Synchrotron Polarization Research Articles

STATISTICAL DESCRIPTION OF SYNCHROTRON INTENSITY FLUCTUATIONS: STUDIES OF ASTROPHYSICAL MAGNETIC TURBULENCE

We provide a theoretical description of synchrotron fluctuations arising from magnetic turbulence. We derive an expression that relates the correlation of synchrotron fluctuations for an arbitrary index of relativistic electrons with the correlations of squared fluctuations of magnetic field component perpendicular to the line of sight. The latter correlations we study assuming that the turbulence is axisymmetric. We obtain general relations valid for an arbitrary model of magnetic turbulence and analyse the relations for particular example of magnetic turbulence that is supported by numerical simulations. We predict that the synchrotron intensity fluctuations are anisotropic with larger correlation present along the direction of magnetic field. This anisotropy is dominated by the quadrupole component with the ratio between quadrupole and monopole parts being sensitive to the compressibility of underlying turbulence. Our work opens avenues for quantitative studies of magnetic turbulence in our galaxy and beyond using synchrotron emission. It also outlines the directions of how synchrotron foreground emission can be separated from cosmological signal, i.e. from CMB or highly redshifted HI emission. For the sake of completeness we also provide the expressions for the synchrotron polarization (Stocks parameters and their combinations) for the model of axisymmetric magnetic turbulence.

THREE-DIMENSIONAL SIMULATIONS OF MAGNETOHYDRODYNAMIC TURBULENCE BEHIND RELATIVISTIC SHOCK WAVES AND THEIR IMPLICATIONS FOR GAMMA-RAY BURSTS

Relativistic astrophysical phenomena such as gamma-ray bursts (GRBs) and active galactic nuclei often require long-lived strong magnetic field. Here, we report on three-dimensional special-relativistic magnetohydrodynamic (MHD) simulations to explore the amplification and decay of macroscopic turbulence dynamo excited by the so-called Richtmyer-Meshkov instability (RMI; a Rayleigh-Taylor type instability). This instability is an inevitable outcome of interactions between shock and ambient density fluctuations. We find that the magnetic energy grows exponentially in a few eddy turnover times, and then, following the decay of kinetic turbulence, decays with a temporal power-law exponent of -0.7. The magnetic-energy fraction can reach $epsilon_B \sim$ 0.1 but depends on the initial magnetic field strength. We find that the magnetic energy grows by at least two orders of magnitude compared to the magnetic energy immediately behind the shock. This minimum degree of the amplification does not depend on the amplitude of the initial density fluctuations, while the growth timescale and the maximum magnetic energy depend on the degree of inhomogeneity in the density. The transition from Kolmogorov cascade to MHD critical balance cascade occurs at $\sim$ 1/10th the initial inhomogeneity scale, which limits the maximum synchrotron polarization to less than 2%. New results include the avoidance of electron cooling with RMI turbulence, the turbulent photosphere model via RMI, and the shallow decay of the early afterglow from RMI. We also performed a simulation of freely decaying turbulence with relativistic velocity dispersion. We find that relativistic turbulence begins to decay much faster than one eddy-turnover time because of fast shock dissipation, which does not support the relativistic turbulence model by Narayan & Kumar.

Poling effect and piezoelectric response in high-strain ferroelectric 0.70Pb(Mg1/3Nb2/3)O3–0.30PbTiO3 crystal

In situ high-resolution synchrotron x-ray diffraction, dielectric permittivity, hysteresis loop, and polarization current, were used to investigate phase transitions of (211)-cut 0.70Pb(Mg1/3Nb2/3)O3–0.30PbTiO3 single crystal before and after an electric (E) field poling. A rhombohedral (R)–tetragonal (T)–cubic transition sequence was observed upon zero-field heating in both unpoled and poled samples. Before the R–T transition takes place, an extra dielectric and polarization current anomalies near 365 K were observed in the poled sample due to a transition of polarization ordering. The direct piezoelectric coefficient d33 exhibits a rapid increase for poling at E=1.0–1.3 kV/cm, followed by an overpoling behavior. The increment of polarization ordering plays an important role while the high piezoelectric response builds up.

