Polarization Of Pulsar Radio Emission Research Articles

Exponential Fluctuations in the Modes of Orthogonal Polarization in Pulsar Radio Emission

A statistical model for the polarization of pulsar radio emission is enhanced to account for the heavy modulation of the emission, the possible covariance of the Stokes parameters, and the observed asymmetries in the distributions of total intensity, polarization, and fractional polarization, by treating the intensities of the orthogonal polarization modes as exponential random variables. The model is used to derive theoretical distributions to compare with what is observed. The resulting distributions are unimodal and generally asymmetric. The unimodality arises from the model’s fundamental assumption that the orthogonal modes are superposed. The asymmetry originates primarily from different fluctuations in mode intensities. The distributions of fractional polarization are truncated at the degree of linear and circular polarization intrinsic to the modes. A number of observable parameters that quantify the statistical properties of the emission and its polarization are derived and are shown to be functions only of the ratio of the modes’ mean intensities, M, suggesting their spectra coevolve according to the frequency dependence of M. This particular implementation of the model requires the modes to fluctuate differently in order to replicate the observations. Given that a single underlying emission mechanism seems unlikely to selectively modulate the mode intensities, the different fluctuations are attributed either to different emission mechanisms for the modes or to mode-dependent propagation or scattering effects in the pulsar magnetosphere.

Orientation Angles of a Pulsar’s Polarization Vector

A statistical model of the polarization of pulsar radio emission is used to derive the general statistics of a polarization vector's orientation angles. The theoretical distributions are compared with orientation angle histograms computed from single-pulse, polarization observations of PSR B2020+28. The favorable agreement between the theoretical and measured distributions lends support to the underlying assumptions of the statistical model, and demonstrates, like recent work on other pulsars, that the handedness of circular polarization is associated with the radiation's orthogonally polarized modes. Comprehensive directional statistics of the vector's orientation angles are also derived, and are shown to follow the Watson bipolar and Fisher distributions in its limiting forms.

An empirical model for the polarization of pulsar radio emission

We present an empirical model for single pulses of radio emission from pulsars based on Gaussian probability distributions for relevant variables. The radiation at a specific pulse phase is represented as the superposition of radiation in two (approximately) orthogonally polarized modes (OPMs) from one or more subsources in the emission region of the pulsar. For each subsource, the polarization states are drawn randomly from statistical distributions, with the mean and the variance on the Poincare sphere as free parameters. The intensity of one OPM is chosen from a lognormal distribution, and the intensity of the other OPM is assumed to be partially correlated, with the degree of correlation also chosen from a Gaussian distribution. The model is used to construct simulated data described in the same format as real data: distributions of the polarization of pulses on the Poincare sphere and histograms of the intensity and other parameters. We concentrate on the interpretation of data for specific phases of PSR B0329+54 for which the OPMs are not orthogonal, with one well defined and the other spread out around an annulus on the Poincare sphere at some phases. The results support the assumption that the radiation emerges in two OPMs with closely correlated intensities, and that in a statistical fraction of pulses one OPM is invisible.

Linear polarization of pulsar radio emission

We suggest that the observed linear polarization of pulsar radio emission is due to features of the emission mechanism that produce electromagnetic waves in a narrow cone around a magnetic field line. As a result, emission collected along the line of sight does not come from a single magnetic field line but from many non-parallel lines, each of which acts on an elementary emitter to generate a single linearly polarized mode. The combined emission from the bunch of rays should be nearly unpolarized at the emission site. When the rays propagate outward, the polarization of the individual ray follows that of the normal mode until the polarization limiting radius (PLR) is reached. The resulting degree of linear polarization will depend on the details of the magnetic field distribution across the emitting bunch at the PLR. We show that for the model of Gedalin et al. the PLR is deep in the magnetosphere, and we demonstrate how the linear polarization forms.

Orthogonal Mode Polarization of Pulsar Radio Emission

We discuss a model for polarization of pulsar radio emission, based on the assumption that waves propagate in two elliptically polarized natural modes. Some results from numerical simulation of single pulses are discussed with emphasis on circular polarization, microstructures and single pulse statistics.

