Polarization Of Radio Sources Research Articles

Evidence of isotropy on large distance scales from polarizations of radio sources

We test the isotropy of radio polarization angle orientations with a robust and reliable dual-frequency polarimetric survey of active galactic nuclei (AGNs). We find that the polarization orientations are consistent with the assumption of isotropy for scales larger than or equal to ∼800 Mpc. This provides further evidence for isotropy at large distance scales and is likely to impose strong constraints on some of the physical mechanisms that may be invoked to explain past observations of alignment of radio and optical polarizations at large distance scales.

MAGNETIC FIELD DISORDER AND FARADAY EFFECTS ON THE POLARIZATION OF EXTRAGALACTIC RADIO SOURCES

ABSTRACT We present a polarization catalog of 533 extragalactic radio sources that have a 2.3 GHz total intensity above 420 mJy from the S-band Polarization All Sky Survey, S-PASS, with corresponding 1.4 GHz polarization information from the NRAO VLA Sky Survey, NVSS. We studied the selection effects and found that fractional polarization, π, of radio objects at both wavelengths depends on the spectral index, the source magnetic field disorder, the source size, and depolarization. The relationship between depolarization, spectrum, and size shows that depolarization occurs primarily in the source vicinity. The median of resolved objects in NVSS is approximately two times larger than that of unresolved sources. Sources with little depolarization are ∼2 times more polarized than both highly depolarized and re-polarized sources. This indicates that intrinsic magnetic field disorder is the dominant mechanism responsible for the observed low fractional polarization of radio sources at high frequencies. We predict that number counts from polarization surveys will be similar at 1.4 GHz and at 2.3 GHz, for fixed sensitivity, although ∼10% of all sources may currently be missing because of strong depolarization. Objects with typically have simple Faraday structures, so they are most useful for background samples. Almost half of flat-spectrum ( ) and ∼25% of steep-spectrum objects are re-polarized. Steep-spectrum, depolarized sources show a weak negative correlation of depolarization with redshift in the range 0 < z < 2.3. Previous non-detections of redshift evolution are likely due the inclusion of re-polarized sources as well.

THE DISTRIBUTION OF POLARIZED RADIO SOURCES >15 μJy IN GOODS-N

We present deep VLA observations of the polarization of radio sources in the GOODS-N field at 1.4 GHz at resolutions of 1.6" and 10". At 1.6", we find that the peak flux cumulative number count distribution is N($>$p) $\sim$ 45 * (p/30$\mu$Jy)$^{-0.6}$ per square degree above a detection threshold of 14.5 $\mu$Jy. This represents a break from the steeper slopes at higher flux densities, resulting in fewer sources predicted for future surveys with the SKA and its precursors. It provides a significant challenge for using background RMs to study clusters of galaxies or individual galaxies. Most of the polarized sources are well above our detection limit, and are radio galaxies which are well-resolved even at 10", with redshifts from $\sim$0.2 - 1.9. We determined a total polarized flux for each source by integrating the 10" polarized intensity maps, as will be done by upcoming surveys such as POSSUM. These total polarized fluxes are a factor of 2 higher, on average, than the peak polarized flux at 1.6"; this would increase the number counts by $\sim$50% at a fixed flux level. The detected sources have rotation measures (RMs) with a characteristic rms scatter of $\sim$11$\frac{rad}{m^2}$ around the local Galactic value, after eliminating likely outliers. The median fractional polarization from all total intensity sources does not continue the trend of increasing at lower flux densities, as seen for stronger sources. The changes in the polarization characteristics seen at these low fluxes likely represent the increasing dominance of star-forming galaxies.

