A New Radio–X-Ray Probe of Galaxy Cluster Magnetic Fields

Clarke, T. E.; Kronberg, P. P.; Böhringer, H.

doi:10.1086/318896

Cited by 531 publications

(616 citation statements)

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“…The FR effect on CMB anisotropies has been considered in the literature as a possible way to generate a Stokes-U contribution from the dominant Stokes-Q contribution both for a primordial [12] and galaxy cluster [13] magnetic fields. In the case of galaxy cluster magnetic fields, the observed rotation measures up to 250 radians m −2 in nearby massive clusters [10], suggest that the CMB linear polarization is rotated, on average, by an angle of order 10 −1 radians at an observed frequency of few GHz. Thus, FR can potentially generate a Stokes-U contribution which is of order 10 −7 in fractional temperature, ∆T pol /T CMB [14], from the dominant Stokes-Q contribution with an rms of order 10 −6 at 10 GHz [13].…”

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confidence: 93%

Is the cosmic microwave background circularly polarized?

2003

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The primordial anisotropies of the cosmic microwave background (CMB) are linearly polarized via Compton-scattering. The Faraday conversion process during the propagation of polarized CMB photons through regions of the large-scale structure containing magnetized relativistic plasma, such as galaxy clusters, will lead to a circularly polarized contribution. Though the resulting Stokes-V parameter is of order 10 −9 at frequencies of 10 GHz, the contribution can potentially reach the level of total Stokes-U at low frequencies due to the cubic dependence on the wavelength. In future, the detection of circular polarization of CMB can be used as a potential probe of the physical properties associated with relativistic particle populations in large-scale structures.The CMB anisotropies are expected to be linearly polarized by anisotropic Compton scattering around the epoch of recombination [1]. This linear polarization field has been widely discussed in the literature [2] and its accurate measurement can ultimately shed new light on the thermal history of the universe and on the primordial gravitational-wave background. It is also well established that, in the standard scenario, no relevant circular polarization should be present. For these reasons, many of the present and near future CMB experiments like MAP and Planck Surveyor have not been designed for a detection of circular polarization.It is therefore quite probable that in the near future, in spite of a continuous incremental knowledge on linear CMB polarization, the experimental bounds on the circular component will not drastically improve from those of early observations [3]. Since post-Planck CMB polarization experiments are already under study [4], it is extremely timely to address the question whether CMB is circularly polarized and what physical information can be extracted from its measurement. This letter represents a discussion in this direction.In order to understand the CMB polarization field, we make use of the Stokes parameters [5]. In the case of a propagating wave in the z direction, E = (E x e iφxx + E y e iφyŷ )e −iωt , with amplitudes E x and E y in the x and y-directions with phases φ x and φ y , respectively, we can write the Stokes parameters as time averaged quantities:Note that the total intensity of the radiation is given by the Stokes-I parameter, while for unpolarized radiation Q = U = V = 0. The linearly polarized radiation is defined by non-zero values for Q and/or U . These latter two Stokes parameters form a spin-2 basis; A rotation of the coordinate system, by an angle θ, leads to a new set of parameters for the same radiation field given by (Q ± iŪ) = (Q ± iU )e 2iθ . This coordinate dependence is avoided in the literature by introducing a new set of orthonormal basis with a part containing the gradient of a scalar field, called grad-or E-modes, and a part containing the curl of a vector field, called curl-or Bmodes [6]. Note that the Stokes-V parameter, which is coordinate-independent similar to Stokes-I, defines the extent to whi...

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confidence: 93%

Is the cosmic microwave background circularly polarized?

2003

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“…They are further revealed by the presence of cluster-wide Mpc scale radio halos in some clusters of galaxies, believed to be the result of synchrotron emission of relativistic electrons accelerated in magnetic fields. The role of the magnetic fields in clusters is still a matter of debate, but some recent works indicate important dynamical influences in the ICM (Clarke, Kronberg, & Böhringer 2001, Vikhlinin, Markevitch, & Murray 2001. Outside clusters, no firm detections of magnetic field in the intergalactic medium (IGM) has yet been reported to our knowledge, but speculations on the magnitude of magnetic fields in IGM range from 10 −7 G to 10 −12 G.…”

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confidence: 99%

Evidence for shock acceleration and intergalactic magnetic fields in a large-scale filament of galaxies ZwCl 2341.1+0000

et al. 2002

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“…There are some measurements of diffuse radio emission from the bridge area between the Coma and Abell superclusters [26], which under assumptions of equipartition allows an estimate of O(0.2 − 0.6) µG for the magnetic field in this region. Fields of O(µG) are also indicated in a more extensive study of 16 low redshift clusters [27]. It is assumed that the observed B-fields result from the amplification of much weaker seed fields.…”

Section: Appendix B: Extragalactic Magnetic Fieldmentioning

confidence: 99%

Update on tests of the Cen A neutron-emission model of highest energy cosmic rays

2011

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We propose that neutron emission from Cen A dominates the cosmic ray sky at the high end of the spectrum. Neutrons that decay generate proton diffusion fronts, whereas those that survive decay produce an angular spike in the direction of the source. We use recent data reported by the Pierre Auger Collaboration to normalize the injection spectrum and estimate the required luminosity in cosmic rays. We find that such a luminosity, LCR ∼ 5 × 10 40 erg/s, is comfortably smaller than the bolometric luminosity of Cen A, L bol ∼ 10 43 erg/s. We compute the incoming current flux density as viewed by an observer on Earth, and we show that the anisotropy amplitude is in agreement with data at the 1σ level. Regardless of the underlying source model, our results indicate that after a decade of data taking the Pierre Auger Observatory will be able to test our proposal.

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A New Radio–X-Ray Probe of Galaxy Cluster Magnetic Fields

Cited by 531 publications

References 14 publications

Is the cosmic microwave background circularly polarized?

Is the cosmic microwave background circularly polarized?

Evidence for shock acceleration and intergalactic magnetic fields in a large-scale filament of galaxies ZwCl 2341.1+0000

Update on tests of the Cen A neutron-emission model of highest energy cosmic rays

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