Rotation Measures Of Extragalactic Sources Research Articles

Structure in the Magnetic Field of the Milky Way Disk and Halo Traced by Faraday Rotation

Magnetic fields in the ionized medium of the disk and halo of the Milky Way impose Faraday rotation on linearly polarized radio emission. We compare two surveys mapping the Galactic Faraday rotation, one showing the rotation measures of extragalactic sources seen through the Galaxy, and one showing Faraday depth of the diffuse Galactic synchrotron emission from the Global Magneto-Ionic Medium Survey. Comparing the two data sets in 5° × 10° bins shows good agreement at intermediate latitudes, 10° < ∣b∣ < 50°, and little correlation between them at lower and higher latitudes. Where they agree, both tracers show clear patterns as a function of Galactic longitude, ℓ; in the northern hemisphere a strong pattern; and in the southern hemisphere a pattern. Pulsars with height above or below the plane ∣z∣ > 300 pc show similar ℓ dependence in their rotation measures. Nearby nonthermal structures show rotation measure shadows as does the Orion–Eridanus superbubble. We describe families of dynamo models that could explain the observed patterns in the two hemispheres. We suggest that a field reversal, known to cross the plane a few hundred parsecs inside the solar circle, could shift to positive z with increasing Galactic radius to explain the pattern in the northern hemisphere. Correlation shows that rotation measures from extragalactic sources are one to two times the corresponding rotation measure of the diffuse emission, implying Faraday complexity along some lines of sight, especially in the southern hemisphere.

Progress in the Global Modeling of the Galactic Magnetic Field

We discuss the global modeling of the properties of the Galactic Magnetic Field (GMF). Several improvements and variations of the model of the GMF from Jansson &amp; Farrar (2012) (JF12) are investigated in an analysis constrained by all-sky rotation measures of extragalactic sources and polarized and unpolarized synchrotron emission data from WMAP and Planck. We present the impact of the investigated model variations on the propagation of ultrahigh-energy cosmic rays in the Galaxy

MAGNETIC FIELD STRUCTURE OF THE LARGE MAGELLANIC CLOUD FROM FARADAY ROTATION MEASURES OF DIFFUSE POLARIZED EMISSION

We present a study of the magnetic field of the Large Magellanic Cloud (LMC), carried out using diffuse polarized synchrotron emission data at 1.4 GHz acquired at the Parkes Radio Telescope and the Australia Telescope Compact Array. The observed diffuse polarized emission is likely to originate above the LMC disk on the near side of the galaxy. Consistent negative rotation measures (RMs) derived from the diffuse emission indicate that the line-of-sight magnetic field in the LMC's near-side halo is directed coherently away from us. In combination with RMs of extragalactic sources that lie behind the galaxy, we show that the LMC's large scale magnetic field is likely to be of quadrupolar geometry, consistent with the prediction of dynamo theory. On smaller scales, we identify two brightly polarized filaments southeast of the LMC, associated with neutral hydrogen arms. The filaments' magnetic field potentially aligns with the direction towards the Small Magellanic Cloud. We suggest that tidal interactions between the Small and the Large Magellanic Clouds in the past 10^9 years is likely to have shaped the magnetic field in these filaments.

Global Structure of the Galactic Magnetic Field from Rotation Measures of Extragalactic Sources

Global Structure of the Galactic Magnetic Field from Rotation Measures of Extragalactic Sources

The Galactic halo magnetic field revisited

Recently, Sun et al. published new Galactic 3D-models of magnetic fields in the disk and halo of the Milky Way and the distribution of cosmic-ray electron density by taking into account the thermal electron density model NE2001 by Cordes & Lazio. The models successfully reproduce observed continuum and polarization all-sky maps and the distribution of rotation measures of extragalactic sources across the sky. However, the model parameters obtained for the Galactic halo, although reproducing the observations, seem physically unreasonable: the magnetic field needs to be significantly stronger in the Galactic halo than in the plane and the cosmic-ray distribution must be truncated at about 1 kpc to avoid excessive synchrotron emission from the halo. The reason for these unrealistic parameters was the low scale-height of the warm thermal gas of about 1 kpc adopted in the NE2001 model. However, this scale-height seemed reasonable and was well studied by numerous investigations. Recently, the scale-height of the warm gas in the Galaxy was revised by Gaensler et al. to about 1.8 kpc, by showing that the 1 kpc scale-height results from a systematic bias in the analysis of pulsar data. This implies a higher thermal electron density in the Galactic halo, which in turn reduces the halo magnetic field strength to account for the observed rotation measures of extragalactic sources. We slightly modified the NE2001 model according to the new scale-height and revised the Sun et al. model parameters accordingly: the strength of the regular halo magnetic field is now 2 μG or lower, and the physically unrealistic cutoff in z for the cosmic-ray electron density is removed. The simulations based on the revised 3D-models reproduce all-sky observations as before.

