Bunch Expansion as a Cause for Pulsar Radio Emissions

Electromagnetic waves due to electron–positron clouds (bunches), created by cascading processes in pulsar magnetospheres, have been proposed to explain the pulsar radio emission. In order to verify this hypothesis, we utilized for the first time Particle-in-Cell (PIC) code simulations to study the nonlinear evolution of electron–positron bunches dependant on the initial relative drift speeds of electrons and positrons, plasma temperature, and distance between the bunches. For this sake, we utilized the PIC code ACRONYM with a high-order field solver and particle weighting factor, appropriate to describe relativistic pair plasmas. We found that the bunch expansion is mainly determined by the relative electron–positron drift speed. Finite drift speeds were found to cause the generation of strong electrostatic superluminal waves at the bunch density gradients that reach up to E ∼ 7.5 × 105 V cm−1 (E/(m e c ω p e −1) ∼ 4.4) and strong plasma heating. As a result, up to 15% of the initial kinetic energy is transformed into the electric field energy. Assuming the same electron and positron distributions, we found that the fastest (in the bunch reference frame) particles of consecutively emitted bunches eventually overlap in momentum (velocity) space. This overlap causes two-stream instabilities that generate electrostatic subluminal waves with electric field amplitudes reaching up to E ∼ 1.9 × 104 V cm−1 (E/(m e c ω p e −1) ∼ 0.11). We found that in all simulations the evolution of electron–positron bunches may lead to the generation of electrostatic superluminal or subluminal waves, which, in principle, can be behind the observed electromagnetic emissions of pulsars in the radio wave range.

HISTORY OF LOW-FREQUENCY RESEARCH OF PULSARS

Purpose: The main most pronounced events, which occurred in the initial period of the pulsars’ study at the decameter wavelength range, are presented. The example of the main scientific problems, which were formulated at the very beginning of pulsar research, shows how the emphasis and priorities of these studies have been changing over time, which tasks have finally been solved, and which are still waiting to be solved. It is shown how the ongoing modernization of the UTR-2 radio telescope have allowed to acquire new qualities in astrophysical research being made with this radio telescope and to identify new scientific directions. The example of the cited references shows how the pulsar research efforts in Ukraine have been developed and how they were integrated into the world astrophysical research of these unique objects. The purpose of this work is to show the relationship between the past and the present on the example of pulsars for longer than a semi-centennial period and to show how the scientific problems that were formulated in the past, and which could not be solved under the then-existing technical conditions, were solved by the subsequent generations of researchers. Design/methodology/approach: The methods of comparison and historical parallels show how the low-frequency studies of pulsars have been developed and evolved almost from their discovery until now. Findings: It is shown how quantitative transformations and technical development, as well as non-standard scientific approaches, unhackneyed thought and international cooperation allow to solve complex radio astronomical problems related to the low-frequency studies of pulsars. Conclusions: The paper provides a historical overview of more than half a century-long radio astronomical studies of pulsars, having been and still being made at the decameter band using the UTR-2 radio telescope. The “old” and current priorities in pulsar research are given, and it is shown how qualitatively the technical parameters of back end facility and computer performance have been changed in studying the coherent pulsar radio emission nature. Key words: aberration; frequency band; pulse; interpulse; dispersion measure; rotation measure; plasma; pulsar; radio telescope

DETERMINATION OF THE ROTATION MEASURE VALUE SIGN WHEN RECEIVING A SINGLE LINEAR POLARIZATION OF THE PULSAR RADIO EMISSION

Purpose: The studies of pulsars allow enriching our knowledge in determination of parameters of both the exotic electron-positron plasma in the pulsar magnetosphere with strong magnetic field and the ordinary ion-electron plasma of the interstellar medium, which exists in a weak magnetic field. To determine the parameters of the both plasma types it is reasonable to use polarization characteristics of a pulsed radio emission of pulsars. An accurate determination of these characteristics is quite a complex problem. For its solving, primarily we have to determine two parameters of the propagation medium – its dispersion and rotation measures. Their absolute values can be determined with the relative precision of 10-4, but the problem of rotation measure value sign determination arises. This sign depends on the interstellar magnetic field direction along the line of sight. Hear, a new method of rotation measure value sign determination is proposed. Design/methodology/approach: Muller polarization matrices are usually used for determination of such a propagation parameter as the rotation measure absolute value. When only one linear polarization is received, using of these matrices allows quite accurate determining the absolute value of the rotation measure, but not the sign of this parameter due to a certain symmetry of these matrices with respect to the direction of the linear polarization rotation plane. If we complement the system of equations, which determines the rotation measure value, with some new additional components, which take into account the contributions of the Earth ionosphere and magnetosphere to the rotation measure value, one can notice that this contribution is always positive in the Southern magnetic hemisphere (the majority of the Northern geographical hemisphere) and is always negative in the Northern magnetic hemisphere (the majority of the Southern geographical hemisphere). Moreover, the absolute value of this contribution is maximal at noon and minimal at midnight, when the concentration of ions in the Earth ionosphere is maximal and minimal, respectively. Accounting for these regularities allows to determine not only the absolute value of the rotation measure, but also its sign by means of two independent time-shifted estimations of the observed absolute value of this parameter for various ionization degrees of the Earth ionosphere. Findings: We show that using of additional equations, which take into account the contribution of the Earth ionosphere and magnetosphere to the value of the rotation measure parameter, allows full determination of this parameter accounting for the sign of this value even for the antennas, which can record a single linear polarization only. This approach allows to determine all polarization parameters of the pulsar radio emission as well as of the pulsed or continuum polarized radio emission of other cosmic sources. Conclusions: The paper presents the results of measurement of the rotation measure for the two closest to the Earth pulsars, namely J0814+7429 (B0809+74), J0953+0755 (B0950+08), and the comparison of the proposed technique for this parameter determination with other existing techniques. Key words: pulse, dispersion measure, rotation measure, plasma, polarization, pulsar, radio telescope

