Clusters of galaxies are the largest bound systems and the most important
link to the large scale structure (LSS) of the Universe. The detailed
properties of clusters, such as the distributions of their various mass
constituents - dark matter, hot intracluster (IC) gas, and galaxies -
dynamics, and thermal structure, are of much interest both intrinsically,
and for the understanding of the formation and evolution of the LSS.
Moreover, detailed astrophysical knowledge of clusters is essential
for their use as precise cosmological probes to measure global
parameters, such as H0,
M, &
, and
parameters characterising the primordial density fluctuation field.
Recent observations of many clusters of galaxies with the Chandra
and XMM satellites at energies
10 keV, have
significantly advanced our knowledge of the morphology and thermal
structure of hot IC gas, the source of the cluster thermal Bremsstrahlung
emission. The improved determinations of the gas temperature, density,
and metal abundances from these observations significantly improve
estimates of such important quantities as the total cluster mass and its
gaseous and baryonic mass fractions.
As has been the case in galaxies, in clusters too a more physically
complete understanding of these systems necessitates knowledge also of
non-thermal (NT) quantities and phenomena in the IC space. Observational
evidence for the relevance of these phenomena in clusters comes mostly
from measurements of extended regions of radio emission, and from Faraday
rotation (FR) of the plane of polarisation of radio sources seen through
(or inside) clusters. Since the observed radio emission is clearly
synchrotron-produced, its level and spectrum yield direct information on
IC relativistic electrons and magnetic fields. Information on cluster
magnetic fields (separately from relativistic electron properties) is
obtained also from FR measurements. Compton scattering of cosmic
microwave background (CMB) photons by the radio-emitting relativistic
electrons boosts photon energies to the
X-and-
regions (e.g.,
Rephaeli
1979).
The search for cluster NT X-ray emission has begun long ago
(Rephaeli et al. 1987),
but first clear indications for emission at energies
20 keV
came only after deep dedicated observations of a few clusters with the
RXTE and BeppoSAX satellites (beginning with analyses of
observations of the Coma cluster
(Rephaeli et
al. 1999,
Fusco-Femiano et al. 1999).
Radio and NT X-ray observations provide quantitative measures of very
appreciable magnetic fields and relativistic electron densities in the
observed clusters. These results open a new dimension in the study of
clusters.
This is a review of cluster NT X-ray observations and their direct
implications, including prospects for the detection of
-ray
emission. The literature on cluster NT phenomena is (perhaps somewhat
surprisingly) quite extensive, including several reviews of radio
emission (e.g.,
Govoni &
Feretti 2004)
and cluster magnetic fields (e.g.,
Carilli &
Taylor 2002),
and a review of the current status of radio observations by
Ferrari et
al. 2008
- Chapter 6, this volume. In
order to properly address the comparison between magnetic field values
deduced from radio observations and jointly from NT X-ray and radio
measurements, we include here a brief summary of cluster radio
observations. Measurements of EUV emission in a few clusters, and claims
that this emission is by energetic electrons, are reviewed by
Durret et
al. 2008 -
Chapter 4, this volume. NT radiation processes are reviewed by
Petrosian et
al. 2008
- Chapter 10, this volume, and relevant aspects
of particle acceleration mechanisms are reviewed by
Petrosian
& Bykov 2008
- Chapter 11, this volume.