865 research outputs found
Polarization of the Sunyaev-Zel'dovich effect: relativistic imprint of thermal and non-thermal plasma
[Abridged] Inverse Compton scattering of CMB fluctuations off cosmic electron
plasma generates a polarization of the associated Sunyaev-Zel'dovich (SZ)
effect. This signal has been studied so far mostly in the non-relativistic
regime and for a thermal electron population and, as such, has limited
astrophysical applications. Partial attempts to extend this calculation for a
thermal electron plasma in the relativistic regime have been done but cannot be
applied to a general relativistic electron distribution. Here we derive a
general form of the SZ effect polarization valid in the full relativistic
approach for both thermal and non-thermal electron plasmas, as well as for a
generic combination of various electron population co-spatially distributed in
the environments of galaxy clusters or radiogalaxy lobes. We derive the
spectral shape of the Stokes parameters induced by the IC scattering of every
CMB multipole, focusing on the CMB quadrupole and octupole that provide the
largest detectable signals in galaxy clusters. We found that the CMB quadrupole
induced Stoke parameter Q is always positive with a maximum amplitude at 216
GHz which increases slightly with increasing cluster temperature. The CMB
octupole induced Q spectrum shows, instead, a cross-over frequency which
depends on the cluster electron temperature, or on the minimum momentum p_1 as
well as on the power-law spectral index of a non-thermal electron population.
We discuss some possibilities to disentangle the quadrupole-induced Q spectrum
from the octupole-induced one which allow to measure these quantities through
the SZ effect polarization. We finally apply our model to the realistic case of
the Bullet cluster and derive the visibility windows of the total,
quandrupole-induced and octupole-induced Stoke parameter Q in the frequency
ranges accessible to SKA, ALMA, MILLIMETRON and CORE++ experiments.Comment: 31 pages, 11 figures, submitted to JCA
Multi-frequency constraints on the non-thermal pressure in galaxy clusters
The origin of radio halos in galaxy clusters is still unknown and is the
subject of a vibrant debate both from the observational and theoretical point
of view. In particular the amount and the nature of non-thermal plasma and of
the magnetic field energy density in clusters hosting radio halos is still
unclear. The aim of this paper is to derive an estimate of the pressure ratio X
between the non-thermal and thermal plasma in radio halo clusters that have
combined radio, X-ray and SZ effect observations. From the simultaneous
P_{1.4}-L_X and P_{1.4}-Y_{SZ} correlations for a sample of clusters observed
with Planck, we derive a correlation between Y_{SZ} and L_X that we use to
derive a value for X. This is possible since the Compton parameter Y_{SZ} is
proportional to the total plasma pressure in the cluster (that we characterize
as the sum of the thermal and non-thermal pressure) while the X-ray luminosity
L_X is proportional only to the thermal pressure of the intracluster plasma.
Our results indicate that the average (best fit) value of the pressure ratio in
a self-similar cluster formation model is X =0.55 \pm 0.05 in the case of an
isothermal beta-model with beta=2/3 and a core radius r_c = 0.3 R_{500} holding
on average for the cluster sample. We also show that the theoretical prediction
for the Y_{SZ}-L_X correlation in this model has a slope that is steeper than
the best fit value for the available data. The agreement with the data can be
recovered if the pressure ratio X decreases with increasing X-ray luminosity as
L_X^{-0.96}. We conclude that the available data on radio halo clusters
indicate a substantial amount of non-thermal pressure in cluster atmospheres
whose value must decrease with increasing X-ray luminosity, or increasing
cluster mass (temperature). (abridged)Comment: A&A, in press; 10 pages; 10 figure
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