A rotation-driven pulsar radio emission mechanism

ABSTRACT We propose and discuss an alternative pulsar radio emission mechanism that relies on rotation-driven plasma oscillations, rather than on a beam-driven instability, and suggest that it may be the generic radio emission mechanism for pulsars. We identify these oscillations as superluminal longitudinal waves in the pulsar plasma and point out that these waves can escape directly in the O mode. We argue that the frequency of the oscillations is ω0 ≈ ωp(2〈γ〉)1/2/γs, where γs is the Lorentz factor of bulk streaming motion and 〈γ〉 is the mean Lorentz factor in the rest frame of the plasma. The dependence of the plasma frequency ωp on radial distance implies a specific frequency-to-radius mapping, ω0∝r−3/2. Escape of the energy in these oscillations is possible if they are generated in overdense, field-aligned regions that we call fibres; the wave energy is initially refracted into underdense regions between the fibres, which act as ducts. Some implications of the model for the interpretation of pulsar radio emission are discussed.

Pulsar radio emission mechanisms: a critique

ABSTRACT We consider critically the three most widely favoured pulsar radio emission mechanisms: coherent curvature emission (CCE), beam-driven relativistic plasma emission (RPE), and anomalous Doppler emission (ADE). We assume that the pulsar plasma is 1D, streaming outwards with a bulk Lorentz factor γs ≫ 〈γ〉 − 1 ≳ 1, where 〈γ〉 is the intrinsic spread in the rest frame of the plasma. We argue that the formation of beams in a multicloud model is ineffective in the intrinsically relativistic case for plausible parameters because the overtaking takes too long. We argue that the default choice for the particle distribution in the rest frame is a Jüttner distribution and that relativistic streaming should be included by applying a Lorentz transformation to the rest-frame distribution, rather than the widely assumed relativistically streaming Gaussian distribution. We find that beam-driven wave growth is severely restricted by (a) the wave properties in pulsar plasma, (b) a separation condition between beam and background, and (c) the inhomogeneity of the plasma in the pulsar frame. The growth rate for the kinetic instability is much smaller and the bandwidth of the growing waves is much larger for a Jüttner distribution than for a relativistically streaming Gaussian distribution. No reactive instability occurs at all for a Jüttner distribution. We conclude that none of CCE, RPE, and ADE is tenable as the generic pulsar radio emission mechanism for ‘plausible’ assumptions about the pulsar plasma.

Pulsar polarimetry with the Parkes ultra-wideband receiver

ABSTRACT Pulsar radio emission and its polarization are observed to evolve with frequency. This frequency dependence is key to the emission mechanism and the structure of the radio beam. With the new ultra-wideband receiver (UWL) on the Parkes radio telescope we are able, for the first time, to observe how pulsar profiles evolve over a broad continuous bandwidth of 700–4000 MHz. We describe here a technique for processing broad-band polarimetric observations to establish a meaningful alignment and visualize the data across the band. We apply this to observations of PSRs J1056–6258 and J1359–6038, chosen due to previously unresolved questions about the frequency evolution of their emission. Application of our technique reveals that it is possible to align the polarization position angle (PA) across a broad frequency range when constrained to applying only corrections for dispersion and Faraday rotation to do so. However, this does not correspond to aligned intensity profiles for these two sources. We find that it is possible to convert these misalignments into emission height range estimates that are consistent with published and simulated values, suggesting that they can be attributed to relativistic effects in the magnetosphere. We discuss this work in the context of the radio beam structure and prepare the ground for a wider study of pulsar emission using broad-band polarimetric data.

Gravitational burst radiation from pulsars in the Galactic centre and stellar clusters

ABSTRACT Pulsars (PSRs) orbiting intermediate or supermassive black holes at the centre of galaxies and globular clusters are known as Extreme Mass Ratio Binaries (EMRBs) and have been identified as precision probes of strong-field GR. For appropriate orbital parameters, some of these systems may also emit gravitational radiation in a ‘burst-like’ pattern. The observation of this burst radiation in conjunction with the electromagnetic radio timing signal would allow for multimessenger astronomy in strong-field gravitational regimes. In this work we investigate gravitational radiation from these PSR-EMRBs, calculating the waveforms and SNRs and explore the influence of this GW on the pulsar radio signal. We find that for typical PSR-EMRBs, gravitational burst radiation should be detectable from both the Galactic centre and the centre of stellar clusters, and that this radiation will not meaningfully affect the pulsar timing signal, allowing PSR-EMRB to remain ‘clean’ test-beds of strong-field GR.