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Radio polarization and RM structure at high Galactic latitudes
Abstract
We present WSRT observations at low frequencies (315–388 MHz) of the diffuse polarized emission in an area of 6° × 6° at high Galactic latitude. Polarized emission is found to be ubiquitous with typical levels of about 3–5 K brightness temperature with a generally mottled structure. The Rotation Measure (RM) of the emission varies between values of –5 to +20 rad m–2. Most of the polarized emission appears Faraday thin with a single unresolved valued for the RM. The data suggest both inhomogeneous as well as dissimilar distribution functions of the synchrotron emitting and Faraday rotating media along the line of sight. The systematic patterns in the RM at this high Galactic latitude of +71° also provide evidence for significant variations in the vertical component of the local Galactic magnetic field at a level of 1 μ Gauss. The potential for Galactic polarimetry at even lower frequencies using LOFAR and SKA is briefly discussed. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
... The "windows" are also not perfectly clean: other effects, notably reflections in cables or antennas, can lead to a sky signal received at time t being detected again at some later time t + ∆t, and thus producing spurious power at τ = ∆t. A similar effect can be produced by linear polarization-to-intensity (Stokes Q, U → I) leakage in the presence of Galactic Faraday rotation: the arrival times of the left-and right-circularly polarized pulse from a distant electron are separated by a relative delay ∆tL−R because of the different indices of refraction for the two circular polarizations, which produces contamination in the Fourier modes corresponding to η = ∆tL−R (e.g., Haverkorn et al. 2003a,b;de Bruyn et al. 2006;Pen et al. 2009;Bernardi et al. 2009Bernardi et al. , 2010Moore et al. 2013) (c.f., for a removal technique, Shaw et al. 2014Shaw et al. , 2015. ...
Redshifted H{\sc\,i} 21 cm emission from unresolved low-redshift large scale structure is a promising window for ground-based Baryon Acoustic Oscillations (BAO) observations. A major challenge for this method is separating the cosmic signal from the foregrounds of Galactic and extra-Galactic origins that are stronger by many orders of magnitude than the former. The smooth frequency spectrum expected for the foregrounds would nominally contaminate only very small modes; however the chromatic response of the telescope antenna pattern at this wavelength to the foreground introduces non-smooth structure, pervasively contaminating the cosmic signal over the physical scales of our interest. Such contamination defines a wedged volume in Fourier space around the transverse modes that is inaccessible for the cosmic signal. In this paper, we test the effect of this contaminated wedge on the future 21 cm BAO surveys using Fisher information matrix calculation. We include the signal improvement due to the BAO reconstruction technique that has been used for galaxy surveys and test the effect of this wedge on the BAO reconstruction as a function of signal to noises and incorporate the results in the Fisher matrix calculation. We find that the wedge effect expected at is very detrimental to the angular diameter distances: the errors on angular diameter distances increased by 3-4.4 times, while the errors on H(z) increased by a factor of 1.5-1.6. We conclude that calibration techniques that clean out the foreground "wedge" would be extremely valuable for constraining angular diameter distances from intensity-mapping 21 cm surveys.
... Polarized Galactic emission of a few Kelvin brightness on angular scales of 1 is ubiquitous in interferometric measurements made at λ ≈ 1 m (de Bruyn et al. 2006). The polarized structure is typically uncorrelated with total intensity structure (Bernardi et al. 2009(Bernardi et al. , 2010 because the bulk of the polarized structure is the result of spatial variations in the Faraday rotation in the intervening magneto-ionic medium, rather than structure intrinsic to the emission itself. ...
The radio source J1819+3845 underwent a period of extreme interstellar scintillation between circa 1999 and 2007. The plasma structure responsible for this scintillation was determined to be just 1-pc from the solar system and to posses a density of cm that is three orders of magnitude higher than the ambient interstellar density (de Bruyn & Macquart 2015). Here we present radio-polarimetric images of the field towards J1819+3845 at wavelengths of 0.2, 0.92 and 2m. We detect an elliptical plasma globule of approximate size (major-axis position angle of ), via its Faraday-rotation imprint (radm) on the diffuse Galactic synchrotron emission. The extreme scintillation of J1819+3845 was most likely caused at the turbulent boundary of the globule (J1819+3845 is currently occulted by the globule). The origin and precise nature of the globule remain unknown. Our observations are the first time plasma structures that likely cause extreme scintillation have been directly imaged.
... Ho we ver, the polarization leakage can still be identified via the polarized beam pattern measurements. A few more studies are focusing on simulating polarization leakage for different H I experiments (de Bruyn et al. 2006 ;Wolleben et al. 2006 ;Schnitzeler, Katgert & de Bruyn 2009 ;Shaw et al. 2015 ;Nunhokee et al. 2017 ). Eliminating the polarization leakage from the full beam pattern is important for separating the foreground from the H I signal. ...
