Spectropolarimetric observations of the CIZA J2242.8+5301 northern radio relic: no evidence of high-frequency steepening

Loi, F.; Murgia, M.; Vacca, V.; Govoni, F.; Melis, A.; Wittor, Denis; Beck, Rainer; Kierdorf, M.; Bonafede, A.; Boschin, W.; Brienza, M.; Carretti, E.; Concu, R.; Feretti, L.; Gastaldello, F.; Paladino, R.; Rajpurohit, K.; Serra, P.; Vazza, Franco

doi:10.1093/mnras/staa2267

Cited by 18 publications

(18 citation statements)

References 44 publications

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“…The spectral index value is consistent with our previous estimates (Rajpurohit et al 2018(Rajpurohit et al , 2020. Recently, the power-law spectrum results are also found for the relic in CIZA J2242.8+5301 (Loi et al 2020). The power-law spectrum is consistent with the standard scenario for the relic formation.…”

Section: Integrated Spectrumsupporting

confidence: 91%

A perfect power-law spectrum even at the highest frequencies: The Toothbrush relic

Rajpurohit

Vazza

Hoeft

et al. 2020

A&A

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Radio relics trace shock fronts generated in the intracluster medium (ICM) during cluster mergers. The particle acceleration mechanism at the shock fronts is not yet completely understood. We observed the Toothbrush relic with the Effelsberg and Sardinia Radio Telescope at 14.25 GHz and 18.6 GHz, respectively. Unlike previously claimed, the integrated spectrum of the relic closely follows a power law over almost three orders of magnitude in frequency, with a spectral index of α58 MHz18.6 GHz = −1.16 ± 0.03. Our finding is consistent with a power-law injection spectrum, as predicted by diffusive shock acceleration theory. The result suggests that there is only little magnetic field strength evolution downstream of the shock. From the lack of spectral steepening, we find that either the Sunyaev–Zeldovich decrement produced by the pressure jump is less extended than ∼600 kpc along the line of sight or, conversely, that the relic is located far behind in the cluster. For the first time, we detect linearly polarized emission from the “brush” at 18.6 GHz. Compared to 8.3 GHz, the degree of polarization across the brush increases at 18.6 GHz, suggesting a strong Faraday depolarization toward lower frequencies. The observed depolarization is consistent with an intervening magnetized screen that arises from the dense ICM containing turbulent magnetic fields. The depolarization, corresponding to a standard deviation of the rotation measures as high as σRM = 212 ± 23 rad m−2, suggests that the brush is located in or behind the ICM. Our findings indicate that the Toothbrush relic can be consistently explained by the standard scenario for relic formation.

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Section: Integrated Spectrumsupporting

confidence: 91%

A perfect power-law spectrum even at the highest frequencies: The Toothbrush relic

Rajpurohit

Vazza

Hoeft

et al. 2020

A&A

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“…We assume the dispersion of the reproducibility of the 3C 286 flux density as an estimate of the systematic uncertainties in our calibration procedure due to errors in pointing, opacity correction, atmospheric fluctuations removal, among others. This systematic error is consistent with what was found by Loi et al (2020) using a much larger data set.…”

Section: Sample Observations and Data Reductionsupporting

confidence: 92%

Searching for anomalous microwave emission in nearby galaxies

Bianchi

Murgia

Melis

et al. 2022

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Aims. We observed four nearby spiral galaxies (NGC 3627, NGC 4254, NGC 4736, and NGC 5055) in the K band with the 64-m Sardinia Radio Telescope, with the aim of detecting anomalous microwave emission (AME), a radiation component presumably due to spinning dust grains, which has been observed thus far in the Milky Way and only in a handful of other galaxies (most notably, M 31). Methods. We mapped the galaxies at 18.6 and 24.6 GHz and studied their global photometry together with other radio-continuum data from the literature in order to find AME as emission in excess of the synchrotron and thermal components. Results. We only found upper limits for AME. These nondetections, and other upper limits in the literature, are nevertheless consistent with the average AME emissivity from a few detections: it is ϵ30 GHzAME = 2.4 ± 0.4 × 10−2 MJy sr−1 (M⊙ pc−2)−1 in units of dust surface density (equivalently, 1.4 ± 0.2 × 10−18 Jy sr−1 (H cm−2)−1 in units of H column density). We finally suggest searching for AME in quiescent spirals with relatively low radio luminosity, such as M 31.