Cassiopeia A: dust factory revealed via submillimetre polarimetry

If Type II supernovae – the evolutionary end points of short-lived, massive stars – produce a significant quantity of dust (>0.1 M⊙) then they can explain the rest-frame far-infrared emission seen in galaxies and quasars in the first Gyr of the Universe. Submillimetre (submm) observations of the Galactic supernova remnant, Cas A, provided the first observational evidence for the formation of significant quantities of dust in Type II supernovae. In this paper, we present new data which show that the submm emission from Cas A is polarized at a level significantly higher than that of its synchrotron emission. The orientation is consistent with that of the magnetic field in Cas A, implying that the polarized submm emission is associated with the remnant. No known mechanism would vary the synchrotron polarization in this way and so we attribute the excess polarized submm flux to cold dust within the remnant, providing fresh evidence that cosmic dust can form rapidly. This is supported by the presence of both polarized and unpolarized dust emission in the north of the remnant where there is no contamination from foreground molecular clouds. The inferred dust polarization fraction is unprecedented (fpol∼ 30 per cent) which, coupled with the brief time-scale available for grain alignment (<300 yr), suggests that supernova dust differs from that seen in other Galactic sources (where fpol= 2−7 per cent) or that a highly efficient grain alignment process must operate in the environment of a supernova remnant.

Magnetic Field Structure from Synchrotron Polarization

Total magnetic fields in spiral galaxies, as observed through their total synchrotron emission, are strongest (up to ≲30 µ G) in the spiral arms. The degree of radio polarization is low; the field in the arms must be mostly turbulent or tangled. Polarized synchrotron emission shows that the resolved regular fields are generally strongest in the interarm regions (up to ≲15 µ G), sometimes forming “magnetic arms” parallel to the optical arms. The field structure is spiral in almost every galaxy, even in flocculent and bright irregular types which lack spiral arms. The observed large-scale patterns of Faraday rotation in several massive spiral galaxies reveal coherent regular fields, as predicted by dynamo models. However, in most galaxies observed so far no simple patterns of Faraday rotation could be found. Either many dynamo modes are superimposed and cannot be resolved by present-day telescopes, or most of the apparently regular field is in fact anisotropic random , with frequent reversals, due to shearing and compressing gas flows. In galaxies with massive bars, the polarization pattern follows the gas flow. However, around strong shocks in bars, the compression of the regular field is much lower than that of the gas; the regular field decouples from the cold gas and is strong enough to affect the flow of the diffuse warm gas. – The average strength of the total magnetic field in the Milky Way is 6µ G near the sun and increases to 20–40 μ G in the Galactic center region. The Galactic field is mostly parallel to the plane, except in the center region. Rotation measure data from pulsars indicate several field reversals, unlike external galaxies, but some reversals could be due to distortions of the nearby field.

Simulated synchrotron emission from pulsar wind nebulae

Aims. A complete set of diagnostic tools aimed at producing synthetic synchrotron emissivity, polarization, and spectral index maps from relativistic MHD simulations is presented. As a first application we consider here the case of the emission from Pulsar Wind Nebulae (PWNe). Methods. The proposed method is based on the addition, on top of the basic set of MHD equations, of an extra equation describing the evolution of the maximum energy of the emitting particles. This equation takes into account adiabatic and synchrotron losses along streamlines for the distribution of emitting particles and its formulation is such that it is easily implemented in any numerical scheme for relativistic MHD. Results. Application to the axisymmetric simulations of PWNe, analogous to those described by Del Zanna et al. (2004), allows direct comparison between the numerical results and observations of the inner structure of the Crab Nebula, and similar objects, in the optical and X-ray bands. We are able to match most of the observed features typical of PWNe, like the equatorial torus and the polar jets, with velocities in the correct range, as well as finer emission details, like arcs, rings and the bright knot, that turn out to arise mainly from Doppler boosting effects. Spectral properties appear to be well reproduced too: detailed spectral index maps are produced for the first time and show softening towards the PWN outer borders, whereas spectral breaks appear in integrated spectra. The emission details are found to strongly depend on both the average wind magnetization, here σeff ≈ 0.02, and on the magnetic field shape. Conclusions. Our method, in spite of its simplicity, provides a realistic modeling of synchrotron emission properties, and twodimensional axisymmetric relativistic MHD simulations appear to be well suited to explain the main observational features of PWNe.