The neutral hydrogen (HI) intensity mapping (IM) survey is regarded as a promising approach for cosmic large-scale structure studies. A major issue for the HI IM survey is to remove the bright foreground contamination. A key to successfully removing the bright foreground is to well control or eliminate the instrumental effects. In this work, we consider the instrumental effects of polarization leakage and use the U-Net approach, a deep learning-based foreground removal technique, to eliminate the polarization leakage effect. The thermal noise is assumed to be a subdominant factor compared with the polarization leakage for future HI IM surveys and ignored in this analysis. In this method, the principal component analysis (PCA) foreground subtraction is used as a preprocessing step for the U-Net foreground subtraction. Our results show that the additional U-Net processing could either remove the foreground residual after the conservative PCA subtraction or compensate for the signal loss caused by the aggressive PCA preprocessing. Finally, we test the robustness of the U-Net foreground subtraction technique and show that it is still reliable in the case of existing constraint error on HI fluctuation amplitude.
... Liao et al. (2016); Nunhokee et al. (2017) calculated the polarization leakage at the centre of the plane of observation by Jones matrix and Mueller matrix, but for the polarization leakage of the complete sky map, we have to calculate it by a large number of convolutions, which is basically difficult to accomplish. There has been some work done to address this issue de Bruyn et al. 2006;Schnitzeler et al. 2009;Wolleben et al. 2006;Nunhokee et al. 2017). ...
The neutral hydrogen (HI) intensity mapping (IM) survey is regarded as a promising approach for cosmic large-scale structure (LSS) studies. A major issue for the HI IM survey is to remove the bright foreground contamination. A key to successfully remove the bright foreground is to well control or eliminate the instrumental effects. In this work, we consider the instrumental effect of polarization leakage and use the U-Net approach, a deep learning-based foreground removal technique, to eliminate the polarization leakage effect.In this method, the principal component analysis (PCA) foreground subtraction is used as a preprocessing step for the U-Net foreground subtraction. Our results show that the additional U-Net processing could either remove the foreground residual after the conservative PCA subtraction or compensate for the signal loss caused by the aggressive PCA preprocessing. Finally, we test the robustness of the U-Net foreground subtraction technique and show that it is still reliable in the case of existing constraint error on HI fluctuation amplitude.
... Despite some empirical observations [e.g. [64][65][66], this effect on radio foregrounds is poorly understood, except perhaps at very low radio frequencies, as purported by the Experiment to Detect the Global Epoch of Reionization Signature (EDGES) [67]. Several models have been proposed to describe this phenomenon. ...
We seek to remove foreground contaminants from 21cm intensity mapping observations. We demonstrate that a deep convolutional neural network (CNN) with a UNet architecture and three-dimensional convolutions, trained on simulated observations, can effectively separate frequency and spatial patterns of the cosmic neutral hydrogen (HI) signal from foregrounds in the presence of noise. Cleaned maps recover cosmological clustering statistics within 10% at all relevant angular scales and frequencies. This amounts to a reduction in prediction variance of over an order of magnitude on small angular scales (ℓ>300), and improved accuracy for small radial scales (k∥>0.17 h Mpc−1) compared to standard Principal Component Analysis (PCA) methods. We estimate posterior confidence intervals for the network's prediction by training an ensemble of UNets. Our approach demonstrates the feasibility of analyzing 21cm intensity maps, as opposed to derived summary statistics, for upcoming radio experiments, as long as the simulated foreground model is sufficiently realistic. We provide the code used for this analysis on Github this https URL as well as a browser-based tutorial for the experiment and UNet model via the accompanying this http URL Colab notebook.
... De-spite some empirical observations [e.g. 15,57,78], this effect on radio foregrounds is poorly understood, except perhaps at very low radio frequencies, as purported by the Experiment to Detect the Global Epoch of Reionization Signature (EDGES) [46]. Several models have been proposed to describe this phenomenon. ...
We seek to remove foreground contaminants from 21cm intensity mapping observations. We demonstrate that a deep convolutional neural network (CNN) with a UNet architecture and three-dimensional convolutions, trained on simulated observations, can effectively separate frequency and spatial patterns of the cosmic neutral hydrogen (HI) signal from foregrounds in the presence of noise. Cleaned maps recover cosmological clustering statistics within 10% at all relevant angular scales and frequencies. This amounts to a reduction in prediction variance of over an order of magnitude on small angular scales (), and improved accuracy for small radial scales ( compared to standard Principal Component Analysis (PCA) methods. We estimate posterior confidence intervals for the network's prediction by training an ensemble of UNets. Our approach demonstrates the feasibility of analyzing 21cm intensity maps, as opposed to derived summary statistics, for upcoming radio experiments, as long as the simulated foreground model is sufficiently realistic. We provide the code used for this analysis on , as well as a browser-based tutorial for the experiment and UNet model via the accompanying .
