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tokens: the shock to larger radii. This means that for a significant fraction of time between the passages through the primary and the secondary shocks, the electrons have lower Lorentz factors than just behind the primary shock, and the magnetic field is also lower. The radiative losses are therefore relatively small (except for the period right after the passage of the primary shock). The electrons then spend a long time in a “freezer,” until the gas passes through the secondary shock (mini-accretion shock), when its density increases back to the initial state (at least approximately). The electrons’ Lorentz factor and the strength of the magnetic field increase accordingly. As a result, the synchrotron emissivity is boosted by a factor (e.g., Markevitch et al. 2005 ), where p is the slope of the relativistic electrons energy spectrum and C is the volume compression factor. In terms of the spectral index α = ( p − 1)∕2 in the radio band, the boost factor is . This expression is based on the assumption that the magnetic field is enhanced by a factor C 2∕3 . For the shock with M ~ 1.5, the compression is C ≈ 1.7. For α = 2, the corresponding boost of the radio flux purely due to adiabatic compression is F ≈ 10. Unlike the “runway” merger shock, the mini-accretion shock does not feature a rarefaction region downstream and, therefore, the radio emission is controlled by radiative losses. Once again the question arises of whether the short radiative times pose a problem. The distance over which the secondary shock front can travel during t max is l max ~ c s t max ≲ 180 kpc (for ν = 1.4 GHz), assuming that the shock moves with the sound speed 5 . At ν = 150 MHz, this distance will be ~500 kpc. This is smaller than the size (radius) of the radio halo of some 700 kpc (not by a huge factor, though), which used to be an argument against the single shock acceleration scenario. However, in the scenario, where secondary shocks are responsible for the compression and, possibly, reacceleration of electrons, this problem can be alleviated. The electron reacceleration scenario implies that there is a pool of nonthermal relativistic electrons produced by MHD shocks and turbulence due to the cluster merging history and fast moving galaxies in the cluster. Then the diffusive shock acceleration (DSA) mechanism would reproduce the slope of the pool electron spectrum in a shock downstream if the pool distribution had a power-law spectrum of index . Otherwise, the spectrum in the shock downstream would have the standard test particle DSA index if the slope of the pool distribution has a steeper spectrum (see e.g., Blandford et al. 1987 ). Therefore, the shock with M ~ 1.5, and the compression ratio C ≈ 1.7 propagatingin the cluster plasma regions without the prominent preexisting nonthermal population have p = 5.2 and the synchrotron radio emission index α = 2.1. On the other hand, energetic particle acceleration by intensive MHD turbulence and multiple shocks may