Pulsar Polarization Research Articles

Wideband polarization and spectral properties of 18 high Galactic latitude pulsars

The broad-spectrum polarization and spectral characteristics of pulsars contain crucial information about the origin of their radio emission. These properties, together with pulsar flux density variations, can also be used to guide future surveys of radio pulsars and probe the Galactic interstellar medium. In this paper, we present studies of 18 pulsars at high Galactic latitudes using the Ultra-Wideband Low (UWL) receiver of the Parkes radio telescope. For these pulsars, we measured their wideband flux densities, spectral indices, and polarization fractions. We obtain seven new rotation measures (RMs) and refine the RMs of another ten pulsars. In this sample of pulsars, we observed significant variations in their flux densities, suggesting that previous shallow surveys were likely to miss a population of pulsars at high galactic latitude. In addition, we identified a previously reported isolated pulsar (PSR J1947−18) as a potential binary system.

Improving pulsar polarization and timing measurements with the Nançay Radio Telescope

Context. Accurate polarimetric calibration of the radio pulse profiles from pulsars is crucial for studying their radiation properties at these wavelengths. Additionally, inaccurate calibration can distort recorded pulse profiles, introducing noise in time of arrival (TOA) data and thus degrading pulsar timing analyses. One method for determining the full polarimetric response of a given telescope is to conduct observations of bright polarized pulsars over wide ranges of parallactic angles, to sample different orientations of their polarization angle and in turn determine the cross-couplings between polarization feeds. Aims. The Nançay decimetric Radio Telescope (NRT) is a 94 m equivalent meridian telescope, capable of tracking a given pulsar for approximately 1 h around transit. The NRT therefore cannot sample wide ranges of parallactic angles when observing a given pulsar, so until late 2019 the polarimetric calibration of 1.4 GHz pulsar observations with the NRT was rudimentary. We therefore aimed to develop a method for improving the calibration of NRT observations, overcoming the above-mentioned limitation. Ultimately, our goal was to improve the quality of NRT pulsar timing, with better calibrated pulsar pulse profiles. Methods. In November 2019, we began conducting regular observations of the bright and highly linearly polarized pulsar PSR J0742−2822, in a special observing mode in which the feed horn rotates by ~180° over the course of the 1 h observation, mimicking wide parallactic angle variations and in principle enabling us to determine the polarimetric response of the NRT at 1.4 GHz. In addition, we assessed the quality of the NRT timing of a selection of millisecond pulsars (MSPs), namely, J1730−2304, J1744−1134, and J1857+0953, with conventional TOAs extracted from total intensity pulse profiles, and TOAs extracted with the Matrix Template Matching (MTM) technique, designed to compensate for putative polarimetric calibration errors. Results. From the analysis of the rotating horn observations of PSR J0742−2822 we could determine the cross-couplings between the polarization feeds and also constrain the Stokes parameters of the noise diode signal, which prior to this work was erroneously assumed to be ideal and was used as the only reference source for the calibration of pulsar observations. The improved polarimetric response of the NRT as determined from these observations was applied to observations of a selection of MSPs with published polarimetric properties. We find that the new polarimetric profiles and polarization position angles are consistent with previous findings, unlike NRT polarimetric results obtained with the previously used method of calibration. The analysis of the timing data shows that the new calibration method improves the quality of the timing, and the MTM method proves very effective at reducing noise from imperfect calibration. For pulsars with sufficient degrees of polarization, the MTM method appears to be the preferred method of extracting TOAs from NRT observations.