... Polarized Galactic emission of a few Kelvin brightness on angular scales of 1 is ubiquitous in interferometric measurements made at λ ≈ 1 m (de Bruyn et al. 2006). The polarized structure is typically uncorrelated with total intensity structure (Bernardi et al. 2009(Bernardi et al. , 2010 because the bulk of the polarized structure is the result of spatial variations in the Faraday rotation in the intervening magneto-ionic medium, rather than structure intrinsic to the emission itself. ...
The radio source J1819+3845 underwent a period of extreme interstellar scintillation between circa 1999 and 2007. The plasma structure responsible for this scintillation was determined to be just 1–3 pc from the solar system and to posses a density of cm⁻³, which is three orders of magnitude higher than the ambient interstellar density. Here we present radio-polarimetric images of the field toward J1819+3845 at wavelengths of 0.2, 0.92, and 2 m. We detect an elliptical plasma globule of approximate size (major-axis position angle of ), via its Faraday-rotation imprint () on the diffuse Galactic synchrotron emission. The extreme scintillation of J1819+3845 was most likely caused at the turbulent boundary of the globule (J1819+3845 is currently occulted by the globule). The origin and precise nature of the globule remain unknown. Our observations represent the first time that plasma structures which likely cause extreme scintillation have been directly imaged.
... Therefore, it is not possible to say whether their data would agree with the above conclusions. Small and varying vertical magnetic field strengths found in a field of view at l = 153 • , 0.5 • < b < 18 • by [77] and at high Galactic latitudes b = 70 • [78] probably reflect smaller-scale magnetic field fluctuations and not the large-scale field. ...
This chapter presents a review of observational studies to determine the
magnetic field in the Milky Way, both in the disk and in the halo, focused on
recent developments and on magnetic fields in the diffuse interstellar medium.
I discuss some terminology which is confusingly or inconsistently used and try
to summarize current status of our knowledge on magnetic field configurations
and strengths in the Milky Way. Although many open questions still exist, more
and more conclusions can be drawn on the large-scale and small-scale components
of the Galactic magnetic field. The chapter is concluded with a brief outlook
to observational projects in the near future.
... Depolarisation is already severe in the inner Galactic plane at 1.4 GHz (Reich et al. 1990;Wolleben et al. 2006;Haverkorn et al. 2006) and it is likely that much of the inner Galactic plane will appear completely depolarised below 300 MHz, although Bernardi et al. (2009) have detected polarized emission at arcminute scales at 150 MHz. Regions at mid and high latitudes, on the other hand, show considerable diffuse Galactic polarised structure at 350 MHz (Haverkorn et al. 2003b;de Bruyn et al. 2006;Schnitzeler et al. 2009). The high sensitivity of the MWA to extended structure will enable it to map the diffuse polarised emission down to very low intensities, thus making visible weak Galactic magnetic fields in the local interstellar medium and Galactic extended disk and halo that are undetectable at higher frequencies. ...
Significant new opportunities for astrophysics and cosmology have been
identified at low radio frequencies. The Murchison Widefield Array is the first
telescope in the Southern Hemisphere designed specifically to explore the
low-frequency astronomical sky between 80 and 300 MHz with arcminute angular
resolution and high survey efficiency. The telescope will enable new advances
along four key science themes, including searching for redshifted 21 cm
emission from the epoch of reionisation in the early Universe; Galactic and
extragalactic all-sky southern hemisphere surveys; time-domain astrophysics;
and solar, heliospheric, and ionospheric science and space weather. The
Murchison Widefield Array is located in Western Australia at the site of the
planned Square Kilometre Array (SKA) low-band telescope and is the only
low-frequency SKA precursor facility. In this paper, we review the performance
properties of the Murchison Widefield Array and describe its primary scientific
objectives.
... As with the ordinary spectral line cube, this third dimension is related to distance, but in a complicated way. Studying this cube tells us about the three dimensional distribution of the magnetized thermal plasma and the polarized emission (de Bruyn et al. 2006). Construction of such cubes from low latitude survey data will allow a much more profound interpretation of the diffuse, polarized Galactic emission, with the goal of separating the structure functions of the electron density and the magnetic field. ...
The warm neutral medium, warm ionized medium, and cool neutral medium all show strong evidence for turbulence as a process dominating their structure and motions on a wide range of scales. The spatial power spectra of density fluctuations in all three phases are consistent with a Kolmogorov slope. Turbulence in the magnetic field in the diffuse medium can also be measured through the structure function of the Faraday rotation measure. With new surveys, new analysis techniques, and new telescopes, in the next few years it will be possible to measure the structure function of the magnetic field over a similarly wide range of scales. This will give a complete picture of the turbulence as a magneto-acoustic process.
