Contents List of Figures

6.4 A 3–Phase Model for Kiloparsec–Scale Jets in Clusters 285
cluster gas, including emission from the unperturbed atmosphere, excess emission
from the shocked ICM surrounding the radio lobes, and a deficit of emission from
the evacuated radio lobes themselves. X–ray observations provide direct constraints
Figure 134: A low frequency radio image (VLA at 330 MHz, blue to red) superposed
on the Chandra image (yellow–red) of the inner 200 kpc of the Cygnus A cluster.
This shows for the first time the interaction of the radio source and the cluster gas.
on the physical conditions in the emitting regions. In contrast, observations of the
non–thermal radio synchrotron emission provide only information on the properties
of the cocoon and beam plasma. The pressure following from X–ray observations
is 1 × 10 10 dyne cm −2 . Fig. 134 also reveals X–ray emission coincident with radio
hot spots. This is non–thermal IC emission from the same population of electrons
emitting the radio synchrotron photons (SSC emission). While the radio emissivity
is a function of the relativistic electron density and the magnetic field strength, IC
X–ray emissivity constrains the number density alone. From this Wilson derives
magnetic fields of 150 µG for the radio hot spots, the minimum energy field would
be 250 µG.
(i) The Sedov–Phase of Jet Evolution: For density contrasts η < 0.01, the
initial expansion of the bow shock can be described in terms of a Sedov wave propagating
into a medium with decreasing density n(r) = n0(Rrc/r) κ with κ 1.4. The
energy in the bubble increases steadily, E = LBt, as long as the jet power does not
fade away. Similarly to the analysis for supernovae, the expansion of the blast wave