FAST Pulsar Database. I. Polarization Profiles of 682 Pulsars

Pulsar polarization profiles form a very basic database for understanding the emission processes in a pulsar magnetosphere. After careful polarization calibration of the 19-beam L-band receiver and verification of beam-offset observation results, we obtain polarization profiles of 682 pulsars from observations by the Five-hundred-meter Aperture Spherical radio Telescope (FAST) during the Galactic Plane Pulsar Snapshot survey and other normal FAST projects. Among them, polarization profiles of about 460 pulsars are observed for the first time. The profiles exhibit diverse features. Some pulsars have a polarization position angle curve with a good S-shaped swing, some with orthogonal modes; some have components with highly linearly polarized components or strong circularly polarized components; some have a very wide profile, coming from an aligned rotator, and some have an interpulse from a perpendicular rotator; some wide profiles are caused by interstellar scattering. We derive geometric parameters for 190 pulsars from the S-shaped position angle curves or with orthogonal modes. We find that the linear and circular polarization or the widths of pulse profiles have various frequency dependencies. Pulsars with a large fraction of linear polarization are more likely to have a large Edot.

Three-dimensional magnetothermal evolution of off-centred dipole magnetic field configurations in neutron stars

ABSTRACT Off-centred dipole configurations have been suggested to explain different phenomena in neutron stars, such as natal kicks, irregularities in polarization of radio pulsars and properties of X-ray emission from millisecond pulsars. Here, for the first time, we model magnetothermal evolution of neutron stars with crust-confined magnetic fields and off-centred dipole moments. We find that the dipole shift decays with time if the initial configuration has no toroidal magnetic field. The decay time-scale is inversely proportional to magnetic field. The octupole moment decreases much faster than the quadrupole. Alternatively, if the initial condition includes strong dipolar toroidal magnetic field, the external poloidal magnetic field evolves from centred dipole to off-centred dipole. The surface thermal maps are very different for configurations with weak B = 1013 G and strong B = 1014 G magnetic fields. In the former case, the magnetic equator is cold while in the latter case, it is hot. We model light curves and spectra of our magnetothermal configurations. We found that in the case of cold equator, the pulsed fraction is small (below a few per cent in most cases) and spectra are well described with a single blackbody. Under the same conditions, models with stronger magnetic fields produce light curves with pulsed fraction of tens of per cent. Their spectra are significantly better described with two blackbodies. Overall, the magnetic field strength has a more significant effect on bulk thermal emission of neutron stars than does the field geometry.

Pulsar polarization: a partial-coherence model

ABSTRACT The population of radio pulsars is observed to demonstrate certain polarization properties not explained by the conventional picture of pulsar polarization, namely frequency evolution of polarization, deviations of the linear polarization angle from a curve of geometric origins, and the presence of features in the circular polarization. We present the partial-coherence model as a way to explain the co-occurrence of these features and to provide an origin for circular polarization in radio pulsar profiles. We describe the mathematics of the model and demonstrate how it can explain these observed features, both on a population level and for the idiosyncrasies of individual pulsars. The partial coherence model can account for complex polarization behaviour, enabling improved access to information about pulsar geometries. We discuss the scientific implications of this for our understanding of pulsar radio emission and propagation.

Meterwavelength Single Pulse Polarimetric Emission Survey. VI. Toward Understanding the Phenomenon of Pulsar Polarization in Partially Screened Vacuum Gap Model

We have observed 123 pulsars with periods longer than 0.1 s in the Meterwavelength Single Pulse Polarimetric Emission Survey. In this work, a detailed study of the polarization behavior of these pulsars has been carried out. We were able to fit the rotating vector model to the polarization position angle sweeps in 68 pulsars, and in 34 pulsars the emission heights could be measured. In all cases the radio emission was constrained to arise below 10% of the light cylinder radius. In pulsars with low spin-down energy loss, erg s−1, we found the mean fractional linear polarization of the individual time samples in single pulses to be around 0.57 (57%), which is significantly larger than the fractional linear polarization of 0.29 (29%) obtained from the average profiles. On the other hand, the mean fractional circular polarization of the individual time samples in single pulses is around 0.08 (8%), similar to the measurements from the average profiles. To explain the observed polarization features, we invoke the partially screened vacuum gap model of pulsars, where dense spark-associated plasma clouds exist with high pair plasma multiplicity, with significant decrease of density in the regions between the clouds that are dominated by iron ions. The coherent radio emission is excited by curvature radiation from charge bunches in these dense plasma clouds and escape as linearly polarized waves near cloud boundaries. We suggest that the circular polarization arises due to propagation of waves in the low-pair-multiplicity, ion-dominated inter-cloud regions.

