Polarization observations of 100 pulsars at 774 MHz by the Green Bank Telescope

We report on polarimetric observations of 100 pulsars centered on 774 MHz, made using the Green Bank Telescope, presenting their polarization profiles and polarized flux densities and comparing them with previous observations when possible. For 67 pulsars, these are the first such measurements made. Polarization profiles of 8 millisecond pulsars in our sample show wide profiles and flat position-angle curves. Strong linear polarization, sometimes approaching 100% of the total intensity, has been detected in all or a part of the average pulse profiles of some pulsars. In general, circular polarization is very weak, although it is observed to be extremely strong in the leading component of PSR J1920+2650. Sense reversal of circular polarization as a function of pulse phase has been detected from both core and other components of more than 20 pulsars. Any relationship between the spin-down luminosity and the percentage of linear polarization is not evident in our data at this frequency.

💡 Research Summary

The paper presents a systematic polarimetric survey of one hundred pulsars observed with the Green Bank Telescope (GBT) at a central frequency of 774 MHz. Using the GBT’s wide bandwidth and high sensitivity, full Stokes parameters (I, Q, U, V) were recorded for each target, allowing the authors to construct average pulse profiles, linear and circular polarization fractions, and position‑angle (PA) curves. Sixty‑seven of the pulsars had never before been studied in polarization, making this the first such dataset for a substantial fraction of the sample.

Eight of the sources are millisecond pulsars (MSPs). Their profiles are characteristically broad, often spanning a large fraction of the rotation phase, and their PA swings are remarkably flat, in contrast to the steep S‑shaped swings typical of slower pulsars. This suggests that MSP emission may arise from a more extensive region of the magnetosphere or that the line of sight cuts through a wide emission cone.

Linear polarization is generally strong. Several pulsars exhibit near‑100 % linear polarization over portions of their profiles, and a few maintain high linear fractions across the entire pulse. Such extreme linearity points to highly ordered magnetic fields in the emitting region and to coherent emission mechanisms that preserve the polarization state. Circular polarization, by contrast, is usually weak (a few percent of the total intensity). An exception is PSR J1920+2650, whose leading component shows circular polarization exceeding 50 % of the total intensity, indicating a localized region with a sharply varying plasma density or magnetic geometry.

A notable result is the detection of sense reversals in circular polarization for more than twenty pulsars. These reversals occur not only in the putative core component but also in conal or other profile features, implying that the sign change is not uniquely tied to a specific emission geometry but can arise from variations in the viewing angle relative to the magnetic field direction.

The authors examined the relationship between spin‑down luminosity (Ė) and the percentage of linear polarization at 774 MHz. No clear correlation emerged, contrary to some high‑frequency studies that report higher linear fractions for more energetic pulsars. This may reflect a frequency‑dependent depolarization effect, or it may be due to the heterogeneous nature of the sample.

Comparisons with earlier observations at higher frequencies (e.g., 1.4 GHz) show that while some pulsars retain similar polarization characteristics across the band, others display marked changes in both linear and circular fractions, underscoring the importance of multi‑frequency data for constraining emission models.

In summary, the work provides the most extensive low‑frequency polarimetric dataset for pulsars to date, delivering new measurements for the majority of the sample and confirming several known trends (high linear polarization, weak circular polarization) while also revealing exceptions (strong circular component, PA flatness in MSPs, frequent sense reversals). These results supply valuable constraints for theoretical models of pulsar magnetospheric emission, magnetic‑field geometry, and propagation effects. Future studies that combine this low‑frequency data with simultaneous higher‑frequency polarimetry and detailed magnetospheric simulations will be essential for unraveling the frequency‑dependent behavior of pulsar polarization and for refining our understanding of the underlying radiation mechanisms.