric behaviour of pulsars, focusing on temporal variations previously unexplored in detail. Researchers engineered a dedicated software package to analyse time-varying spectral behaviour across all Stokes parameters, deploying it to observe the millisecond pulsar J2144-5237, a binary system with an approximate 10-day orbital period. Observations were conducted using the Parkes radio telescope’s ultra-wide-bandwidth low-frequency receiver, capturing data across a frequency range of 704-4032MHz with a time resolution of 30 seconds and a frequency channel resolution of 1MHz, folded into 1024 phase bins. Initial data processing employed PSRPYPE, utilising CLFD3 for radio frequency interference (RFI) removal, followed by standard flux and polarization calibration procedures.

A pulsed signal was injected into the feed to precisely measure relative gains and phases between the receiver’s linear probes, while flux calibration relied on observations of the radio galaxy PKS 1943, 638. The pulsar J0437, 4715 served as a calibration source for establishing a polarization calibration model, implemented within the PAC command of PSRCHIVE. Calibrated data, saved in the standard PSRFITS format, was then segmented to manage computational demands. Scientists harnessed the python package astropy.io.fits to read the PSRFITS files, performing dispersion measure correction and baseline removal before transforming the data into Stokes parameters.

Rotation measure (RM) determination, crucial for accurate polarization analysis, was achieved using a modified version of RM-Tools on roughly 20-minute time chunks. To mitigate ionospheric effects, the team integrated spinifex, which utilises publicly available global ionospheric maps in IONEX format to estimate and remove ionospheric RM contributions. The research team visualised signal changes across the pulse phase by representing the Stokes parameters on the Poincaré sphere, a technique recently applied to fast radio bursts. This approach provides an intuitive way to visualise changes in the polarization state. The research team developed a novel package for analysing time-varying spectral behaviour across all Stokes parameters, successfully demonstrating its capabilities on the millisecond pulsar J2144-5237, which resides in a binary system with an approximate orbital period of 10 days. This detailed analysis opens new avenues for exploring pulsar emission, magnetic field geometry, and propagation effects within the magnetosphere.

Experiments revealed significant variation in the rotation measure (RM) across the pulsar’s orbital phase, ranging from 22 to 31 rad m−2, as determined from 20-minute time chunks. The team meticulously calibrated the data using established procedures, including flux and polarization calibration, and accounting for ionospheric contributions with the Spinifex software, which utilises publicly available global ionospheric maps. Measurements confirm that while no systematic trend was immediately apparent in the RM variation, the accuracy of RM determination is crucial for minimising errors in derived Stokes parameters. The study presents a Poincaré sphere representation of the polarization properties of J2144-5237, showcasing a systematic temporal change in the sphere’s location corresponding to the main component with phase. The team processed data from observations taken on 29 May 2023, 22 June 2023, 26 August 2023 and 28 May 2025, utilising the PSRPYPE and CLFD3 software for initial data analysis and RFI removal. Unlike some pulsars exhibiting sign-changing Stokes parameters across pulse phase, J2144-5237 does not demonstrate this behaviour, meaning pulse phase averaging does not lead to depolarization of the Stokes parameters. This breakthrough delivers a foundation for probing the spectro-temporal nature of full Stokes data on a larger sample of pulsars and millisecond pulsars, potentially yielding vital information on emission mechanisms within the magnetosphere, interstellar propagation, and binary interactions.