... Because the distribution of density, column density, and emission measure in the models is relatively insensitive to the magnetic field strengths we considered, these data alone cannot effectively constrain the properties of the magnetic field. However, our study's density structure of the WIM effectively fixes the density, one of the free parameters in interpreting Faraday rotation studies of the ionized medium, which depends on the convolution of both the density and the magnetic field structure of the ISM at high Galactic latitude (Boulares & Cox 1990;Zweibel & Heiles 1997;Ferrière 2001;Han 2006;de Bruyn et al. 2006). Comparison of MHD models such as those we considered to the combination of density and magnetic field probes will be a powerful way to study the magnetic field structure of the WIM in the local solar neighborhood. ...
We present an analysis of the distribution of H-alpha emission measures for the warm ionized medium (WIM) of the Galaxy using data from the Wisconsin H-Alpha Mapper (WHAM) Northern Sky Survey. Our sample is restricted to Galactic latitudes |b| > 10. We removed sightlines intersecting nineteen high-latititude classical H II regions, leaving only sightlines that sample the diffuse WIM. The distribution of EM sin |b| for the full sample is poorly characterized by a single normal distribution, but is extraordinarily well fit by a lognormal distribution, with <log EM sin |b|> = 0.146 +/- 0.001 and standard deviation 0.190 +/- 0.001. <log EM sin |b|> drops from 0.260 +/- 0.002 at Galactic latitude 10<|b|<30 to 0.038 +/- 0.002 at Galactic latitude 60<|b|<90. The distribution may widen slightly at low Galactic latitude. We compare the observed EM distribution function to the predictions of three-dimensional magnetohydrodynamic simulations of isothermal turbulence within a non-stratified interstellar medium. We find that the distribution of EM sin |b| is well described by models of mildy supersonic turbulence with a sonic Mach number of ~1.4-2.4. The distribution is weakly sensitive to the magnetic field strength. The model also successfully predicts the distribution of dispersion measures of pulsars and H-alpha line profiles. In the best fitting model, the turbulent WIM occupies a vertical path length of 400-500 pc within the 1.0-1.8 kpc scale height of the layer. The WIM gas has a lognormal distribution of densities with a most probable electron density n_{pk} = 0.03 cm^{-3}. We also discuss the implications of these results for interpreting the filling factor, the power requirement, and the magnetic field of the WIM.
Recent technological advances have led to a resurgence of interest in
low frequency radio astronomy. Ionospheric distortion of cosmic
radiation has, however, been a challenge for high fidelity and high
sensitivity measurements at these long wavelengths. Several new and
innovative low radio frequency interferometers are currently in varying
stages of development, construction and commissioning across the globe.
They will pursue a broad range of scientific objectives, and precise
ionospheric calibration over the wide field-of-view of these new
generation instruments will be a prerequisite for achieving these
science goals. The task of calibration is made more difficult by the
Faraday rotation (FR) of polarized flux as it passes through the
magnetized plasma of the ionosphere, the plasmasphere, the
magnetosphere, and the heliosphere. To quantify these effects, we
present a survey of the order of magnitude of FR associated with these
media and their spatial and temporal variations.
Faraday rotation towards polarised pulsars and extragalactic sources is the best observable for determining the configuration of the magnetic field of the Galaxy in its plane and also at high latitudes. The Galactic magnetic field plays an important role in numerous astrophysical processes, including star formation and propagation of ultrahigh-energy cosmic rays; it is also an important component in measurements of the cosmological microwave background. This review article provides a brief overview of the latest advancements in the field, from an observer’s point of view. The most recent results based on pulsar rotation measures are discussed, which show that we have begun to confidently resolve the main features of the Galactic magnetic field on kiloparsec scales, both in the Solar neighbourhood and at larger distances. As we are currently in great anticipation of polarisation observations with new, state-of-the-art telescopes and hardware, a brief overview of how much this field of research will benefit from the upcoming pulsar surveys is also given.
Measurement of the spatial distribution of neutral hydrogen via the redshifted 21-cm line promises to revolutionize our knowledge of the epoch of reionization and the first galaxies, and may provide a powerful new tool for observational cosmology from redshifts 1) < z < 4. In this review we discuss recent advances in our theoretical understanding of theepoch of reionization (EoR), the application of 21-cm tomography to cosmology and measurements of the dark energy equationof state after reionization, and the instrumentation and observational techniques shared by 21-cm EoR and postreionizationcosmology machines.We place particular emphasis on the expected signal and observational capabilities of first generation 21-cm fluctuation instruments.
