The detection of cluster magnetic fields via radio source depolarisation

Osinga, E.; Weeren, R. J. van; Andrade-Santos, Felipe; Rudnick, Lawrence; Bonafede, A.; Duncan, K. J.; Giacintucci, S.; Mroczkowski, Tony; Röttgering, H. J. A.

doi:10.1051/0004-6361/202243526

Cited by 7 publications

(2 citation statements)

References 95 publications

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“…Firstly, FR Is are more commonly found in galaxy clusters (Best 2009 ;Gendre et al 2013 ;Croston et al 2019 ) and the strong Faraday depolarization observed there (e.g. Osinga et al 2022 ) means that polarized sources embedded in (and in the background of) these environments are not found at 144 MHz (Carretti et al 2022(Carretti et al , 2023. Secondly, the brightest and most highly polarized regions of FR IIs are at the source extremities, extending well beyond the host galaxy environment in many cases, where they are known to experience lower depolarization (e.g.…”

Section: The Nature Of Polarized Sources At Mhz Frequenciesmentioning

confidence: 99%

The Faraday Rotation Measure Grid of the LOFAR Two-metre Sky Survey: Data Release 2

O’Sullivan

Shimwell

Hardcastle

et al. 2023

Monthly Notices of the Royal Astronomical Society

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A Faraday rotation measure (RM) catalogue, or RM Grid, is a valuable resource for the study of cosmic magnetism. Using the second data release (DR2) from the LOFAR Two-metre Sky Survey (LoTSS), we have produced a catalogue of 2,461 extragalactic high-precision RM values across 5,720 deg2 of sky (corresponding to a polarized source areal number density of ∼0.43 deg−2). The linear polarization and RM properties were derived using RM synthesis from the Stokes Q and U channel images at an angular resolution of 20″ across a frequency range of 120 to 168 MHz with a channel bandwidth of 97.6 kHz. The fraction of total intensity sources (>1 mJy beam−1) found to be polarized was ∼0.2 per cent. The median detection threshold was 0.6 mJy beam−1 (8σQU), with a median RM uncertainty of 0.06 rad m−2 (although a systematic uncertainty of up to 0.3 rad m−2 is possible, after the ionosphere RM correction). The median degree of polarization of the detected sources is 1.8 per cent, with a range of 0.05 per cent to 31 per cent. Comparisons with cm-wavelength RMs indicate minimal amounts of Faraday complexity in the LoTSS detections, making them ideal sources for RM Grid studies. Host galaxy identifications were obtained for 88 per cent of the sources, along with redshifts for 79 per cent (both photometric and spectroscopic), with the median redshift being 0.6. The focus of the current catalogue was on reliability rather than completeness, and we expect future versions of the LoTSS RM Grid to have a higher areal number density. In addition, 25 pulsars were identified, mainly through their high degrees of linear polarization.

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Section: The Nature Of Polarized Sources At Mhz Frequenciesmentioning

confidence: 99%

The Faraday Rotation Measure Grid of the LOFAR Two-metre Sky Survey: Data Release 2

O’Sullivan

Shimwell

Hardcastle

et al. 2023

Monthly Notices of the Royal Astronomical Society

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“…In Figure 11, we showed that the residual power, indicating complexity, was approximately 2× higher in Φ 𝑓 𝑢𝑙𝑙 for continuous distributions with Faraday widths in the range 6 -27 rad m −2 . In comparison, the values of 𝜎 𝑅𝑀 , derived by Osinga et al (2022) based on the depolarization of 819 sources, fell into this range 70% of the time, with a median value of 10 rad m −2 .…”