Pulsar Polarization Arrays.

Pulsar timing arrays (PTAs) consisting of widely distributed and well-timed millisecond pulsars can serve as a galactic interferometer to measure gravitational waves. With the same data acquired for PTAs, we propose to develop pulsar polarization arrays (PPAs), to explore astrophysics and fundamental physics. As in the case of PTAs, PPAs are best suited to reveal temporal and spatial correlations at large scales that are hard to mimic by local noise. To demonstrate the physical potential of PPAs, we consider detection of ultralight axionlike dark matter (ALDM), through cosmic birefringence induced by its Chern-Simons coupling. Because of its tiny mass, the ultralight ALDM can be generated as a Bose-Einstein condensate, characterized by a strong wave nature. Incorporating both temporal and spatial correlations of the signal, we show that PPAs have a potential to probe the Chern-Simons coupling up to ∼10^{-14}-10^{-17} GeV^{-1}, with a mass range ∼10^{-27}-10^{-21} eV.

Pulsar polarization: a broad-band population view with the Parkes Ultra-Wideband receiver

ABSTRACTThe radio polarization properties of the pulsar population are only superficially captured by the conventional picture of pulsar radio emission. We study the broadband polarization of 271 young radio pulsars, focusing particularly on circular polarization, using high-quality observations made with the Ultra-Wideband Low receiver on Murriyang, the Parkes radio telescope. We seek to encapsulate polarization behaviour on a population scale by defining broad categories for frequency- and phase-dependent polarization evolution, studying the co-occurrences of these categorizations and comparing them with average polarization measurements and spin-down energy ($\dot{E}$). This work shows that deviations of the linear polarization position angle from the rotating vector model are linked to the presence of circular polarization features and to frequency evolution of the polarization. Polarization fraction, circular polarization contribution, and profile complexity all evolve with $\dot{E}$ across the population, with the profiles of high-$\dot{E}$ pulsars being simple and highly linearly polarized. The relationship between polarization fraction and circular contribution is also seen to evolve such that highly polarized profiles show less variation in circular contribution with frequency than less strongly polarized profiles. This evolution is seen both across the population and across frequency for individual sources. Understanding pulsar radio polarization requires detailed study of individual sources and collective understanding of population-level trends. For the former, we provide visualizations of their phase- and frequency-resolved polarization parameters. For the latter, we have highlighted the importance of including the impact of circular polarization and of $\dot{E}$.

Source of circular polarization in radio pulsars

ABSTRACT It is known that the concept of limiting polarization introduced 70 yr ago by K. G. Budden has the capacity to explain the magnitude of circular polarization seen in normal pulsars with light-cylinder radii of the order of 109–10 cm under the assumption of a high-multiplicity electron–positron plasma. However, a review of limiting polarization under the same assumption in millisecond pulsars indicates that it is inapplicable there because the region of limiting polarization lies far outside the light cylinder. This paper, using the ion–proton model, evaluates circular polarization both generally and specifically for J2144−3933, and gives a fairly detailed understanding of the observations in normal pulsars including the change of sign as a function of frequency seen in J0908−4913. But it also fails to explain circular polarization in millisecond pulsars owing to the smaller particle number densities and birefringence of the magnetosphere in these objects. However, the review of limiting polarization finds that, within the ion–proton model, this distinct process can describe their circular polarization. It is argued that certain features of millisecond pulsar Stokes profiles are clearly consistent with limiting polarization.

A broad-band perspective on circular polarization in radio pulsar observations

AbstractWe investigate the broad-band behaviour of circular polarization in radio pulsar profiles and show the relationship between polarization fraction and what proportion of that polarization is circular, both across frequency, and for a large number of pulsars viewed collectively. The behaviour observed may be explained by pulsar polarization originating from the partially-coherent combination of two linearly-polarized orthogonal modes with different flux spectral indices. (See also the poster in the “supplementary information”.)

