Investigations of Faraday Rotation Maps of Extended Radio Sources ...

18 CHAPTER 1. INTRODUCTION
In general, equipartition magnetic fields in relics are determined to be ∼ 0.5 . . . 2µG
(Govoni et al. 2001a). However due to the believed origin of the relics, the magnetic
fields might be compressed (as discussed below) in these sources and, thus, might not
be indicative for the overall magnetic field in the peripheral regions of the cluster.
There is growing evidence that relics are results of ongoing merger events. Enßlin
et al. (1998) suggest that relics are caused by first order Fermi acceleration of relativistic
particles in shocks produced during cluster merger events. Another scenario
suggests that relics are fossil radio sources which were revived by compression associated
with mergers (Enßlin & Gopal-Krishna 2001). Both types of relics might exist
in reality.
As described in Sect. 1.2.3, apart from equipartition arguments there is also the
comparison of synchrotron radio emission with the Inverse Compton X-ray emission.
Hard X-ray excess was detected in the spectrum of Coma (e.g. Fusco-Femiano et al.
1999; Rephaeli et al. 1999). A measurement using BeppoSAX data detected a nonthermal
excess with respect to the thermal radiation of Hard X-ray at a ∼ 4.8σ level
(Fusco-Femiano et al. 2004). Although, this detection is currently under debate (see
footnote in Enßlin & Hansen 2004), applying Eq. (1.25), however, yields a volume
averaged intra-cluster magnetic field of ∼ 0.2µG. The IC and equipartition estimates
for the Coma cluster agree very well with each other.
Another example where excess Hard X-ray emission has been detected is in the
cluster Abell 2256 (Fusco-Femiano et al. 2000; Rephaeli & Gruber 2003). The analysis
of this excess leads to magnetic field strength of 0.05µG in the relic region at the
periphery. Higher fields of about ∼ 0.5µG in the region of the halo in the cluster
centre could be present.
1.3.2 Rotation Measures
In order to study cluster magnetic fields through RM measurements, polarised radio
sources embedded or behind cluster environments have to be observed. In principle,
any extragalactic RM consists of different components
RM = RM intr + RM ICM + RM gal + RM ionsp , (1.49)
where RM intr is the source intrinsic Faraday rotation, RM gal is the Faraday rotation
due to the galactic foreground, RM ionosp is the Faraday rotation due to the ionosphere
of the Earth and RM ICM is the Faraday rotation associated with the cluster gas. The
contribution from the Ionosphere of the Earth is in general very small and can be
neglected. The galactic component can be subtracted by comparison with RM point
source catalogues of pulsars tracing the RM distribution of our Galaxy which is of
the order of 10 rad m −2 up to 300 rad m −2 for sources at low galactic latitudes (e.g.
Simard-Normandin et al. 1981; Han et al. 1999).
Once the galactic contribution is subtracted from the RM signal, the contribution
from the ICM should dominate the residual RM signal. High resolution RM studies
of Cygnus A (Dreher et al. 1987) reveal high RM values with large gradients on arcsec
scale which cannot be attributed to a galactic origin or to a thermal gas mixed with the
synchrotron emitting plasma. It is concluded that the RM must arise in an external
screen of magnetised, ionised plasma. This high resolution studies have been performed
on several other extended sources resulting in the same conclusion. Another