Aims: We investigate the properties of the Galactic ISM by applying Faraday tomography to a radio polarization data set in the direction of the Galactic anti-centre. Methods: We address the problem of missing large-scale structure in our data, and show that this does not play an important role for the results we present. Results: The main peak of the Faraday depth spectra in our data set is not measurably resolved for about 8% of the lines of sight. An unresolved peak indicates a separation between the regions with Faraday rotation and synchrotron emission. However, cosmic rays pervade the ISM, and synchrotron emission would therefore also be produced where there is Faraday rotation. We suggest that the orientation of the magnetic field can separate the two effects. By modelling the thermal electron contribution to the Faraday depth, we map the strength of the magnetic field component along the line of sight. Polarized point sources in our data set have rotation measures that are comparable to the Faraday depths of the diffuse emission in our data. Our Faraday depth maps show narrow canals of low polarized intensity. We conclude that depolarization over the telescope beam produces at least some of these canals. Finally, we investigate the properties of one conspicuous region in this data set and argue that it is created by a decrease in line-of-sight depolarization compared to its surroundings.
Aims:We investigate the distribution and properties of Faraday rotating
and synchrotron emitting regions in the Galactic ISM in the direction of
the Galactic anti-centre. Methods: We apply Faraday tomography
to a radio polarization dataset that we obtained with the WSRT. We
developed a new method to calculate a linear fit to periodic data, which
we use to determine rotation measures from our polarization angle data.
From simulations of a Faraday screen + noise we could determine how
compatible the data are with Faraday screens. Results: An
unexpectedly large fraction of 14% of the lines-of-sight in our dataset
show an unresolved main component in the Faraday depth spectrum. For
lines-of-sight with a single unresolved component we demonstrate that a
Faraday screen in front of a synchrotron emitting region that contains a
turbulent magnetic field component can explain the data.
Faraday rotation measure (RM) synthesis is an important tool to study and
analyze galactic and extra-galactic magnetic fields. Since there is a Fourier
relation between the Faraday dispersion function and the polarized radio
emission, full reconstruction of the dispersion function requires knowledge of
the polarized radio emission at both positive and negative square wavelengths
. However, one can only make observations for .
Furthermore observations are possible only for a limited range of wavelengths.
Thus reconstructing the Faraday dispersion function from these limited
measurements is ill-conditioned. In this paper, we propose three new
reconstruction algorithms for RM synthesis based upon compressive
sensing/sampling (CS). These algorithms are designed to be appropriate for
Faraday thin sources only, thick sources only, and mixed sources respectively.
Both visual and numerical results show that the new RM synthesis methods
provide superior reconstructions of both magnitude and phase information than
RM-CLEAN
Magnetic fields on a range of scales play a large role in the ecosystems of
galaxies, both in the galactic disk and in the extended layers of gas away from
the plane. Observing magnetic field strength, structure and orientation is
complex, and necessarily indirect. Observational data of magnetic fields in the
halo of the Milky Way are scarce, and non-conclusive about the large-scale
structure of the field. In external galaxies, various large-scale
configurations of magnetic fields are measured, but many uncertainties about
exact configurations and their origin remain. There is a strong interaction
between magnetic fields and other components in the interstellar medium such as
ionized and neutral gas and cosmic rays. The energy densities of these
components are comparable on large scales, indicating that magnetic fields are
not passive tracers but that magnetic field feedback on the other interstellar
medium components needs to be taken into account.