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“…Merger-shock models predict the relic should be highly polarized only when viewed close to edge-on (Skillman et al 2013). For the Sausage and the Toothbrush relics, which are very likely seen edge-on, high polarization fractions (55%-70%) have been reported (van Weeren et al 2010(van Weeren et al , 2012Loi et al 2020;Rajpurohit et al 2020b;Di Gennaro et al 2021). It is believed that the high polarization fraction in relics is due to shock waves, which compress and hence align isotropically distributed magnetic fields.…”

Section: Intrinsic Polarization Angle At the Shockmentioning

confidence: 99%

“…Radio relics are strongly polarized at frequencies above 1 GHz, some with a polarization fraction as high as 65% (e.g., van Weeren et al 2010van Weeren et al , 2012Owen et al 2014;Kierdorf et al 2017;Loi et al 2020;Rajpurohit et al 2020b;Di Gennaro et al 2021). The inferred magnetic field directions are often found to be well aligned with the shock surface.…”

Section: Introductionmentioning

confidence: 99%

Turbulent magnetic fields in the merging galaxy cluster MACS J0717.5+3745

Rajpurohit

Hoeft

Wittor

et al. 2021

A&A

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We present wideband (1 − 6.5 GHz) polarimetric observations, obtained with the Karl G. Jansky Very Large Array, of the merging galaxy cluster MACS J0717.5+3745, which hosts one of the most complex known radio relic and halo systems. We used both rotation measure synthesis and QU-fitting to find a reasonable agreement of the results obtained with these methods, particularly when the Faraday distribution is simple and the depolarization is mild. The relic is highly polarized over its entire length (850 kpc), reaching a fractional polarization > 30% in some regions. We also observe a strong wavelength-dependent depolarization for some regions of the relic. The northern part of the relic shows a complex Faraday distribution, suggesting that this region is located in or behind the intracluster medium (ICM). Conversely, the southern part of the relic shows a rotation measure very close to the Galactic foreground, with a rather low Faraday dispersion, indicating very little magnetoionic material intervening along the line of sight. Based on a spatially resolved polarization analysis, we find that the scatter of Faraday depths is correlated with the depolarization, indicating that the tangled magnetic field in the ICM causes the depolarization. We conclude that the ICM magnetic field could be highly turbulent. At the position of a well known narrow-angle-tailed galaxy (NAT), we find evidence of two components that are clearly separated in the Faraday space. The high Faraday dispersion component seems to be associated with the NAT, suggesting the NAT is embedded in the ICM while the southern part of the relic lies in front of it. If true, this implies that the relic and this radio galaxy are not necessarily physically connected and, thus, the relic may, in fact, not be powered by the shock re-acceleration of fossil electrons from the NAT. The magnetic field orientation follows the relic structure indicating a well-ordered magnetic field. We also detected polarized emission in the halo region; however, the absence of significant Faraday rotation and a low value of Faraday dispersion suggests the polarized emission that was previously considered as the part of the halo does, in fact, originate from the shock(s).

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Spectropolarimetric observations of the CIZA J2242.8+5301 northern radio relic: no evidence of high-frequency steepening

Cited by 18 publications

References 44 publications

A perfect power-law spectrum even at the highest frequencies: The Toothbrush relic

A perfect power-law spectrum even at the highest frequencies: The Toothbrush relic

Searching for anomalous microwave emission in nearby galaxies

Turbulent magnetic fields in the merging galaxy cluster MACS J0717.5+3745

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