Polarization of synchrotron radiation from conical shock waves

Simulated images of synchrotron intensity and polarization are presented for a simple, semi-dynamical model of conical shock waves in an astrophysical jet. Earlier work is extended by inclusion of a component of upstream magnetic field parallel to the jet in addition to the tangled (or disordered) component considered in the earlier paper. Results for several cases representing shocks of moderate strength are shown. It is found that the on-axis polarization reflects the upstream magnetic field structure. Off-axis, the electric field of polarization is oblique to the axis and covers a range depending on the shock cone angle and viewing angle. The results are compared with the structure of a bright knot about 0.8 arcsec from the nucleus in the quasar 3C 380, which may be an example of this kind of structure.

The symmetry of the Be(0 0 0 1) surface states as determined by ARPES with variable polarization synchrotron radiation

We measured the electronic band structure of the Be(0001) surface using high-resolution angle resolved photoemission with variable polarization synchrotron radiation. We used photons of 32.5eV delivered by the beamline APE at Elettra, with the two orthogonal linear polarizations. The electronic band structure along the two (Γ¯–K¯andΓ¯–M¯) high symmetry directions was probed. As a function of photon polarization either states of even or odd symmetry with respect to the crystal reflection plane(s) contribute to the photoemission intensity. We find that the two surface states have different reflection symmetry, according to the theoretical predictions.

Transrelativistic Synchrotron Emissivity, Cross Section, and Polarization

The spectrum and polarization produced by particles spiraling in a magnetic field undergo dramatic changes as the emitters transition from nonrelativistic to relativistic energies. However, none of the currently available methods for calculating the characteristics of this radiation field are adequate for the purpose of sustaining accuracy and speed of computation in the intensity, and none even attempt to provide a means of determining the polarization fraction other than in the cyclotron or synchrotron limits. But the transrelativistic regime, which we here find to lie between $5\times 10^7$ K and $5\times 10^9$ K for a thermal plasma, is becoming increasingly important in high-energy astrophysical environments, such as in the intra-cluster medium, and in the accretion flows of supermassive black holes. In this paper, we present simple, yet highly accurate, fitting formulae for the magnetobremsstrahlung (also known as cyclo-synchrotron) emissivity, its polarization fraction (and content), and the absorption cross-section. We demonstrate that both the harmonic and high-energy limiting behavior are well represented, incurring at most an error of $\sim 5%$ throughout the transition region.

Polarization in the inner region of pulsar wind nebulae

We present here the first effort to compute synthetic synchrotron polarization maps of Pulsar Wind Nebulae (PWNe). Our goal is to highlight how polarization can be used as an additional diagnostic tool for the flow structure in the inner regions of these nebulae. Recent numerical simulations suggest the presence of flow velocities ∼0.5 c in the surroundings of the termination shock, where most of the high energy emission comes from. We construct polarization maps taking into account relativistic effects like Doppler boosting and position angle swing. The effect of different bulk velocities is clarified with the help of a toy-model consisting of a uniformly emitting torus. We also present a map based on recent numerical simulations of the entire nebula and compare it with presently available data. The comparison with upcoming high resolution observations could provide new insight into the inner structure of the nebula and put constraints on the geometrical properties of the magnetic field.

Surface electron bands and Fermi surface of Be(0001)

The three surface electronic states of Be(0001) have different symmetry as probed by high-resolution angle resolved photoemission (ARPES) with variable polarization synchrotron radiation. We probed the two ($\overline{\ensuremath{\Gamma}}\text{\ensuremath{-}}\overline{M}$ and $\overline{\ensuremath{\Gamma}}\text{\ensuremath{-}}K$) high symmetry directions and measured the Fermi surface cuts. Electron states of even or odd symmetry with respect to the crystal (and/or surface) mirror planes contribute to the photoemission intensity as a function of the light polarization and experimental geometry. The effects of the large surface relaxation of Be(0001) are directly reflected in the Fermi surface.

