RELATIVISTIC PAIR BEAMS FROM TeV BLAZARS: A SOURCE OF REPROCESSED GeV EMISSION RATHER THAN INTERGALACTIC HEATING

The interaction of TeV photons from blazars with the extragalactic background light produces a relativistic beam of electron-positron pairs streaming through the intergalactic medium (IGM). The fate of the beam energy is uncertain. By means of two-and three-dimensional particle-in-cell simulations, we study the nonlinear evolution of dilute ultra-relativistic pair beams propagating through the IGM. We explore a wide range of beam Lorentz factors.gamma(b) >> 1 and beam-to-plasma density ratios a << 1, so that our results can be extrapolated to the extreme parameters of blazar-induced beams (gamma(b) similar to 10(6) and a similar to 10 (15), for powerful blazars). For cold beams, we show that the oblique instability governs the early stages of evolution, but its exponential growth terminates-due to self-heating of the beam in the transverse direction-when only a negligible fraction similar to (a/gamma(b))(1/3) similar to 10(-7) of the beam energy has been transferred to the IGM plasma. Further relaxation of the beam proceeds through quasi-longitudinal modes, until the momentum dispersion in the direction of propagation saturates at Delta Pb,vertical bar vertical bar /gamma bm(e)c similar to 0.2. This corresponds to a fraction similar to 10% of the beam energy-irrespective of gamma(b) or a-being ultimately transferred to the IGM plasma (as compared to the heating efficiency of similar to 50% predicted by one-dimensional models, which cannot properly account for the transverse broadening of the beam). For the warm beams generated by TeV blazars, the development of the longitudinal relaxation is suppressed, since the initial dispersion in beam momentum is already Delta Pb0,vertical bar vertical bar /gamma bm(e)c >= 1. Here, the fraction of beam energy ultimately deposited into the IGM is only similar to a gamma(b) similar to 10(-9). It follows that most of the beam energy is still available to power the GeV emission produced by inverse Compton up-scattering of the cosmic microwave background by the beam pairs.