ANTISYMMETRY IN THE FARADAY ROTATION SKY CAUSED BY A NEARBY MAGNETIZED BUBBLE

Rotation measures of pulsars and extragalactic point sources have been known to reveal large-scale antisymmetries in the Faraday rotation sky with respect to the Galactic plane and halo that have been interpreted as signatures of the mean magnetic field in the Galactic halo. We describe Faraday rotation measurements of the diffuse Galactic polarized radio emission over a large region in the northern Galactic hemisphere. Through application of Rotation Measure Synthesis we achieve sensitive Faraday rotation maps with high angular resolution, capable of revealing fine-scale structures of about 1 deg in the Faraday rotation sky. Our analysis suggests that the observed antisymmetry in the Faraday rotation sky at b > 0 deg is dominated by the magnetic field around a local HI bubble at a distance of approx. 100 pc, and not by the magnetic field of the Galactic halo. We derive physical properties of the magnetic field of this shell, which we find to be 20 - 34 uG strong. It is clear that the diffuse polarized radio emission contains important information about the local magneto-ionic medium, which cannot yet be derived from Faraday rotation measures of extragalactic sources or pulsars alone.

Modelling the Galactic magnetic field on the plane in two dimensions

We present a method for parametric modelling of the physical components of the Galaxy's magnetised interstellar medium, simulating the observables, and mapping out the likelihood space using a Markov Chain Monte-Carlo analysis. We then demonstrate it using total and polarised synchrotron emission data as well as rotation measures of extragalactic sources. With these three datasets, we define and study three components of the magnetic field: the large-scale coherent field, the small-scale isotropic random field, and the ordered field. In this first paper, we use only data along the Galactic plane and test a simple 2D logarithmic spiral model for the magnetic field that includes a compression and a shearing of the random component giving rise to an ordered component. We demonstrate with simulations that the method can indeed constrain multiple parameters yielding measures of, for example, the ratios of the magnetic field components. Though subject to uncertainties in thermal and cosmic ray electron densities and depending on our particular model parametrisation, our preliminary analysis shows that the coherent component is a small fraction of the total magnetic field and that an ordered component comparable in strength to the isotropic random component is required to explain the polarisation fraction of synchrotron emission. We outline further work to extend this type of analysis to study the magnetic spiral arm structure, the details of the turbulence as well as the 3D structure of the magnetic field.

Constraining models of the large scale Galactic magnetic field with WMAP5 polarization data and extragalactic rotation measure sources

We introduce a method to quantify the quality-of-fit between data and observables depending on the large scale Galactic magnetic field. We combine WMAP5 polarized synchrotron data and rotation measures of extragalactic sources in a joint analysis to obtain best fit parameters and confidence levels for GMF models common in the literature. None of the existing models provide a good fit in both the disk and halo regions, and in many instances best-fit parameters are quite different than the original values. We note that probing a very large parameter space is necessary to avoid false likelihood maxima. The thermal and relativistic electron densities are critical for determining the GMF from the observables but they are not well constrained. We show that some characteristics of the electron densities can already be constrained using our method and with future data it may be possible to carry out a self-consistent analysis in which models of the GMF and electron densities are simultaneously optimized.

Magnetic field near the central region of the Galaxy: rotation measure of extragalactic sources

Aims. We determine the properties of the Faraday screen and the magnetic field near the central region of the Galaxy. Methods. We measured the Faraday rotation measure (RM) towards 60 background extragalactic source components through the −6 ◦ < l < 6 ◦ , −2 ◦ < b < 2 ◦ region of the Galaxy using the 4.8 and 8.5 GHz bands of the ATCA and VLA. Here we use the measured RMs to estimate the systematic and the random components of the magnetic fields. Results. The measured RMs are found to be mostly positive for the sample sources in the region. This is consistent with either a large scale bisymmetric spiral magnetic fields in the Galaxy or with fields oriented along the central bar of the Galaxy. The outer scale of the RM fluctuation is found to be about 40 pc, which is much larger than the observed RM size scales towards the non thermal filaments (NTFs). The RM structure function is well-fitted with a power law index of 0.7 ± 0.1 at length scales of 0.3 to 100 pc. If Gaussian random processes in the ISM are valid, the power law index is consistent with a two dimensional Kolmogorov turbulence. If there is indeed a strong magnetic field within ∼1 ◦ (radius 150 pc) from the GC, the strength of the random field in the region is estimated to be ∼20 µG. Conclusions. Given the highly turbulent magnetoionic ISM in this region, the strength of the systematic component of the magnetic fields would most likely be close to that of the random component. This suggests that the earlier estimated milliGauss magnetic field near the NTFs is localised and does not pervade the central 300 pc of the Galaxy.

