On the Pulse Intensity Modulation of PSR B0823+26

We investigate the radio emission behaviour of PSR B0823+26, a pulsar which is known to undergo pulse nulling, using an 153-d intensive sequence of observations. The pulsar is found to exhibit both short (~min) and unusually long-term (~hours or more) nulls, which not only suggest that the source possesses a distribution of nulling timescales, but that it may also provide a link between conventional nulling pulsars and longer-term intermittent pulsars. Despite seeing evidence for periodicities in the pulsar radio emission, we are uncertain whether they are intrinsic to the source, due to the influence of observation sampling on the periodicity analysis performed. Remarkably, we find evidence to suggest that the pulsar may undergo pre-ignition periods of 'emission flickering', that is rapid changes between radio-on (active) and -off (null) emission states, before transitioning to a steady radio-emitting phase. We find no direct evidence to indicate that the object exhibits any change in spin-down rate between its radio-on and -off emission modes. We do, however, place an upper limit on this variation to be <= 6 % from simulations. This indicates that emission cessation in pulsars does not necessarily lead to large changes in spin-down rate. Moreover, we show that such changes in spin-down rate will not be discernible in the majority of objects which exhibit short-term (<= 1 d) emission cessation. In light of this, we predict that many pulsars could exhibit similar magnetospheric and emission properties to PSR B0823+26, but which have not yet been observed.Comment: 13 pages, 11 figures, accepted for publication in MNRAS; 1 reference correcte

On the pulse-width statistics in radio pulsars. III. Importance of the conal profile components

This work is a continuation of two previous papers of a series, in which we examined the pulse-width statistics of normal radio pulsars. In the first paper we compiled the largest ever database of pulsars with interpulses in their mean profiles. In the second one we confirmed the existence of the lower boundary in the scatter plot of core component pulse-widths versus pulsar period W50 sim 2.5 P^{-0.5}[deg], first discovered by Rankin using much smaller number of interpulse cases. In this paper we show that the same lower boundary also exists for conal profile components. Rankin proposed a very simple method of estimation of pulsar inclination angle based on comparing the width W50 of its core component with the period dependent value of the lower boundary. We claim that this method can be extended to conal components as well. To explain an existence of the lower boundary Rankin proposed that the core emission originates at or near the polar cap surface. We demonstrated clearly that no coherent pulsar radio emission can originate at altitudes lower than 10 stellar radii, irrespective of the actual mechanism of coherence. We argue that the lower boundary reflects the narrowest angular structures that can be distinguished in the average pulsar beam. These structures represent the core and the conal components in mean pulsar profiles. The P^{-0.5} dependence follows from the dipolar nature of magnetic field lines in the radio emission region, while the numerical factor of about 2.5 deg reflects the curvature radius of a non-dipolar surface magnetic field in the partially screened gap above the polar cap, where dense electron-positron plasma is created. Both core and conal emission should originate at altitudes of about 50 stellar radii in a typical pulsar, with a possibility that the core beam is emitted at a slightly lower heights than the conal ones.Comment: 13 pages, 5 figure

The Parkes Pulsar Timing Array Project

A "pulsar timing array" (PTA), in which observations of a large sample of pulsars spread across the celestial sphere are combined, allows investigation of "global" phenomena such as a background of gravitational waves or instabilities in atomic timescales that produce correlated timing residuals in the pulsars of the array. The Parkes Pulsar Timing Array (PPTA) is an implementation of the PTA concept based on observations with the Parkes 64-m radio telescope. A sample of 20 millisecond pulsars is being observed at three radio-frequency bands, 50cm (~700 MHz), 20cm (~1400 MHz) and 10cm (~3100 MHz), with observations at intervals of 2 - 3 weeks. Regular observations commenced in early 2005. This paper describes the systems used for the PPTA observations and data processing, including calibration and timing analysis. The strategy behind the choice of pulsars, observing parameters and analysis methods is discussed. Results are presented for PPTA data in the three bands taken between 2005 March and 2011 March. For ten of the 20 pulsars, rms timing residuals are less than 1 microsec for the best band after fitting for pulse frequency and its first time derivative. Significant "red" timing noise is detected in about half of the sample. We discuss the implications of these results on future projects including the International Pulsar Timing Array (IPTA) and a PTA based on the Square Kilometre Array. We also present an "extended PPTA" data set that combines PPTA data with earlier Parkes timing data for these pulsars