Starquake ends hiatus in soft X-ray polarimetry

Measurements with a CubeSat gas pixel detector reveal a change in the Crab pulsar polarization after a glitch in the spin period, suggesting that starquakes alter the magnetosphere.

Artefacts of circumpolar cartography in radio pulsar polarization

ABSTRACT Single-pulse data on radio pulsar polarization are traditionally presented in the form of two-dimensional grey-scale patterns with the pulse longitude and polarization angle (PA) on the horizontal and vertical axis, respectively. Such diagrams reveal several enigmatic polarization effects: (1) bifurcations and loops of PA curve under central pulse components, (2) vertical spread of flux at all PA values, (3) exchange of power content between PA tracks of two orthogonal polarization modes (OPMs), and (4) peripherically flat PA swings that span more than 180 deg. It is shown that all these phenomena result from passage of observed polarization state near the pure-V pole of Poincaré sphere. Much of their complexity results from cartographic transformation from Poincaré sphere to the longitude–PA plane. An odd number of near-pole passages produce apparent replacement of OPM power in the profile wings, although the same amount of flux keeps staying in each modal patch on the Poincaré sphere. The fitting of pulsar PA curves should therefore allow for transitions between the primary (strong) and secondary (weak) PA tracks. The Stokes space (or Poincaré sphere) representation of pulsar polarization data contains crucial polarization information and needs to accompany the traditional viewing if the published figures are to be fully useful for interpretation.

The Parkes Pulsar Timing Array project: second data release

AbstractWe describe 14 yr of public data from the Parkes Pulsar Timing Array (PPTA), an ongoing project that is producing precise measurements of pulse times of arrival from 26 millisecond pulsars using the 64-m Parkes radio telescope with a cadence of approximately 3 weeks in three observing bands. A comprehensive description of the pulsar observing systems employed at the telescope since 2004 is provided, including the calibration methodology and an analysis of the stability of system components. We attempt to provide full accounting of the reduction from the raw measured Stokes parameters to pulse times of arrival to aid third parties in reproducing our results. This conversion is encapsulated in a processing pipeline designed to track provenance. Our data products include pulse times of arrival for each of the pulsars along with an initial set of pulsar parameters and noise models. The calibrated pulse profiles and timing template profiles are also available. These data represent almost 21 000 h of recorded data spanning over 14 yr. After accounting for processes that induce time-correlated noise, 22 of the pulsars have weighted root-mean-square timing residuals of$<\!\!1\,\mu\text{s}$in at least one radio band. The data should allow end users to quickly undertake their own gravitational wave analyses, for example, without having to understand the intricacies of pulsar polarisation calibration or attain a mastery of radio frequency interference mitigation as is required when analysing raw data files.

Radio pulsar polarization as a coherent sum of orthogonal proper mode waves

ABSTRACT Radio pulsar polarization exhibits a number of complex phenomena that are classified into the realm of ‘beyond the rotating vector model’ (RVM). It is shown that these effects can be understood in geometrical terms, as a result of coherent and quasi-coherent addition of elliptically polarized natural mode waves. The coherent summation implies that the observed tracks of polarization angle (PA) do not always correspond to the natural propagation mode (NPM) waves. Instead, they are statistical average of coherent sum of the NPM waves, and can be observed at any (and frequency-dependent) distance from the natural modes. Therefore, the observed tracks of PA can wander arbitrarily far from the RVM, and may be non-orthogonal. For equal amplitudes of the NPM waves two pairs of orthogonal polarization modes (OPMs), displaced by 45°, can be observed, depending on the width of lag distribution. Observed pulsar polarization mainly results from two independent effects: the change of mode amplitude ratio and the change of phase lag. In the core region both effects are superposed on each other, which can produce so complex behaviour as observed in the cores of PSR B1933+16, B1237+25, and J0437−4715. Change of the phase lag with frequency ν is mostly responsible for the observed strong evolution of these features with ν. The coherent addition of orthogonal natural waves is a useful interpretive tool for the observed radio pulsar polarization.