Rotation Measures of Extragalactic Sources behind the Southern Galactic Plane: New Insights into the Large‐Scale Magnetic Field of the Inner Milky Way

We present new Faraday rotation measures (RMs) for 148 extragalactic radio sources behind the southern Galactic plane (253o < l < 356o, |b| < 1.5o), and use these data in combination with published data to probe the large-scale structure of the Milky Way's magnetic field. We show that the magnitudes of these RMs oscillate with longitude in a manner that correlates with the locations of the Galactic spiral arms. The observed pattern in RMs requries the presence of at least one large-scale magnetic reversal in the fourth Galactic quadrant, located between the Sagittarius- Carina and Scutum-Crux spiral arms. To quantitatively compare our measurements to other recent studies, we consider all available extragalactic and pulsar RMs in the region we have surveyed, and jointly fit these data to simple models in which the large-scale field follows the spiral arms. In the best-fitting model, the magnetic field in the fourth Galactic quadrant is directed clockwise in the Sagittarius-Carina spiral arm (as viewed from the North Galactic pole), but is oriented counter- clockwise in the Scutum-Crux arm. This contrasts with recent analyses of pulsar RMs alone, in which the fourth-quadrant field was presumed to be directed counter-clockwise in the Sagittarius- Carina arm. Also in contrast to recent pulsar RM studies, our joint modeling of pulsar and extragalactic RMs demonstrates that large numbers of large-scale magnetic field reversals are not required to account for observations.

The Strength and Structure of the Galactic Center Magnetic Field

This paper summarizes recently obtained, strong evidence for a weak global field in the Galactic center (GC): the existence of a large-scale region of diffuse, low-frequency, non- thermal emission coincident with the central molecular zone. The overall energetics of this emission, considered along with constraints on GC cosmic ray energy density and diffusion, indicate clearly that the magnetic field pervading this region is ∼ 10 μG. For completeness, additional points on the orientation of the GC nonthermal filaments, rotation measures of extragalactic sources seen through the GC, and comparison with other normal spiral galaxies are also reviewed.

Small-scale variations in the galactic magnetic field - The rotation measure structure function and birefringence in interstellar scintillations

The structure function of rotation measures of extragalactic sources and birefringence in interstellar scintillations are used to investigate variations in the interstellar magnetic field on length scales of about 0.01-100 pc and 10 to the 11th cm, respectively. Model structure functions are derived for the case of a power-law power spectrum of irregularities in the quantity (n(e)B), and an estimate for the structure function is computed for several regions of the sky using data on extragalactic sources. The results indicate an outer angular scale for rotation measure (RM) variations of not less than about 5 deg (a linear scale of about 9-90 pc at a distance of 0.1-1 kpc). There is also evidence for RM variations on angular scales as small as 1 arcmin, but it cannot be determined whether these are intrinsic to the source or caused by the interstellar medium. The effect of a random, Faraday-active medium on the diffraction of radio waves is derived, and an upper limit to the variations in n(e)B on a length scale of 10 to the 11th cm is obtained from available observations.

Faraday rotation and the turbulent structure of the Galaxy

We extend Jokipii and Lerche's analysis of the turbulent structure of our Galaxy by means of a study of the rotation measure of extragalactic sources. Like them we use a simple, statistically homogeneous and isotropic disc model of the Galaxy and assume that the magnetic field has both an average component and a fluctuating one. We assume that the electron density is proportional to some power of the magnetic field (NeαBn with 1≤n≤2). Using the rotation measure data on 242 extragalactic sources given by Vallee and Kronberg we consider both an exponential and a Gaussian two-point correlation function for the (Gaussian) fluctuating component of the magnetic field with a correlation lengthL. We find reasonable agreement between theory and observations for an average magnetic field of about 3 μG, a fluctuating magnetic field component with an amplitude of about 2.6μG, an average electron density of about 0.03 cm−3, a fluctuating density component of about 0.05 cm−3, and a correlation length of about 300 pc.

Structure of the Local Galactic Magnetic Field

view Abstract Citations (171) References (42) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Structure of the Local Galactic Magnetic Field Manchester, R. N. Abstract Rotation measures have been determined for 18 pulsars, which brings the total number known to 38. It is concluded from these results that the magnetic field in the local region consists of a longitudinal component of strength 2.2 1 0.4 microgauss directed toward 1 = 94 1 ii together with superposed irregularities of comparable field strength. The longitudinal component appears to be uniform on a large scale, having the same sense and direction out to more than 1 kpc from the Sun. Scales of irregularities are typically a few hundred parsecs, with smaller-scale structure apparently not an important contribution to the average field. These conclusions are compared with those from observations of rotation measures of extragalactic sources and optical polarization, and their implications on proposed models for the origin of the galactic field are discussed. Subject headings: galactic structure - magnetic fields - pulsars Publication: The Astrophysical Journal Pub Date: March 1974 DOI: 10.1086/152757 Bibcode: 1974ApJ...188..637M full text sources ADS | data products SIMBAD (39)