Curved Radio Spectra of Weak Cluster Shocks
release_nyf547y5ijanvmcpjkcwg5ci4u
[as of editgroup_7ymtjomtu5gjvl3vrtl47mvpte]
by
Hyesung Kang (1 Pusan National University,
Korea, 2 UNIST, Korea)
2015
Abstract
In order to understand certain observed features of arc-like giant radio
relics such as the rareness, uniform surface brightness, and curved integrated
spectra, we explore a diffusive shock acceleration (DSA) model for radio relics
in which a spherical shock impinges on a magnetized cloud containing fossil
relativistic electrons. Toward this end, we perform DSA simulations of
spherical shocks with the parameters relevant for the Sausage radio relic in
cluster CIZA J2242.8+5301, and calculate the ensuing radio synchrotron emission
from re-accelerated electrons. Three types of fossil electron populations are
considered: a delta-function like population with the shock injection momentum,
a power-law distribution, and a power-law with an exponential cutoff. The
surface brightness profile of radio-emitting postshock region and the
volume-integrated radio spectrum are calculated and compared with observations.
We find that the observed width of the Sausage relic can be explained
reasonably well by shocks with speed u_s ∼ 3× 10^3 and sonic
Mach number M_s ∼ 3. These shocks produce curved radio spectra that
steepen gradually over (0.1-10) ν_ br with break frequency ν_
br∼ 1 GHz, if the duration of electron acceleration is ∼ 60 - 80 Myr.
However, the abrupt increase of spectral index above ∼ 1.5 GHz observed in
the Sausage relic seems to indicate that additional physical processes, other
than radiative losses, operate for electrons with γ_e ≳ 10^4.
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