Pulsar polarization: the view from the southern hemisphere

AbstractPulsar polarization has been a fruitful area of study since the first discovery of pulsars 50 years ago. Polarization gives information on the geometry of the star, the location of the radio emission in the magnetosphere, the physics behind the radio emission mechanism and a plethora of phenomenology. Here, I will restrict myself to a brief outline of recent work in pulsar polarization using observations taken with the Parkes radio telescope over the past decade.

Coherent origin of peculiar polarization in radio pulsars

AbstractThe observed polarization of radio pulsars involves several peculiar effects, such as comparable amount of orthogonal polarization modes (OPMs) which often bear the same handedness of circular polarisation V. In the average profiles of B1913+16 and B1933+16, orthogonal jumps of polarization angle (PA) are observed to occur at the maximum V, instead of V = 0. High levels of V are also observed in core components (eg. in B1237+25), where they are accompanied by strong distortions of PA from the rotating vector model. In weakly polarized emission, PA jumps by 45° are observed in B1919+21 and B0823+26. It is shown that all these peculiarities can be interpreted in a model which assumes coherent addition of waves in natural propagation modes.

The origin of radio pulsar polarization

The origin of radio pulsar polarization

Multiwavelength Polarization of Rotation-powered Pulsars

Abstract Polarization measurements provide strong constraints on models for emission from rotation-powered pulsars. We present multiwavelength polarization predictions showing that measurements over a range of frequencies can be particularly important for constraining the emission location, radiation mechanisms, and system geometry. The results assume a generic model for emission from the outer magnetosphere and current sheet in which optical to hard X-ray emission is produced by synchrotron radiation (SR) from electron–positron pairs and γ-ray emission is produced by curvature radiation (CR) or SR from accelerating primary electrons. The magnetic field structure of a force-free magnetosphere is assumed and the phase-resolved and phase-averaged polarization is calculated in the frame of an inertial observer. We find that large position angle (PA) swings and deep depolarization dips occur during the light-curve peaks in all energy bands. For synchrotron emission, the polarization characteristics are strongly dependent on photon emission radius with larger, nearly 180°, PA swings for emission outside the light cylinder (LC) as the line of sight crosses the current sheet. The phase-averaged polarization degree for SR is less that 10% and around 20% for emission starting inside and outside the LC, respectively, while the polarization degree for CR is much larger, up to 40%–60%. Observing a sharp increase in polarization degree and a change in PA at the transition between X-ray and γ-ray spectral components would indicate that CR is the γ-ray emission mechanism.

Effects of gravitational waves on the polarization of pulsars

The polarization of electromagnetic waves in the presence of a gravitational wave is analyzed. The rotation of the polarization angle and the Stokes parameters are deduced. A possible application to the detection of stochastic background of gravitational waves is proposed as a complement to the pulsar timing method.

Intrinsic instrumental polarization and high-precision pulsar timing

Radio telescopes are used to accurately measure the time of arrival (ToA) of radio pulses in pulsar timing experiments that target mostly millisecond pulsars (MSPs) due to their high rotational stability. This allows for detailed study of MSPs and forms the basis of experiments to detect gravitational waves. Apart from intrinsic and propagation effects, such as pulse-to-pulse jitter and dispersion variations in the interstellar medium, timing precision is limited in part by the following: polarization purity of the telescope's orthogonally polarized receptors, the signal-to-noise ratio (S/N) of the pulsar profile, and the polarization fidelity of the system. Using simulations, we present how fundamental limitations in recovering the true polarization reduce the precision of ToA measurements. Any real system will respond differently to each source observed depending on the unique pulsar polarization profile. Using the profiles of known MSPs we quantify the limits of observing system specifications that yield satisfactory ToA measurements, and we place a practical design limit beyond which improvement of the system results in diminishing returns. Our aim is to justify limits for the front-end polarization characteristics of next generation radio telescopes, leading to the Square Kilometre Array (SKA).