Discovery of Phase-Locked Intermittent Brightenings in PSR B1919+21

Abstract

Context. In single-pulse observations of PSR B1919+21, we identify a previously unreported emission phenomenon, which we term phase-locked intermittent brightenings (PLIBs).

Aims. We aim to characterise the phenomenology and polarimetric properties of PLIBs and to explore possible physical interpretations of their origin.

Methods. Using highly sensitive, high time-resolution polarimetric observations of PSR B1919+21 obtained with the Arecibo Observatory, we analysed flux-density variations, pulse-phase stability, and polarisation-vector trajectories in the Stokes Q–U plane, with particular emphasis on episodes of enhanced emission.

Results. Phase-locked intermittent brightenings are characterised by flux enhancements up to an order of magnitude above the mean level, occurring intermittently but at a fixed rotational phase (l = −5°). They appear in both polarised and unpolarised emission and are associated with a ∼90° change in the linear polarisation position angle, a transition of the Q–U vector from quadrant IV to quadrant II, and a reversal in the handedness of circular polarisation (Stokes V). Following a PLIB event, the Q–U vector executed a complete 360° clockwise rotation before returning to quadrant II; the characteristic Q–U-plane ‘torus’ structure at l = 0° is likewise consistently confined to quadrant II.

Conclusions. We interpret PLIBs as signatures of spatially localised but temporally intermittent processes operating within the neutron-star magnetosphere. Possible mechanisms include magnetic reconnection or surface instabilities, acting within a magnetic configuration that can be represented schematically as a double-helix structure embedded in a cylindrical magnetic tube. Within this phenomenological framework, the rotation of the double-helix structure with a period of 2 P3 provides a qualitative explanation for the observed evolution of the polarisation plane. Meanwhile, the rotational distortion of magnetic field lines associated with the neutron star’s spin offers a plausible interpretation of the observed orthogonal polarisation mode transitions. The occurrence and modulation properties further suggest that the underlying structure has relatively low magnetic helicity. Overall, we propose a phenomenological framework that integrates these elements and provides a unified, qualitative interpretation of the emission and polarisation behaviour associated with PLIBs.