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

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. 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. 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 of less than 35 deg. 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. 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.Comment: 16 pages, 9 figures, accepted for publication in A&

Phase-locked modulation delay between the poles of pulsar B1055-52

We present a detailed single pulse study of PSR B1055-52 based on observations at the Parkes radio telescope. The radio emission is found to have a complex modulation dominated by a periodicity of ~20 times its rotational period P (0.197s), whose phase and strength depends on pulse longitude. This periodicity exhibits a phase-locked delay of about 2.5P between the main pulse (MP) and interpulse (IP), presumed to be the opposite poles of the pulsar. This delay corresponds to a light travel distance of many times the light cylinder radius. More complex modulations are found within the MP on timescales down to about 9P, and both these and the principal modulation vary strongly across the (at least) 7 components which the MP and IP exhibit. The nature of the single pulse emission, which ranges from smooth and longitudinally extended to `spiky', is also component-dependent. Despite these disparities, the total pulse intensity distributions at the MP and IP are virtually identical in shape, suggesting a common emission mechanism. In an attempt to account for the complex modulations we examine a number of physical models, both intrinsic (which presuppose the pulsar to be an isolated neutron star) and extrinsic (appealing to the presence of circumstellar material to modulate the emission). Significant objections can be made to each model, so this pulsar's behaviour patterns remain a crucial challenge to theorists.Comment: 13 pages, 8 figures, submitted to MNRA

Pulsar bi-drifting: implications for polar cap geometry

For many years it has been considered puzzling how pulsar radio emission, supposedly created by a circulating carousel of sub-beams, can produce the driftbands demonstrated by PSR J0815+09, and more recently PSR B1839-04, which simultaneously drift in opposing directions. Here we suggest that the carousels of these pulsars, and hence their beams, are not circular but elliptical with axes tilted with respect to the fiducial plane. We show that certain relatively unusual lines of sight can cause bi-drifting to be observed, and a simulation of the two known exemplars is presented. Although bi-drifting is rare, non-circular beams may be common among pulsars and reveal themselves by having profile centroids displaced from the fiducial plane identified by polarisation position angle swings. They may also result in profiles with asymmetric and frequency-dependent component evolution. It is further suggested that the carousels may change their tilt by specific amounts and later reverse them. This may occur suddenly, accompanying a mode change (e.g. PSR B0943+10), or more gradually and short-lived as in "flare" pulsars (e.g. PSR B1859+07). A range of pulsar behaviour (e.g. the shifting drift patterns of PSRs B0818-41 and B0826-34) may also be the result of non-circular carousels with varying orientation. The underlying nature of these carousels - whether they are exclusively generated by polar cap physics or driven by magnetospheric effects - is briefly discussed.Comment: 13 pages, 6 figures, accepted for publication in MNRAS, updated acknowledgement

Single pulse modeling and the bi-drifting subpulses of radio pulsar B1839-04

We study the bi-drifting pulsar B1839-04, where the observed subpulse drift direction in the two leading pulse components is opposite from that in the two trailing components. Such diametrically opposed apparent motions challenge our understanding of an underlying structure. We find that for the geometry spanned by the observer and the pulsar magnetic and rotation axes, the observed bi-drifting in B1839-04 can be reproduced assuming a non-dipolar configuration of the surface magnetic field. Acceptable solutions are found to either have relatively weak $(\sim 10^{12} \,{\rm G})$ or strong $(\sim 10^{14} \,{\rm G})$ surface magnetic fields. Our single pulse modeling shows that a global electric potential variation at the polar cap that leads to a solid-body-like rotation of spark forming regions is favorable in reproducing the observed drift characteristics. This variation of the potential additionally ensures that the variability is identical in all pulse components resulting in the observed phase locking of subpulses. Thorough and more general studies of pulsar geometry show that a low ratio of impact factor to opening angle $(\beta / \rho)$ increases the likelihood of bi-drifting to be observed. We thus conclude that bi-drifting is visible when our line of sight crosses close to the magnetic pole.Comment: 15 pages, 14 figures, accepted for publication in Ap