Experiments designed to measure the redshifted 21~cm line from the Epoch of Reionization (EoR) are challenged by strong astrophysical foreground contamination, ionospheric distortions, complex instrumental response and other different types of noise (e.g. radio frequency interference). The astrophysical foregrounds are dominated by diffuse synchrotron emission from our Galaxy. Here we present a simulation of the Galactic emission used as a foreground module for the LOFAR- EoR key science project end-to-end simulations. The simulation produces total and polarized intensity over maps of the Galactic synchrotron and free-free emission, including all observed characteristics of the emission: spatial fluctuations of amplitude and spectral index of the synchrotron emission, together with Faraday rotation effects. The importance of these simulations arise from the fact that the Galactic polarized emission could behave in a manner similar to the EoR signal along the frequency direction. As a consequence, an improper instrumental calibration will give rise to leakages of the polarized to the total signal and mask the desired EoR signal. In this paper we address this for the first time through realistic simulations. Comment: 14 pages, 8 figures, accepted to be published in MNRAS
We present the first results from a series of observations conducted with the Westerbork telescope in the 140--160 MHz range with a 2 arcmin resolution aimed at characterizing the properties of the foregrounds for epoch of reionization experiments. For the first time we have detected fluctuations in the Galactic diffuse emission on scales greater than 13 arcmin at 150 MHz, in the low Galactic latitude area known as Fan region. Those fluctuations have an rms of 14 K. The total intensity power spectrum shows a power--law behaviour down to with slope . The detection of diffuse emission at smaller angular scales is limited by residual point sources. We measured an rms confusion noise of 3 mJy beam. Diffuse polarized emission was also detected for the first time at this frequency. The polarized signal shows complex structure both spatially and along the line of sight. The polarization power spectrum shows a power--law behaviour down to with slope . The rms of polarization fluctuations is 7.2 K on 4 arcmin scales. By extrapolating the measured spectrum of total intensity emission, we find a contamination on the cosmological signal of K on 5 arcmin scales and a corresponding rms value of 18.3 K at the same angular scale. The level of the polarization power spectrum is K on 5 arcmin scales. Given its exceptionally bright polarized signal, the Fan region is likely to represent an upper limit on the sky brightness at moderate and high Galactic latitude. Comment: Minor corrections made to match the final version printed on A&A. A version with high resolution figures is available at http://www.astro.rug.nl/~bernardi/FAN/fan.pdf
The GMRT reionization effort aims to map out the large scale structure of the Universe during the epoch of reionization (EoR). Removal of polarized Galactic emission is a difficult part of any 21 cm EoR program, and we present new upper limits to diffuse polarized foregrounds at 150 MHz. We find no high significance evidence of polarized emission in our observed field at mid galactic latitude (J2000 08h26m+26). We find an upper limit on the 2-dimensional angular power spectrum of diffuse polarized foregrounds of [l^2 C_l/(2 PI)]^{1/2}< 3K in frequency bins of width 1 MHz at 300<l<1000. The 3-dimensional power spectrum of polarized emission, which is most directly relevant to EoR observations, is [k^3 P_p(k)/(2 PI^2)]^{1/2}< 2K at k_perp > 0.03 h/Mpc, k < 0.1 h/Mpc. This can be compared to the expected EoR signal in total intensity of [k^3 P(k)/ (2 PI^2) ]^{1/2} ~ 10 mK. We find polarized structure is substantially weaker than suggested by extrapolation from higher frequency observations, so the new low upper limits reported here reduce the anticipated impact of these foregrounds on EoR experiments. We discuss Faraday beam and depth depolarization models and compare predictions of these models to our data. We report on a new technique for polarization calibration using pulsars, as well as a new technique to remove broadband radio frequency interference. Our data indicate that, on the edges of the main beam at GMRT, polarization squint creates ~ 3% leakage of unpolarized power into polarized maps at zero rotation measure. Ionospheric rotation was largely stable during these solar minimum night time observations. Comment: 17 pages, 6 figures, 2 tables; changed figures, added appendices. To appear in MNRAS
We have just completed the observational stage of a 2·4 GHz survey of the Southern Galactic Plane, using the Parkes radiotelescope, between 238° ≤ l ≤ 365° and with a latitude range of at least | b | ≤ 5° (beamwidth ~9 arcmin). The survey details both continuum emission and linear polarisation down to rms noises of approximately 12 and 4 mJy/beam respectively. It is the most sensitive survey to date of the southern plane at this frequency and should nicely complement the Effelsberg northern plane surveys in addition to the recent southern surveys such as the 843 MHz MOST survey and the 5 GHz PMN survey. The total-power maps are now completed, and reduction of the polarisation data is still in progress. In addition to compact HII regions and extragalactic sources, we are detecting on our total-power images a considerable amount of large-scale structure, and a significant number of new SNR candidates and spur-like features.
We present the first section of a radio continuum and polarization survey at medium Galactic latitudes carried out with the Effelsberg 100-m telescope at 1.4 GHz. Four large fields have been observed, which all together cover an area of about 1100 square degree. The rms-sensitivity is about 15 mK TB (about 7 mJy/beam area) in total intensity and is limited by confusion. A sensitivity of 8 mK TB is obtained in linear polarization. The angular resolution of the observations is 9.35 arcminute. The maps in total intensity and linear polarization have been absolutely calibrated by low resolution data where available. Significant linear polarization is seen in all the maps. In general, the intensity fluctuations measured in linear polarization are not correlated with total intensity structures. Areas of high polarization of some degrees extent are seen, again with no apparent corresponding total intensity feature. Modulation of polarized background emission by spatially varying Faraday rotation seems the most likely explanation. Quite unexpected is the detection of filamentary and ring-like depolarization structures in the direction of the anticentre region, whose extents are up to about 3 degree. Comment: Latex2 document, 17 pages, 14 figures. Accepted for publication in Astronomy and Astrophysics Supp. Ser
Angular power spectra and structure functions of the Stokes parameters Q and
U and polarized intensity P are derived from three sets of radio polarimetric
observations. Two of the observed fields have been studied at multiple
frequencies, allowing determination of power spectra and structure functions of
rotation measure RM as well. The third field extends over a large part of the
northern sky, so that the variation of the power spectra over Galactic latitude
and longitude can be studied. The power spectra of Q and U are steeper than
those of P, probably because a foreground Faraday screen creates extra
structure in Q and U, but not in P. The extra structure in Q and U occurs on
large scales, and therefore causes a steeper spectrum. The derived slope of the
power spectrum of P is the multipole spectral index, and is consistent with
earlier estimates. The multipole spectral index decreases with Galactic
latitude (i.e. the spectrum becomes flatter), but is consistent with a constant
value over Galactic longitude. Power spectra of the rotation measure RM show a
spectral index of about 1, while the structure function of RM is approximately
flat. The structure function is flatter than earlier estimates from polarized
extragalactic sources, which could be due to the fact that extragalactic source
RM probes the complete line of sight through the Galaxy, whereas as a result of
depolarization diffuse radio polarization only probes the nearby ISM.
