OBLIQUE MIXED SHOCKS IN EXTRAGALACTIC JETS

Analytical calculations of the spectra obtained from the first-order Fermi acceleration mechanism with oblique shocks are presented. They are a generalization of the quasi-parallel configuration, where the streaming instability modifies the acceleration process, and of the quasi-perpendicular configuration, where the magnetic pressure is significant. We show that the influence of the streaming instability on the compression ratio of the fluid is always negligible, and it changes the spectral index only through the modification of the effective compression ratio of the scattering centers. The observational constraints on the cutoff frequency imply that the magnetic turbulence in extragalactic jets must be weak and most probably of Kraichnan type. The application of our calculations to the jet of 3C 273 shows that the theory of diffusive acceleration in oblique shocks works well with a nonrelativistic flow velocity and yields narrow ranges for the different parameters.