4.2. Relics

Radio relics are a class of diffuse sources typically located near the periphery of the cluster. Unlike the halos, they show an elongated or irregular shape and are strongly polarized. The prototype of this class is 1253+275, in the Coma cluster (see Fig. 3), first classified by Ballarati et al. [76]. The polarization of 1253+275 at 20 cm is 25 - 30% [37, 77]. The magnetic field is oriented along the source major axis and the magnetic field strength derived from minimum energy arguments of the synchrotron emitting plasma is 0.55 µG.

A complex radio emission is detected in A2256 [78, 79], which contains several head-tail radio galaxies, two large regions of diffuse radio relic emission and a central radio halo. The relics are highly polarized [80], with the linear fractional polarization at 20 cm above 30% for the majority of the region, reaching values up to 50%. The intrinsic magnetic field direction reveals that there is a large-scale order to the fields, and it appears to trace the bright filaments in the relics.

In addition to Coma and A2256, other clusters present both a central halo and a peripheral relic, e.g. A2255 [81, 82], A1300 [83], A2744 [40] and A754 [43, 84]. A spectacular example of two almost symmetric relics in the same cluster is found in A3667 [85, 86].

A puzzling relic source is 0917+75 [41, 87], located at 5 to 8 Mpc from the centers of the closest clusters (A762, A786, A787), thus not unambiguously associated with any of them. It has a high fractional polarization (up to 48% at 20 cm and 60% at 6 cm for the brightest parts of the source) and the magnetic field direction appears to be coherent over scales of at least several hundreds kpc.

The equipartition magnetic fields in the relics, computed with standard assumptions (k = 1, = 1, ₁ = 10 MHz, ₂ = 10 GHz), are in the range 0.5 - 2 µG [40, 77, 88]. We note, however, that they refer to regions where the cluster magnetic fields are expected to be compressed (see below), thus they are not indicative of the overall magnetic field intensity in the peripheral cluster regions.

In recent years, there has been increasing evidence that the relics are related to ongoing merger events. It has been suggested that relics result from first order Fermi acceleration of relativistic particles in shocks produced during cluster merger events [55, 88, 89, 90]. Enßlin and Brüggen [91] presented 3-D magneto hydrodynamic (MHD) simulations of electrons reaccelerated by adiabatic compression [92] of existing cocoons of radio plasma that traverses a shock wave. The passage of a radio cocoon through a shock wave and the consequent evolution of the gas density and magnetic field energy density can be seen in Fig. 4. This model is consistent with the relic elongated structure almost perpendicular to the merger axis. Moreover, the derived maps obtained in the simulations reproduce very well the filamentary structure seen in relic sources at high resolution [93].

Figure 4. 3-D MHD simulations [91] of electrons reaccelerated by compression of existing cocoons of radio plasma that traverses a shock wave. In figure is shown the passage of a radio cocoon through a shock wave and the consequent evolution of the gas density (top) and magnetic field energy density (bottom).