CURVED RADIO SPECTRA OF WEAK CLUSTER SHOCKS

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 the 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) similar to 3 x 10(3) km s(-1) and sonic Mach number M-s similar to 3. These shocks produce curved radio spectra that steepen gradually over (0.1-10)v(br) with a break frequency v(br) similar to 1GHz if the duration of electron acceleration is similar to 60-80 Myr. However, the abrupt increase in the spectral index above similar to 1.5 GHz observed in the Sausage relic seems to indicate that additional physical processes, other than radiative losses, operate for electrons with gamma(e) greater than or similar to 10(4)open1