POLARIZATION ALIGNMENT IN JVAS/CLASS FLAT SPECTRUM RADIO SURVEYS

We present a detailed statistical analysis of the alignment of polarizations of radio sources at high redshift. This study is motivated by the puzzling signal of alignment of polarizations from distant quasars at optical frequencies. We use a coordinate invariant measure of dispersion to test for alignment of polarizations of widely separated sources. The data from JVAS/CLASS 8.4 GHz surveys are used for our study. We find that the data set with polarization flux greater than 1 mJy shows a significant signal of alignment at distance scales of order 150 Mpc. The significance of alignment decreases as we go to larger distances. In contrast the data set with polarization flux less than 1 mJy does not show significant alignment at short distances. However this set, as well as the full data sample, shows a very significant signal at large distances of order 500–850 Mpc. Due to the presence of relatively large error in the low polarization sample, this signal needs to be tested further with more refined data. We also study the signal by imposing a cut on the error in polarization. We find that the significance of alignment increases with decrease in fractional error. We are unable to attribute our results to known sources of bias. We discuss a possible physical explanation of our results.

A Fundamental Figure of Merit for Radio Polarimeters

This paper has been withdrawn to allow publication elsewhere.

Fractional polarization as a probe of magnetic fields in the intra-cluster medium

The aim of this work is to investigate the average properties of the intra-cluster medium (ICM) magnetic fields, and to search for possible correlations with the ICM thermal properties and cluster radio emission. We have selected a sample of 39 massive galaxy clusters from the HIghest X-ray FLUx Galaxy Cluster Sample, and used Northern VLA Sky Survey data to analyze the fractional polarization of radio sources out to 10 core radii from the cluster centers. Following Murgia et al (2004), we have investigated how different magnetic field strengths affect the observed polarized emission of sources lying at different projected distances from the cluster center. In addition, statistical tests are performed to investigate the fractional polarization trends in clusters with different thermal and non-thermal properties. We find a trend of the fractional polarization with the cluster impact parameter, with fractional polarization increasing at the cluster periphery and decreasing toward the cluster center. Such trend can be reproduced by a magnetic field model with central value of few $\mu$G. The logrank statistical test indicates that there are no differences in the depolarization trend observed in cluster with and without radio halo, while the same test indicates significant differences when the depolarization trend of sources in clusters with and without cool core are compared. The comparison between clusters with high and low temperatures does not yields significant differences. Although therole of the gas density should be better accounted for, these results give important indications for models that require a role of the ICM magnetic field to explain the presence of cool core and radio halos in galaxy clusters.

The role of magnetic fields in cluster cooling flows

Radial inflow and shear of the intracluster magnetic field will produce a dramatic increase in the strength of the field and will cause the field to become more radial within cooling flows. The magnetic pressure becomes comparable to the thermal pressure within a radius of typically 10 kpc. Within this region, reconnection of the magnetic field is efficient, and maintains the field in equipartition with the gas pressure. Buoyancy is ineffective in transporting the magnetic field out of the cooling flow. Reconnection releases the magnetic energy at a rate of 10^42^-10^43^ ergs s^-1^. Part of this energy may accelerate nonthermal, relativistic particles, resulting in diffuse radio emission from the central regions of cooling flows. The largest portion of the reconnection energy is likely to be dissipated in heating the cooling flow gas and may provide a major source of ionization and heating to the emission-line filaments in cooling flows. One dimensional, steady state, numerical models for cooling flows with magnetic fields show that the field has no dramatic effect on the structure of the cooling flow. The primary effect in the inner regions of cooling flows is to effectively increase the heat capacity of the gas by a factor of ~ 2. Also affected by a similar factor are the total gravitational mass derived by applying the hydrostatic equation to the gas pressure, and the accretion rates derived from the gas density and temperature profiles. The most important directly observable effect of the magnetic field in cooling flows is likely to be very strong Faraday rotation of the polarization of radio sources occurring within or behind the cooling flow. The rotation measure should increase rapidly towards the center of the cooling flow (roughly as r^-2^) and should reach values of RM ~ 10^3-4^ rad m^-2^ at projected radii within the equipartition radius. Comparison to several recent observations of the polarization of radio sources associated with the central galaxies in cooling flow clusters suggests that such large rotation measures may be common in cooling flows.