Evidence for magnetospheric effects on the radiation of radio pulsars

We have conducted the largest investigation to date into the origin of phase resolved, apparent RM variations in the polarized signals of radio pulsars. From a sample of 98 pulsars based on observations at 1.4 GHz with the Parkes radio telescope, we carefully quantified systematic and statistical errors on the measured RMs. A total of 42 pulsars showed significant phase resolved RM variations. We show that both magnetospheric and scattering effects can cause these apparent variations. There is a clear correlation between complex profiles and the degree of RM variability, in addition to deviations from the Faraday law. Therefore, we conclude that scattering cannot be the only cause of RM variations, and show clear examples where magnetospheric effects dominate. It is likely that, given sufficient signal-to-noise, such effects will be present in all radio pulsars. These signatures provide a tool to probe the propagation of the radio emission through the magnetosphere.Comment: 20 pages, 8 Figures, accepted for publication in MNRA

Correlated emission and spin-down variability in radio pulsars

The recent revelation that there are correlated period derivative and pulse shape changes in pulsars has dramatically changed our understanding of timing noise as well as the relationship between the radio emission and the properties of the magnetosphere as a whole. Using Gaussian processes we are able to model timing and emission variability using a regression technique that imposes no functional form on the data. We revisit the pulsars first studied by Lyne et al. (2010). We not only confirm the emission and rotational transitions revealed therein, but reveal further transitions and periodicities in 8 years of extended monitoring. We also show that in many of these objects the pulse profile transitions between two well-defined shapes, coincident with changes to the period derivative. With a view to the SKA and other telescopes capable of higher cadence we also study the detection limitations of period derivative changes.Comment: 4 pages, 2 Figures, Proceedings of IAU Symposium 337 "Pulsar Astrophysics - The Next 50 Years" held at Jodrell Bank Observatory, UK Sept. 4-8 201

The Subpulse Modulation Properties of Pulsars and its Frequency Dependence

A large sample of about two hundred pulsars have been observed to study their subpulse modulation at an observing wavelength of (when achievable) both 21 and 92 cm using the Westerbork Synthesis Radio Telescope. For 57 pulsars drifting subpulses are discovered for the first time and are confirmed for many others. This leads to the conclusion that it could well be that the drifting subpulse mechanism is an intrinsic property of the emission mechanism itself, although for some pulsars it is difficult or impossible to detect. It appears that the youngest pulsars have the most disordered subpulses and the subpulses become more and more organized into drifting subpulses as the pulsar ages. Drifting subpulses are in general found at both frequencies and the measured values of P3 at the two frequencies are highly correlated, showing the broadband nature of this phenomenon. Also the modulation indices measured at the two frequencies are clearly correlated, although at 92 cm they are on average possibly higher. The correlations with the modulation indices are argued to be consistent with the picture in which the radio emission is composed out of a drifting subpulse signal plus a quasi-steady signal which becomes, on average, stronger at high observing frequencies. There is no obvious correlation found between P3 and the pulsar age (or any other pulsar parameter) contrary to reports in the past.Comment: Proceedings of the 40 Years of Pulsars: Millisecond Pulsars, Magnetars and More conference in Montrea

The glitch-induced identity changes of PSR J1119-6127

We demonstrate that the high-magnetic field pulsar J1119-6127 exhibits three different types of behaviour in the radio band. Trailing the "normal" profile peak there is an "intermittent" peak and these components are flanked by two additional components showing very erratic "RRAT-like" emission. Both the intermittent and RRAT-like events are extremely rare and are preceded by a large amplitude glitch in the spin-down parameters. The post-glitch spin-down rate is smaller than the pre-glitch rate. This type of relaxation is very unusual for the pulsar population as a whole, but is observed in the glitch recovery of a RRAT. The abnormal emission behaviour in PSR J1119-6127 was observed up to three months after the epoch of the large glitch, suggestive of changes in the magnetospheric conditions during the fast part of the recovery process. We argue that both the anomalous recoveries and the emission changes could be related to reconfigurations of the magnetic field. Apart from the glitches, the spin-down of PSR J1119-6127 is relatively stable, allowing us to refine the measurement of the braking index (n=2.684\pm0.002) using more than 12 years of timing data. The properties of this pulsar are discussed in light of the growing evidence that RRATs do not form a distinct class of pulsar, but rather are a combination of different extreme emission types seen in other neutron stars. Different sub-classes of the RRATs can potentially be separated by calculating the lower limit on the modulation index of their emission. We speculate that if the abnormal behaviour in PSR J1119-6127 is indeed glitch induced then there might exist a population of neutron stars which only become visible in the radio band for a short duration in the immediate aftermath of glitch activity. These neutron stars will be visible in the radio band as sources that only emit some clustered pulses every so many years.Comment: 20 pages, 10 figures, Accepted for publication in MNRA