Investigation of the bi-drifting subpulses of radio pulsar B1839−04 utilising the open-source data-analysis project PSRSALSA

Aims. The usefulness and versatility of the PSRSALSA open-source pulsar data-analysis project is demonstrated through an analysis of the radio pulsar B1839−04. This study focuses on the phenomenon of bi-drifting, an effect where the drift direction of subpulses is systematically different in different pulse profile components. Bi-drifting is extremely rare in the pulsar population, and the theoretical implications are discussed after comparing B1839−04 with the only other known bi-drifter. Methods. Various tools in PSRSALSA, including those allowing quantification of periodicities in the subpulse modulation, their flux distribution, and polarization properties, are exploited to obtain a comprehensive picture of the radio properties of PSR B1839−04. In particular, the second harmonic in the fluctuation spectra of the subpulse modulation is exploited to convincingly demonstrate the existence of bi-drifting in B1839−04. Bi-drifting is confirmed with a completely independent method allowing the average modulation cycle to be determined. Polarization measurements were used to obtain a robust constraint on the magnetic inclination angle. Results. The angle between the rotation and magnetic axis is found to be smaller than 35°. Two distinct emission modes are discovered to be operating, with periodic subpulse modulation being present only during the weaker mode. Despite the variability of the modulation cycle and interruption by mode-changes, the modulation pattern responsible for the bi-drifting is strictly phase locked over a timescale of years such that the variability is identical in the different components. Conclusions. The phase locking implies that a single physical origin is responsible for both drift directions. Phase locking is hard to explain for many models, including those specifically proposed in the literature to explain bi-drifting, and they are therefore shown to be implausible. It is argued that within the framework of circulating beamlets, bi-drifting could occur if the circulation were severely distorted, possibly by distortions in the magnetic field