Solar noise storms coordinated observations: May 16?24, 1981

We present the main results obtained from the coordinated observations of solar radio noise storms organized by C.E.S.R.A. in May 1981. They concern the structure and polarization of radio sources, and the relation with the photospheric magnetic field. A model of coronal magnetic field, accounting for the observations, is briefly discussed.

Polarization of radio sources. VI - an oscillatory behavior of the intensity in a general solution of the radiation transfer problem in a plasma

view Abstract Citations (10) References (12) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Polarization of radio sources. VI. An oscillatory behavior of the intensity in a general solution of the radiation transfer problem in a plasma. Pacholczyk, A. G. ; Swihart, T. L. Abstract The solution for a general form of the transfer equation is given. The total intensity is found to oscillate under general conditions, although the oscillations vanish under most special cases discussed previously in the literature. The oscillations are a manifestation of the Faraday effects and their interaction with polarized absorption. Subject headings: plasmas - polarization - radiative transfer - radio sources Publication: The Astrophysical Journal Pub Date: February 1975 DOI: 10.1086/153399 Bibcode: 1975ApJ...196..125P Keywords: Plasma-Electromagnetic Interaction; Polarized Electromagnetic Radiation; Radiant Flux Density; Radiative Transfer; Radio Sources (Astronomy); Astronomical Models; Boundary Value Problems; Matrix Theory; Oscillations; Astrophysics full text sources ADS |

Measurements of the Circular Polarization of Radio Sources at Frequencies of 0·63, 1·4, 5·0 and 8·9 GHz

Circular polarization measurements were made with the Parkes 64 m telescope on 66 mainly extragalactic sources, 33 of which were studied at two or more frequencies. Circular polarization was definitely found in the eight quasi-stellar sources PKS0237-23, 0537-441, 1127-14, 1226+02 (3C273), 1253-05 (3C279), 2134+004, 2145+06 and 2345-16, as it was measured at the 4u level or higher in at least two observing sessions.

The Polarization of Radio Sources at 31 cm

Measurements are presented of the integrated polarizations and flux densities of 226 extragalactic radio sources at λ31 cm. Four sources with flat radio spectra have varied significantly in both polarization and total intensity over a period of one year.

Polarization of radio sources. V - Absorption effects on circular repolarization in compact sources

view Abstract Citations (11) References (18) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Polarization of radio sources. V. Absorption effects on circular repolarization in compact sources. Pacholczyk, A. G. ; Swihart, T. L. Abstract There are known difficulties in attributing the observed circular polarization of compact radio sources to synchrotron emission. Circular repolarization is a plausible alternative: it is consistent with the observed frequency dependence of polarization, with the magnetic field values resulting from synchrotron self-absorption theory, and avoids the known polarization sign-reversal difficulties in the neighborhood of the turnover. Application of the circular repolarization theory leads to the conclusion that the ratio of relativistic to thermal electron density in compact radio sources is between one and 100. Publication: The Astrophysical Journal Pub Date: September 1974 DOI: 10.1086/153094 Bibcode: 1974ApJ...192..591P Keywords: Circular Polarization; Electromagnetic Absorption; Polarization Characteristics; Radio Sources (Astronomy); Synchrotron Radiation; Electron Density (Concentration); Faraday Effect; Frequency Response; High Energy Electrons; Magnetic Fields; Quasars; Astrophysics full text sources ADS |

Observations of the linear polarization of radio sources at 2.8 and 4.5 CM

We present in this paper the results of a survey of the linear polarization at 2.8 and 4.5 cm of 22 QSR's, l0 galaxies, ll unidentified sources and one supermova remnant. The observations were made with the Algonquin Radio Observatory 46m telescope during the period May 1968-November 1970. Of the 25 variable flux density sources observed, seven exhibited statistically significant temporal variations in polarization, while l6 were marginally significant and two showed no variations. The polarization of the source PKS 2l34 + 004 was found to vary at both 2.8 and 4.5 cm even though its flux density did not undergo temporal changes. (auth)