The large-scale polarization of the microwave foreground

Most of the useful information about inflationary gravitational waves and reionization is on large angular scales where Galactic foreground contamination is the worst, so a key challenge is to model, quantify and remove polarized foregrounds. We use the Leiden radio surveys to quantify the polarized synchrotron radiation at large angular scales, which is likely to be the most challenging polarized contaminant for the WMAP satellite. We find that the synchrotron E- and B-contributions are equal to within 10% from 408–820 MHz with a hint of E-domination at higher frequencies. We quantify Faraday rotation and depolarization effects and show that they cause the synchrotron polarization percentage to drop both towards lower frequencies and towards lower multipoles.

Large-scale polarization of the microwave background and foreground

The DASI discovery of cosmic microwave background (CMB) polarization has opened a new chapter in cosmology. Most of the useful information about inflationary gravitational waves and reionization is on large angular scales where galactic foreground contamination is the worst, so a key challenge is to model, quantify, and remove polarized foregrounds. We use the POLAR experiment, COBE/DMR and radio surveys to provide the strongest limits to date on the TE cross-power spectrum of the CMB on large angular scales and to quantify the polarized synchrotron radiation, which is likely to be the most challenging polarized contaminant for the WMAP satellite. We find that the synchrotron E and B contributions are equal to within 10% from 408--820 MHz with a hint of E domination at higher frequencies. We quantify Faraday rotation and depolarization effects in the two-dimensional $(\mathcal{l},\ensuremath{\nu})$ plane and show that they cause the synchrotron polarization percentage to drop both towards lower frequencies and towards lower multipoles.

A submillimetre imaging polarimeter at the James Clerk Maxwell Telescope

A polarimeter has been built for use with the Submillimetre Common-User Bolometer Array (SCUBA), on the James Clerk Maxwell Telescope (JCMT) in Hawaii. SCUBA is the first of a new generation of highly sensitive submillimetre cameras, and the UK/Japan Polarimeter adds a polarimetric imaging/photometry capability in the wavelength range 350 to 2000 μm. Early science results range from measuring the synchrotron polarization of the black hole candidate Sgr A* to mapping magnetic fields inferred from polarized dust emission in Galactic star-forming clouds. We describe the instrument design, performance, observing techniques and data reduction processes, along with an assessment of the current and future scientific capability.

Macromolecules in the galaxy and the cosmic microwave background

A deep search for the predicted Galactic microwave dipole emission of spinning interstellar fullerene-type molecules has been conducted using the RATAN-600 radio telescope. This effect is of interest both in its own right and as a new form of Galactic screen between the cosmic microwave background (CMB) and the observer. The power of this noise component on angular scales of about 0.1° (l=1000) is estimated. These scales are of primary interest for the “Cosmological Gene of the Universe” (RATAN-600), PLANCK Surveyor Mission, and other projects. At the frequencies of the expected emission peak of this dust component, the small-scale noise is a factor of 20 weaker than the predicted noise. It is shown that the role of this dust component is negligible, at least at the main PLANCK frequencies. The first estimates of the contribution from Galactic polarization noise are given for λ=3.9 cm, where the theory predicts a maximum in the CMB polarization (l∼1000). At wavelengths of ∼1 cm and shorter, macromolecules contribute less than 1 µK, and should not hinder CMB polarization studies. Galactic synchrotron polarization, likewise, should not prevent CMB polarization experiments at the main PLANCK frequencies.

