Large Rotation Measure Research Articles

The Production of Rotation Measure Gradients in the Lobes of Extragalactic Radio Sources

It is proposed that the large rotation measure gradients observed in the lobes of some extragalactic radio sources are produced by non–linear Kelvin–Helmholtz surface waves. Related small scale turbulence is responsible for a “diffuse spray” of relativistic plasma and magnetic field into the surrounding medium. The mixture of magnetic field and thermal plasma causes large rotation measure variations on the scale of the Kelvin–Helmholtz waves. An order of magnitude prediction of the effect is in good agreement with the observations of Cygnus A and PKS 2104–25 N.

Search for Extragalactic Radio Sources with Large Rotation Measure

Intrinsic large rotation measures (RM) were searched for 128 extragalactic sources based on polarization data obtained at the NRO 45–m telescope. The number of sources with RM > 500 rad m−2 is only seven.

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.

High Frequency Observations of a Large Faraday Rotation Galaxy in a Cluster

We present 22 and 49 GHz interferometric observations of Hyd A (3C218). The source was found to have a very large Faraday rotation measure (RM) (Kato et al. 1987), and to be a dominant member of a luminous X-ray cluster with a large cooling flow (David et al. 1988). These characteristics are very similar to those of Cyg A which is suggested to produce a large RM within a dense sheath around the radio lobes as a result of somehow an interaction between dense intracluster medium (ICM) and radio jets and/or lobes (Dreher et al. 1987). Hyd A is the second example of Cyg A type source. In case of Cyg A, hot spots are the place where the interaction between jets and ICM occurs (Carilli et al. 1988). We then expect in Hyd A that similar interaction also occurs to form hot spots, and consequently that high frequency observations reveal structures of the interaction.

The local Galactic magnetic field

The Galactic magnetic field is analyzed based on the large amount of pulsar rotation measures (RMs) now available, and regions of the sky suspected of producing anomalous RMs are discussed. The North Polar Spur appears to cause large positive RMs in the region l between 0 and 60 deg and b between 0 and 60 deg, and thus causes a systematic bias in modeling if the pulsars in the region are not flagged. Using only unflagged pulsars within D = 3 kpc and a longitudinal model of the local field, the strength is found to be B(0) = 1.6 + or - 0.3 microgauss in the direction l(B) = 96 deg + or - 4 deg, with a reversal at D(r) = 600 + or - 80 pc toward the inner Galaxy. A concentric-ring model is found to be superior to a bisymmetric spiral model as a fit to the data without the 3 kpc restriction, with a strength of B0 = 1.3 + or - 0.2 microgauss at the position of the sun and a reversal at D(r) = 650 + or - 90 pc toward the Galactic center. 33 refs.

Extragalactic radio sources with very large Faraday rotation

There are several radio sources with Faraday rotation measures larger than 500 rad m−2 (ref. 1). Because most of these sources are located near the galactic plane, the large rotation measures can be attributed to the galactic medium2. However, a few sources at high galactic latitude also show large rotation, implying intrinsic Faraday rotation within the source. Until now, no sources have been discovered to have a rotation measure > 1,000 rad m−2. Using a multichannel polarimeter at 10 GHz attached to the 45-m radio telescope at Nobeyama, we have observed about 100 extragalactic radio sources which were suspected to have large rotation from our polarization catalogue1. Four of these 100 sources were found to have an intrinsic rotation measure > 1,000 rad m−2. Here we report rotation measures which are two orders of magnitude larger than those observed so far for most extragalactic radio sources. Two of these four sources are classified as compact steep-spectrum sources.

New evidence on the origin of rotation measures in extragalactic radio sources

The causes of the Faraday rotation in extragalactic radio sources have been reinvestigated in the light of new polarisation data which extend to large redshifts. Our results show convincingly that at nearly all galactic latitudes, the large rotation measures are generated outside of the Milky Way. Furthermore, any contribution from the intergalactic medium must be small when compared with the Faraday rotation generated at or in the extragalactic sources themselves. Having rotation measures for 108 QSOs of which 48 have redshift (z) greater than unity, we have been able to place the lowest and most reliable upper limits on any contribution from an intergalactic magneto-ionic medium at early cosmological epochs. The data show promise that it should soon be possible to ascertain whether or not the density of intergalactic ionised gas follows the cosmological scale factor.

On the Linear Polarization of the Microwave Radiation from Radio Sources.

view Abstract Citations (18) References (23) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS On the Linear Polarization of the Microwave Radiation from Radio Sources. Seielstad, G. A. ; Morris, D. ; Radhakrishnan, V. Abstract Measurements of the wavelength dependence of the linear polarization of twenty-nine radio sources have been compiled. It is confirmed that in the decimeter wavelength range the observed Faraday rotation of the plane of polarization arises in the Galaxy. Although the magnetic field in the local spiral arm must be highiy ordered, the data do not allow a definite field configuration to be deduced. The simple model due to Chandrasekhar and Fermi (1953) is shown to be untenable, however. The hypothesis that depolarization at decimeter wavelengths is caused by differential Faraday rotation in the Galaxy is rejected for lack of supporting evidence. An inverse correlation between emissivity (or magnetic-field strength) and fractional polarization at 10-cm wavelength is presented. This is interpreted as confirmation of the suggestion by Woltjer (1962) that depolarization is produced by Faraday rotation effects within the radio sources themselves. Sources with high magnetic fields will therefore only exhibit detectable polarization at centimeter wavelengths. At these wavelengths they will have large depolarization rates and consequently large rotation measures due to their internal Faraday rotation. Such sources,in contrast to the majority of those studied hitherto, will therefore show poor correlation between rotation measure and galactic latitude and an enhanced correlation with depolarization rate. Publication: The Astrophysical Journal Pub Date: July 1964 DOI: 10.1086/147892 Bibcode: 1964ApJ...140...53S full text sources ADS | data products NED (29)

Depolarization of Radio Sources.

view Abstract Citations References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Depolarization of Radio Sources. Morris, D. ; Radhakrishnan, V. ; Seielstad, G. A. Abstract Measurements of the wavelength dependence of the linear polarization of 29 radio sources have been compiled. It is confirmed that in the decimeter wavelength range the observed Faraday rotation of the plane of polarization arises in the Galaxy. Although the magnetic field in the local spiral arm must be highly ordered the data do not allow a definite field configuration to be deduced. The simple model due to Chandrasekhar and Fermi (Chandrasekhar, S., and Fermi, F., Astrophys. J. 118,113,1953) is shown to be untenable, however. The hypothesis that depolarization at decimeter wavelengths is caused by differential Faraday rotation in the Galaxy is rejected for lack of supporting evidence. An inverse correlation between emissivity (or magnetic field strength) and fractional polarization at 10-cm wavelength is presented. This is interpreted as confirmation of the suggestion by Woltjer (Woltjer, L., Astrophys. J. 136, 1152,1962) that depolarization is produced by Faraday rotation effects within the radio sources themselves. Sources with high magnetic fields will therefore only exhibit detectable polarization at centimeter wavelengths. At these wavelengths they will have large depolarization rates and consequently large rotation measures due to their internal Faraday rotation. Such sources in contrast to the majority of those studied hitherto will therefore show poor correlation between rotation measure and Galactic latitude and an enhanced correlation with depolarization rate. This work was done under contract Nonr 220(19). Publication: The Astronomical Journal Pub Date: 1964 DOI: 10.1086/109477 Bibcode: 1964AJ.....69S.145M full text sources ADS |