We discuss how the interaction between the electrons in a relativistic jet and the Cosmic Microwave Background (CMB) affects the observable properties of radio-loud AGN at early epochs. At high z the magnetic energy density in the radio lobes of powe
rful radio-loud quasars can be exceeded by the energy density of the CMB (because of its (1+z)^4 dependance). In this case, relativistic electrons cool preferentially by scattering off CMB photons, rather than by synchrotron. Thus, sources sharing the same intrinsic properties have different extended radio and X-ray luminosities when located at different z: more distant sources are less luminous in radio and more luminous in X-rays than their closer counterparts. Instead, in compact regions where the local magnetic field still exceeds the CMB in terms of energy density, synchrotron radiation would be unaffected by the presence of the CMB. Such regions include the compact inner jet and the so-called hot spots in the radio lobes. The decrease in radio luminosity is larger in misaligned sources, whose radio flux is dominated by the extended isotropic component. These sources can fail detection in current flux limited radio surveys, and therefore they are possibly under-represented in the associated samples. As the cooling time is longer for lower energy electrons, the radio luminosity deficit due to the CMB photons is less important at low radio frequencies. Therefore objects not detected so far in current surveys at a few GHz could be picked up by low frequency deep surveys, such as LOFAR and SKA. Until then, we can estimate the number of high redshift radio-loud AGNs through the census of their aligned proxies, i.e., blazars. Indeed, their observed radio emission arises in the inner and strongly magnetized compact core of the relativistic jet, and not affected by inverse Compton scattering off CMB photons.
We consider the interaction between a relativistic fireball and material assumed to be still located just outside the progenitor star. Only a small fraction of the expected mass is sufficient to efficiently decelerate the fireball, leading to dissipa
tion of most of its kinetic energy. Since the scattering optical depths are still large at distances comparable to the progenitor radius, the dissipated energy is trapped in the system, accelerating it to relativistic velocities. The process resembles the birth of another fireball at radii R~1e11 cm, not far from the transparency radius, and with a starting bulk Lorentz factors Gamma_c~10. As seen in the observer frame, this re--generated fireball appears collimated within an angle theta_j=1/Gamma_c. If the central engine works intermittently, the funnel can, at least partially, refill and the process can repeat itself. We discuss how this idea can help solving some open issues of the more conventional internal shock scenario for interpreting the Gamma-Ray Burst properties.