The polarized component of the diffuse radio synchrotron emission of our Galaxy shows structure, which is apparently unrelated to the structure in total intensity, on many scales. The structure in the polarized emission can be due to several processes or mechanisms. Some of those are related to the observational setup, such as beam depolarization -- the vector combination and (partial) cancellation of polarization vectors within a synthesized beam --, or the insensitivity of a synthesis telescope to structure on large scales, also known as the 'missing short spacings problem'. Other causes for structure in the polarization maps are intrinsic to the radiative transfer of the emission in the warm ISM, which induces Faraday rotation and depolarization. We use data obtained with the Westerbork Synthesis Radio Telescope at 5 frequencies near 350 MHz to estimate the importance of the various mechanisms in producing structure in the linearly polarized emission. In the two regions studied here, which are both at positive latitudes in the second Galactic quadrant, the effect of 'missing short spacings' is not important. The properties of the narrow depolarization 'canals' that are observed in abundance lead us to conclude that they are mostly due to beam depolarization, and that they separate regions with different rotation measures. As beam depolarization only creates structure on the scale of the synthesized beam, most of the structure on larger scales must be due to depth depolarization. We do not discuss that aspect of the observations here, but in a companion paper we derive information about the properties of the ISM from the structure of the polarized emission. Comment: 12 pages, 10 figures, accepted for publication by A&A
We report on full-polarization radio observations of the Perseus cluster (Abell 426) using the Westerbork Synthesis Radio Telescope (WSRT) at wavelengths from 81-95 cm. We have employed a novel technique, Rotation Measure synthesis (Brentjens and de Bruyn, 2005) to unravel the polarization properties of the emission across the full field of view and detect polarized emission over a wide range of RM from about 0 to 90 rad m^-2. The low RM emission is associated with our Galaxy, while the high RM emission is associated with the Perseus cluster. The latter reaches typical surface brightness levels of 0.5-1 mJy per beam and must be rather highly polarized. Most of the peripheral polarized emission appears too bright, by about 1-2 orders of magnitude, to be explainable as Thomson scattered emission of the central radio source off the thermal electrons in the cluster. The bulk of the emission associated with the Perseus cluster is probably related to buoyant bubbles of relativistic plasma, probably relics from still active or now dormant AGN within the cluster. A lenticular shaped structure measuring 0.5-1 Mpc is strikingly similar to the structures predicted by Ensslin et al. (1998). At the western edge of the cluster, we detect very long, linear structures that may be related to shocks caused by infall of gas into the Perseus cluster. Comment: 18 pages, 17 figures, accepted by A&A, corrected small typo, added reference
We extend the rotation measure work of Burn (1966) to the cases of limited sampling of lambda squared space and non-constant emission spectra. We introduce the rotation measure transfer function (RMTF), which is an excellent predictor of n-pi ambiguity problems with the lambda squared coverage. Rotation measure synthesis can be implemented very efficiently on modern computers. Because the analysis is easily applied to wide fields, one can conduct very fast RM surveys of weak spatially extended sources. Difficult situations, for example multiple sources along the line of sight, are easily detected and transparently handled. Under certain conditions, it is even possible to recover the emission as a function of Faraday depth within a single cloud of ionized gas. Rotation measure synthesis has already been successful in discovering widespread, weak, polarized emission associated with the Perseus cluster (De Bruyn and Brentjens, 2005). In simple, high signal to noise situations it is as good as traditional linear fits to polarization angle versus lambda squared plots. However, when the situation is more complex or very weak polarized emission at high rotation measures is expected, it is the only viable option. Comment: 17 pages, 14 figures, accepted by A&A, added references, corrected typos
A study is made of the implications of the recent polarization measurements for the structures of discrete radio sources and the source–observer media. Simple models of wavelength-dependent depolarizing mechanisms are investigated and it is found that most are incompatible with the observations of Gardner & Whiteoak. The models of internal Faraday dispersion predict a lower polarization at 30 cm than is observed. It is suggested that the depolarization of the Crab nebula is produced by Faraday rotation in the filamentary shell that surrounds the nebula. Such filaments could also exist in the outer regions of extragalactic sources.