Section: 𝑚𝑖𝑛mentioning

confidence: 91%

Full resolution deconvolution of complex Faraday spectra

Rudnick

Cotton

2023

Monthly Notices of the Royal Astronomical Society

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Polarized synchrotron emission from multiple Faraday depths can be separated by calculating the complex Fourier transform of the Stokes’ parameters as a function of the wavelength squared, known as Faraday Synthesis. As commonly implemented, the transform introduces an additional term $\lambda _0^2$, which broadens the real and imaginary spectra, but not the amplitude spectrum. We use idealized tests to investigate whether additional information can be recovered with a clean process restoring beam set to the narrower width of the peak in the real “full” resolution spectrum with $\lambda _0^2=0$. We find that the $\lambda _0^2$ choice makes no difference, except for the use of a smaller restoring beam. With this smaller beam, the accuracy and phase stability are unchanged for single Faraday components. However, using the smaller restoring beam for multiple Faraday components we find a) better discrimination of the components, b) significant reductions in blending of structures in tomography images, and c) reduction of spurious features in the Faraday spectra and tomography maps. We also discuss the limited accuracy of information on scales comparable to the width of the amplitude spectrum peak, and note a clean-bias, reducing the recovered amplitudes. We present examples using MeerKAT L-band data. We also revisit the maximum width in Faraday depth to which surveys are sensitive, and introduce the variable Wmax, the width for which the power drops by a factor of 2. We find that most surveys cannot resolve continuous Faraday distributions unless the narrower full restoring beam is used.

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The diffuse radio emission in the high-redshift cluster PSZ2 G091.83+26.11: Total intensity and polarisation analysis with Very Large Array 1–4 GHz observations

Gennaro¹,

Brüggen²,

Weeren³

et al. 2023

A&A

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Context. Diffuse radio emission in galaxy clusters, namely radio halos and radio relics, is usually associated with merger events. Despite the tremendous advances in observations in the last decades, the particle (re-)acceleration and magnetic field amplification mechanisms and the connection with the stage and geometry of the cluster merger are still uncertain. Aims. In this paper, we present the peculiar case of PSZ2 G091.83+26.11 at z = 0.822. This cluster hosts a megaparsec-scale radio halo and an elongated radio source whose morphology resembles that of a radio relic. However, the location of this diffuse radio source with respect to the intracluster medium (ICM) distribution and to the cluster centre is not consistent with a simple merger scenario. Methods. We use Karl Jansky Very Large Array data at 1–4 GHz to investigate the spectral and polarisation properties of the diffuse radio emission. We combine these data with previously published data from the Low Frequency Array (LOFAR) in the 120–168 MHz band and from the upgraded Giant Metrewave Radio Telescope (uGMRT) at 250–500 and 550–900 MHz. Finally, we complement the radio data with Chandra X-ray observations in order to compare the thermal and non-thermal emission of the cluster. Results. The elongated radio emission east of the cluster is visible up to 3.0 GHz and has an integrated spectral index of α144 MHz3.0 GHz = −1.24 ± 0.03, with a steepening from −0.89 ± 0.03 to −1.39 ± 0.03. These values correspond to Mach numbers ℳradio, int = 3.0 ± 0.19 and ℳradio, inj = 2.48 ± 0.15. Chandra data revealed a surface brightness discontinuity at the location of the radio source with a compression factor of C = 2.22 −0.30+0.39 (i.e. MXray = 1.93−0.32+0.42). We also found that the source is polarised at GHz frequencies. Using QU-fitting, we estimate an intrinsic polarisation fraction of p0 ∼ 0.2, a Rotation Measure of ∼50 rad m−2 (including the Galactic contribution), and an external depolarisation of σRM ∼ 60 rad m−2. The polarisation B-vectors are aligned with the major axis of the source, suggesting magnetic field compression. Hence, we classify this source as a radio relic. Finally, we found a trend consistent with a linear or super-linear correlation between the non-thermal and thermal emission. Conclusions. We propose an off-axis merger and/or multiple merger events to explain the position and orientation of the relic with the respect to the ICM emission. Given the properties of the radio relic, we speculate that PSZ2 G091.83+26.11 is in a fairly young merger state.

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The detection of cluster magnetic fields via radio source depolarisation

Cited by 7 publications

References 95 publications

The Faraday Rotation Measure Grid of the LOFAR Two-metre Sky Survey: Data Release 2

The Faraday Rotation Measure Grid of the LOFAR Two-metre Sky Survey: Data Release 2

Full resolution deconvolution of complex Faraday spectra

The diffuse radio emission in the high-redshift cluster PSZ2 G091.83+26.11: Total intensity and polarisation analysis with Very Large Array 1–4 GHz observations

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