ITAL]Chandra[/ITAL] Detection of the Forward and Reverse Shocks in Cassiopeia A

We report the localization of the forward and reversed shock fronts in the young supernova remnant Cas-A using X-ray data obtained with the Chandra Observatory. High resolution X-ray maps resolve a previously unseen X-ray feature encompassing the extremity of the remnant. This feature consists of thin, tangential wisps of emission bordering the outer edge of the thermal X-ray and radio remnant, forming a circular rim, approx. 2.7 in radius. Radio images show a sharp rise in brightness at this X-ray rim, along with a large jump in the synchrotron polarization angle. These characteristics suggest that these wisps are the previously unresolved signature of the forward, or outer, shock. Similarly, we identify the sharp rise in emissivity of the bright shell for both the radio and X-ray line emission associated with the reverse shock. The derived ratio of the averaged forward and reverse shock radii of approx. 3:2 constrains the remnant to have swept up roughly the same amount of mass as was ejected; this suggests that Cas-A is just entering the Sedov phase. Comparison of the X-ray spectra from the two shock regions shows that the equivalent widths of prominent emission lines are significantly lower exterior to the bright shell, as expected if they are respectively identified with the shocked circumstellar material and shocked ejecta. Furthermore, the spectrum of the outer rim itself is dominated by power-law emission, likely the counterpart of the non-thermal component previously seen at energies above 10 keV.

Galactic and extragalactic magnetic fields

The current state of research of the Galactic magnetic field is reviewed critically. The average (equipartition) strength of the total field derived from radio synchrotron data is 6 +/- 2 muG locally and about 10 +/- 3 muG at 3 kpc Galactic radius. These values agree well with the estimates using the locally measured cosmic-ray energy spectrum and the radial variation of protons derived from gamma-rays. Optical and synchrotron polarization data yield a strength of the local regular field of 4 +/- 1 muG, but this value is an upper limit if the field strength fluctuates within the beam or if anisotropic fields are present. Pulsar rotation measures, on the other hand, give only 1.4 +/- 0.2 muG, a lower limit if fluctuations in regular field strength and thermal electron density are anticorrelated along the pathlength. The local regular field may be part of a 'magnetic arm' between the optical arms. However, the global structure of the regular Galactic field is not yet known. Several large-scale field reversals in the Galaxy were detected from rotation measure data, but a similar phenomenon was not observed in external galaxies. The Galactic field may be young in terms of dynamo action so that reversals from the chaotic seed field are preserved, or a mixture of dynamo modes causes the reversals, or the reversals are signatures of large-scale anisotropic field loops. The Galaxy is surrounded by a thick disk of radio continuum emission of similar extent as in edge-on spiral galaxies. While the local field in the thin disk is of even symmetry with respect to the plane (quadrupole), the global thick-disk field may be of dipole type. The Galactic center region hosts highly regular fields of up to milligauss strength which are oriented perpendicular to the plane.

Total electron yield exafs studies of (001) Au/Co monocrystalline multilayers

Total Electron Yield (TEY) Co K edge Extended X-ray Absorption Fine Structure (EXAFS) experiments have been performed on (001) monocrystalline Au/Co multilayers. The interface is oriented parallel to the synchrotron polarization vector in order to probe preferentially the in-plane local environment. This study shows that the thin Co layers have a body centered tetragonal structure (bct), i.e. a flattened [001] bcc structure. A Co-Au mixing at the interface has also been put in evidence.

Magnetic field in a turbulent galactic disk

Abstract A simple kinematic model has been applied to simulate the evolution of the interstellar magnetic field permanently twisted by turbulent gas motions accompanied by effects of the field diffusion. The magnetic field was found to develop well-ordered twisted structures over the whole gas parcel analyzed. This field configuration has a preferred sense of twisting dependent on the helicity of the gas flow. The model of synchrotron emission and polarization of M31-like galaxy containing magnetic field structures taken from our simulations reproduces quite well its basic observed radio and polarization properties.

Characterisation of a source of x-ray synchrotron radiation

Abstract The spectral and angular distribution of synchrotron radiation is sensitive to the motion of the electrons within the electron beam source. By measuring the energy dependence of the X-ray intensity scattered from a powder sample at different heights above and below the centre of a synchrotron beam, the source can be characterised. Measurements were carried out on the Daresbury synchrotron radiation source operating with two different values of vertical source emittance, which allows the electron motion to be modelled in both cases when calculating the synchrotron radiation flux and polarisation.