A complex number representation is used for the state of linear polarization and a Faraday dispersion function is defined to describe the distribution of polarized radiation with respect to Faraday depth. The persistence of polarization at 30 cm, after partial depolarization between 10 cm and 20 cm, implies that the radiation is spread over a large range of Faraday depths. The observed linearity of the plot of the angle of polarization against wavelength squared for most sources implies that it is justifiable to make an assumption which enables one to calculate the Faraday dispersion function of a source from the dependence of its polarization on wavelength.
Estimates are given for upper limits to the densities of internal ionized gases in the sources for which we have polarization measurements.
We present low frequency radio polarimetric observations of the Perseus cluster. The data were taken with the Westerbork Synthesis Radio Telescope (WSRT) between 315–360 MHz. We have discovered faint, extended, highly polarized emission that we associate with the Perseus cluster (de Bruyn & Brentjens 2005). We propose that at least one of these structures is associated with large-scale structure formation gas inflow from the Perseus-Pisces supercluster. The extragalactic emission is seen through a polarized Galactic synchrotron foreground screen. The discovery was made possible by a novel rotation measure analysis technique, called Faraday Rotation Measure Synthesis, or RM-synthesis for short (Brentjens & de Bruyn 2005). (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Faraday rotation and depolarization of synchrotron radio emission are considered in a consistent general approach, under conditions typical of spiral galaxies, i.e. when the magneto-ionic medium and relativistic electrons are non-uniformly distributed in a layer containing both regular and fluctuating components of magnetic field, thermal electron density and synchrotron emissivity. We demonstrate that non-uniformity of the magneto-ionic medium along the line of sight strongly affects the observable polarization patterns. The degree of polarization p and the observed Faraday rotation measure RM are very sensitive to whether or not the source is symmetric along the line of sight. The RM may change sign in a certain wavelength range in an asymmetric slab even when the line-of-sight magnetic field has no reversals. Faraday depolarization in a purely regular magnetic field can be much stronger than suggested by the low observed rotation measures. A twisted regular magnetic field may result in p increasing with λ— a behaviour detected in several galaxies. We derive expressions for statistical fluctuations in complex polarization and show that random fluctuations in the degree of polarization caused by Faraday dispersion are expected to become significantly larger than the mean value of p at λ ≳ 20 − 30 cm. We also discuss depolarization arising from a gradient of Faraday rotation measure across the beam, both in the source and in an external Faraday screen. We briefly discuss applications of the above results to radio polarization observations. We discuss how the degree of polarization is affected by the scaling of synchrotron emissivity ɛ with the total magnetic field strength B. We derive formulae for the complex polarization at λ → 0 under the assumption that ɛ ∝ B2B2⊥, which may arise under energy equipartition or pressure balance between cosmic rays and magnetic fields. The resulting degree of polarization is systematically larger than for the usually adopted scaling ɛ ∝ B2⊥; the difference may reach a factor of 1.5.
Thesis (Ph. D.)--University of Texas at Austin, 1999. Vita. Includes bibliographical references (leaves 110-116).
We describe a first attempt to derive properties of the regular and turbulent Galactic magnetic field from multi-frequency polarimetric observations of the diffuse Galactic synchrotron background. A single-cell-size model of the thin Galactic disk is constructed which includes random and regular magnetic fields and thermal and relativistic electrons. The disk is irradiated from behind with a uniform partially polarized background. Radiation from the background and from the thin disk is Faraday rotated and depolarized while propagating through the medium. The model parameters are estimated from a comparison with 350 MHz observations in two regions at intermediate latitudes done with the Westerbork Synthesis Radio Telescope. We obtain good consistency between the estimates for the random and regular magnetic field strengths and typical scales of structure in the two regions. The regular magnetic field strength found is a few microGauss, and the ratio of random to regular magnetic field strength is 0.7 +/- 0.5, for a typical scale of the random component of 15 +/- 10 pc. Furthermore, the regular magnetic field is directed almost perpendicular to the line of sight. This modeling is a potentially powerful method to estimate the structure of the Galactic magnetic field, especially when more polarimetric observations of the diffuse synchrotron background at intermediate latitudes become available. Comment: 12 pages, 6 figures, accepted by A&A