Polarization of Radio Sources at 49 cm and 73 cm

Polarization of Radio Sources at 49 cm and 73 cm

The Polarization of Radio Sources with Appreciable Redshift

The Polarization of Radio Sources with Appreciable Redshift

Measurements of the Circular Polarization of Radio Sources at 1.4 and 5 GHz

Recently reported measurements of the circular polarization of the radiation from extragalactic radio sources have indicated that the polarization may be variable1 and that the polarization is greatest for sources with flat or inverted spectra2,3. To confirm these findings and to clarify the relationship between observation and theory4−6 we have initiated a series of measurements using the Parkes 64-m telescope.

Polarization of Radio Sources. III. Absorption Effects on Circular Polarization in a Synchrotron Source

view Abstract Citations (28) References (7) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Polarization of Radio Sources. III. Absorption Effects on Circular Polarization in a Synchrotron Source Pacholczyk, A. G. ; Swihart, T. L. Abstract The transfer problem for polarized radiation is solved for a uniform synchrotron source. The absolute value of the degree of circular polarization V/I has the same dependence on frequency and magnetic field in the two extremes of very large and very small optical thickness of the source. It is remarkable that V reverses sign at an intermediate optical thickness of the source. Publication: The Astrophysical Journal Pub Date: December 1971 DOI: 10.1086/151227 Bibcode: 1971ApJ...170..405P full text sources ADS | Related Materials (2) Part 1: 1967ApJ...150..647P Part 2: 1970ApJ...161..415P

Polarization of Radio Sources. II. Faraday Effect in the Case of Quasi-Transverse Propagation

view Abstract Citations (45) References (6) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Polarization of Radio Sources. II. Faraday Effect in the Case of Quasi-Transverse Propagation Pacholczyk, A. G. ; Swihart, T. L. Abstract Under the conditions of quasi-transverse propagation of electromagnetic waves in a magnetoionic plasma, the effect analogous to Faraday rotation in the quasi-longitudinal case (which we will call Faraday pulsation) can produce a large elliptical polarization of radiation that originally was linearly polarized. Therefore, the presence of elliptical polarization in the radiation cannot serve as a means for distinguishing between a synchrotron mechanism with high-energy electrons and any other type of emission process in radio sources in which conditions suggesting quasi- propagation (large magnetic fields) may take place. The Faraday pulsation has a wavelength dependence through which it can be identified. Publication: The Astrophysical Journal Pub Date: August 1970 DOI: 10.1086/150548 Bibcode: 1970ApJ...161..415P full text sources ADS | Related Materials (2) Part 1: 1967ApJ...150..647P Part 3: 1971ApJ...170..405P

The Linear Polarization of Radio Sources Between 11 and 20 cm Wavelength. I. Observations

Observations are presented of the linear polarization of 366 sources observed with the Parkes 210 ft telescope at frequencies of 2650, 1660, and 1410 MHz. Parameters derived from the results, namely rotation measure, intrinsic polarization angle, and depolarization, are tabulated together with other pertinent physical properties. The source list includes 64 quasars, 119 radio or normal spiral galaxies, and 31 galactic objects; the remainder are unidentified objects or doubtful identifications.

The Linear Polarization of Radio Sources Between 11 and 20 cm Wavelength. III. Influence of the Galaxy on Source Depolarization and Faraday Rotation

The dependence on galactic latitude of the 11 and 20 cm polarization of 355 extraga.la.ctic sources reveals no definite evidence of depolarization within the Ga.laxy. The distribution of Faraday rotation can be explained in terms of a. field along the local spiral arm, deformed by magnetic "loops" in the solar neighbourhood.