Investigation of polarization characteristics of stable and evolutionary sub-pulse drifting in pulsars

Abstract

Even after decades of their discovery, pulsar emission mechanism remains an enigmatic phenomenon. Radio emission from pulsars exhibits a plethora of phenomena that modulate their pulse-to-pulse emission, observed in the formof subpulse drifting, nulling, mode changing, etc., thereby providing a range of avenues to understand the complex physical processes governing pulsar emission. Subpulse drifting is a phenomenon wherein individual substructures within the pulses march in phase across the pulse window, revealing information about the pulsar beam intricacies. Full polarimetric studies of subpulse drifting, though challenging and hence limited, can, in principle, shed light on the ‘spark’ geometry and its evolution required to explain this phenomenon. PSR J0026–1955 is a newly discovered pulsar, which has been observed to exhibit highly peculiar subpulse drifting behavior, including mode switching It also exhibits a high nulling fraction of approximately 58%. Our study presents the first polarization analysis of the subpulse drifting behavior in this pulsar utilizing the high-sensitivity full polarimetric data from the upgraded Giant Meterwave Radio Telescope (uGMRT). In this investigation, we focus on the subpulse drifting in full polarimetric mode, orthogonal polarization mode (OPM) switching, and drift-synchronous polarization modulation. We examine the polarization properties and associated parameters and compare their evolution over the various subpulse drifting modes, and in particular, we compare the stable and evolutionary drift modes. Our work has revealed notable changes in the polarization angle (PA) at peak longitude, supporting the rotating vector model proposed by Radhakrishnan and Cooke. Observations of drift rate changes suggest the dominance of the evolutionary modes, which are transient (short-lasting) compared to the stable drifting modes. Using the Stokes parameters and derived polarization plots, we have identified synchronous drift modulation of the position angle, alongside periodic behaviour in ellipticity angle plots and circular polarisation Plots. We analyzed the averaged position angle (PA) curve for modes A0 and B. Mode A0 displayed a shallower slope, while mode B exhibited a steeper slope. This observation supports the nested carousel model, which suggests that B modes originate from sparks closer to the magnetic axis, whereas A0 modes are generated by sparks located farther away. Additionally, our examination of the ellipticity angle plots for both A0 and B modes indicates that the behavior is more circular in mode B compared to mode A0. This suggests that regions closer to the magnetic axis exhibit greater circularity, while circularity in polarisation diminishes as we move away from the magnetic axis.