Radio Polarization of the Young High–Magnetic‐Field Pulsar PSR J1119−6127

We have investigated the radio polarization properties of PSR J1119� 6127, a recently discovered young radio pulsar with a large magnetic field. Using pulsar-gated radio imaging data taken at a center frequency of 2496 MHz with the Australia Telescope Compact Array, we have determined a rotation measure for the pulsar of +842 � 23 rad m � 2 . These data, combined with archival polarimetry data taken at a center frequency of 1366 MHz with the Parkes telescope, were used to determine the polarization characteristics of PSR J1119� 6127 at both frequencies. The pulsar has a fractional linear polarization of � 75% and � 55% at 1366 and 2496 MHz, respectively, and the profile consists of a single, wide component. This pulse morphology and high degree of linear polarization are in agreement with previously noticed trends for young pulsars (e.g., PSR J1513� 5908). A rotating vector (RV) model fit of the position angle of linear polarization over pulse phase using the Parkes data suggests that the radio emission comes from the leading edge of a conal beam. We discuss PSR J1119� 6127 in the context of a recent theoretical model of pulsar spin-down, which can in principle be tested with polarization and timing data from this pulsar. Geometric constraints from the RV fit are currently insufficient to test this model with statistical significance, but additional data may allow such a test in the future. Subject headings: polarization — pulsars: individual (PSR J1119� 6127) — radio continuum: stars — stars: neutron Pulsar polarimetry is one of the keys to understanding the process and geometry of radio emission from pulsars. In the rotating vector (RV) model (Radhakrishnan & Cooke 1969) the polarization of pulsar radio emission is linked to the emission geometry in such a way that as the pulsar rotates, the axis of linear polarization is aligned with the projected direction on the sky of the pulsar’s magnetic dipole axis. The pulsar’s emission geometry itself may be described by two angles, each measured from the pulsar’s angular

Three‐Dimensional Statistics of Radio Polarimetry

The measurement of radio polarization may be regarded as a three-dimensional statistical problem because the large photon densities at radio wavelengths allow the simultaneous detection of the three Stokes parameters which completely describe the radiation's polarization. The statistical nature of the problem arises from the fluctuating instrumental noise, and possibly from fluctuations in the radiation's polarization. A statistical model is used to derive the general, three-dimensional statistics that govern radio polarization measurements. The statistics are derived for specific cases of source-intrinsic polarization, with an emphasis on the orthogonal polarization modes in pulsar radio emission. The statistics are similar to those commonly found in other fields of the physical, biological, and Earth sciences. Given the highly variable linear and circular polarization of pulsar radio emission, an understanding of the three-dimensional statistics may be an essential prequisite to a thorough interpretation of pulsar polarization data.

Wave dispersion in gyrotropic relativistic pulsar plasmas.

Wave dispersion in a gyrotropic relativistic pulsar plasma is discussed. A pulsar plasma in general contains electrons, positrons, and possibly ions. Although electron-positron pairs are dominant in number density, charge neutrality is generally not satisfied and the gyrotropic terms need to be considered. These gyrotropic terms can lead to elliptical polarization that may be relevant for the observed circular polarization of pulsar radio emission. The wave dispersion and polarization are obtained numerically by calculating the response in terms of the three relativistic plasma dispersion functions. For waves propagating at an oblique angle (to the ambient magnetic field) a significant ellipticity requires the plasma to deviate substantially from the neutrality condition.

