SHOCK-DRIFT PARTICLE-ACCELERATION IN SUPERLUMINAL SHOCKS - A MODEL FOR HOT SPOTS IN EXTRAGALACTIC RADIO-SOURCES

Efficient acceleration of relativistic particles must occur in the hot spots of extragalactic radio sources Recent evidence that jets in Fanaroff-Riley type II (F-R II) radio sources are relativistic even on kiloparsec scales suggests that the hotspots in these sources are the downstream regions of relativistic shocks. Additional evidence that "compact" hot spots exhibit relativistic beaming suggests that the shocks responsible for these hot spots are often highly oblique. Oblique, relativistic shocks are likely to be "superluminal," implying that they cannot be the sites of first-order Fermi acceleration. As an alternative mechanism for particle acceleration, we study "shock-drift" acceleration at relativistic shocks, a mechanism by which particles are accelerated in a single shock crossing by drifting parallel (or antiparallel) to the electric field. By following the trajectories of individual charged particles, we show that adiabatic invariance of the magenetic moment ceases to be a good approximation when the upstream shock speed is relativistic, and that the acceleration exceeds that predicted by simple adiabatic theory. Resulting enhancements in synchrotron emissivity are more than adequate to explain the observed contrasts in surface brightness between hot spots and the upstream flow. We discuss the morphology, polarization, and spectra of compact hot spots in the context of the oblique relativistic shock model focusing particularly on the spectral cutoffs which have recently been observed.