On the pulse--width statistics in radio pulsars. I. Importance of the interpulse emission

We performed Monte Carlo simulations of different properties of pulsar radio emission, such as: pulsar periods, pulse-widths, inclination angles and rates of occurrence of interpulse emission (IP). We used recently available large data sets of the pulsar periods P, the pulse profile widths W and the magnetic inclination angle alpha. We also compiled the largest ever database of pulsars with interpulse emission, divided into the double-pole (DP-IP) and the single-pole (SP-IP) cases. Their distribution on the P - Pdot diagram strongly suggests a secular alignment of the magnetic axis from the originally random orientation. We derived possible parent distribution functions of important pulsar parameters by means of the Kolmogorov-Smirnov significance test using the available data sets (P, W, alpha and IP), different models of pulsar radio beam rho = rho(P) as well as different trial distribution functions of pulsar period and the inclination angles. The best suited parent period distribution function is the log-normal distribution, although the gamma function distribution cannot be excluded. The strongest constraint on derived model distribution functions was the requirement that the numbers of interpulses were exactly (within 1sigma errors) at the observed level of occurrences. We found that a suitable model distribution function for the inclination angle is the complicated trigonometric function which has two local maxima, one near 0 deg and the other near 90 deg. The former and the latter implies the right rates of IP occurrence. It is very unlikely that the pulsar beam deviates significantly from the circular cross-section. We found that the upper limit for the average beaming factor fb describing a fraction of the full sphere (called also beaming fraction) covered by a pulsar beam is about 10%. This implies that the number of the neutron stars in the Galaxy might be underestimated.Comment: 35 pages, 18 figure

On the mean profiles of radio pulsars I: Theory of the propagation effects

We study the influence of the propagation effects on the mean profiles of radio pulsars using the Kravtsov-Orlov method of the wave propagation in the inhomogeneous media. This approach allows us firstly to include into consideration the transition from geometrical optics to vacuum propagation, the cyclotron absorption, and the wave refraction simultaneously. In addition, arbitrary non-dipole magnetic field configuration, drift motion of plasma particles, and their realistic energy distribution are taken into account. The one-to-one correspondence between the signs of circular polarization and position angle (p.a.) derivative along the profile for both ordinary and extraordinary waves is predicted. Using the numerical integration we now can model the main profiles of radio pulsars. It is shown that standard S-shape form of the p.a. swing can be realized for small enough pair production multiplicity and large enough bulk plasma Lorentz factor only. It is also shown that the value of p.a. maximum derivative, that is often used for determination the angle between magnetic dipole and rotation axis, depends on the plasma parameters and could differ from the rotation vector model (RVM) prediction.Comment: 20 pages, 16 figures, accepted MNRA

Pulsar Coherent Radio Emission from Solitons: Average Emission Properties

Observations have established that coherent radio emission from pulsars arises at a few hundred kilometers above the stellar surface. Recent polarization studies have further demonstrated that plasma instabilities are necessary for charge bunching that gives rise to coherent emission. The formation of charged solitons in the electron–positron plasma is the only known bunching mechanism that can be realized at these heights. More than five decades of observations have revealed a number of emission features that should emerge from any valid radio emission mechanism. We have carried out numerical calculations to find the features of average emission from curvature radiation due to charged solitons. The characteristic curvature radiation spectrum has been updated from the well-known one-dimensional dependence to a general two-dimensional form, and the contribution from each soliton along the observer’s line of sight (LOS) has been added to reproduce the pulsar emission. The outflowing plasma is formed by sparking discharges above the stellar surface that are located within concentric rings resembling the core–cone emission beam, and uniform distribution of solitons along any LOS has been assumed. The observed effects of radius-to-frequency mapping, where the lower-frequency emission originates from higher altitudes, are seen in this setup. The power-law spectrum and relative steepening of the core spectra with respect to the cones also emerge. The estimated polarization position angle reflects the geometrical configuration of pulsars as expected. These studies demonstrate the efficacy of coherent curvature radiation from charged solitons to reproduce the average observational features of pulsars