Pulsar Studies at High Radio Frequencies

AbstractPulsars were discovered at 81.5 MHz and a lot of the studies of these exciting objects have been made up to the present time at radio frequencies below 1.6 GHz. The reasons for this concentration on the low radio frequency characteristics of pulsars is the fact that the spectra are very steep and that very few radio telescopes exist that are capable of efficient operations at high radio frequencies. The Effelsberg 100-m radio telescope of the Max-Planck-Institut fur Radioastronomie operates regularly up to the frequency of 50 GHz and was used to study pulsars at cm/mm-wavelengths. In the southern skies the Parkes 64-m telescope has been used to study pulsars up to 8.4 GHz. One pulsar has been detected at 87 GHz with the 30-m Pico Veleta telescope of IRAM.The studies of pulsars over the whole frequency range are of great importance because this is necessary for the elucidation of the mechanism that is responsible for the pulsar emission. The high polarization of pulsar radio emission at lower radio frequencies has supported the hypothesis of a coherent emission mechanism, which is required to generate the high luminosity. It has been known for some time that pulsars, unlike other radio sources, have a lower polarization at high radio frequencies. Recently a change of pulsar spectrum, a flattening or possibly an inversion has been observed at the highest radio frequencies. The inversion of the pulsar spectrum seems to coincide with a complete depolarization of some pulsars.Millisecond pulsars are less luminous than normal pulsars. This makes them even more difficult to detect at higher radio frequencies. Recent observations have extended the spectra of ten millisecond pulsars up to 4.85 GHz. The results imply that millisecond pulsars have properties very similar to normal (slow) pulsars, which suggests similar emission mechanisms.

Propagation effects on polarization of pulsar radio emission

We consider the role of a propagation effect such as stimulated Raman scattering on the polarization of radio pulses. When an intense electromagnetic wave with frequency close to the plasma frequency interacts with the plasma in the pulsar magnetosphere, the incident wave undergoes stimulated Raman scattering. Using typical plasma and magnetic field parameters, we compute the growth rate and estimate the polarization properties of the scattered mode. Under some conditions, we find that the polarization properties of the scattered mode can become significantly different from those of the incident wave. The frequencies at which strong Raman scattering occurs in the outer parts of the magnetosphere fall into the observable radio band. In some pulsars, for example PSR B0628−28 and B1914+13 , a large and symmetric type of circular polarization has been observed. We propose that such an unusual circular polarization is produced by the propagation effects.

The Distribution of Fractional Linear Polarization and the Random Component of Polarization Position Angle

One of the most fascinating features of the polarization of pulsar radio emission is the occurrence of orthogonal modes of polarization. The origin of the modes is not clear, but, whatever their underlying nature, the modes will affect observables such as the instantaneous linear polarization. We derive an idealized distribution of fractional linear polarization assuming that the modes are superposed and compare our result with observations. We discuss how superposed modes may affect the position angle distributions.Consider a subpulse composed of superposed modes of orthogonal polarization. We assume that each mode is completely linearly-polarized with no dispersion in position angle. If the flux densities of the individual modes are represented by the independent random variablesX1andX2, then the linear polarization of the subpulse is related toX=X1−X2and its total intensity isY=X1+X2. We denote the means of the mode distributions byμ1andμ2and their standard deviations byσ1andσ2. The means are generally not the same because observed distributions of position angle (e.g. Stinebring et al. 1984) show that the modes generally do not occur with equal frequency. If the individual modes are normally-distributed with identical standard deviations,XandYare also normally-distributed, independent random variables. In this ideal case, the distribution of fractional polarization,Z=|X/Y|, is

Propagation Effects on the Polarization of Pulsar Radio Emission

The properties of the natural wave modes of a pulsar magnetosphere are derived in a simple way by appealing to the 'low-density limit'. The properties are evaluated explicitly for a general version of the cold plasma model which includes relativistic streaming motions, and it is argued that this model is probably adequate for pulsar magnetospheres. The observed circular polarization in pulsar radio emission could arise as a propagation effect; the conditions under which initially linear polarization could be converted into partially circularly polarized radiation are summarized. The observed 'orthogonal modes' of polarization could be due to components in the two natural modes having slightly different ray paths. The angular separation of the two rays is found to be a strong function of frequency (!!(), ~ 4 x 108 ne/ /2), and it is suggested that a study of the frequency dependence of 'orthogonal modes' could provide useful information.