Particle acceleration efficiencies in astrophysical shear flows

The acceleration of energetic particles in astrophysical shear flows is analyzed. We show that in the presence of a non‐relativistic gradual velocity shear, power law particle momentum distributions f(p) ∝ p−(3+α) may be generated, assuming a momentum‐dependent scattering time τ ∝ pα, with α > 0. We consider possible acceleration sites in astrophysical jets and study the conditions for efficient acceleration. It is shown, for example, that in the presence of a gradual shear flow and a gyro‐dependent particle mean free path, synchrotron radiation losses no longer stop the acceleration once it has started to work efficiently. This suggests that shear acceleration may naturally account for a second, non‐thermal population of energetic particles in addition to a shock‐accelerated one. The possible relevance of shear acceleration is briefly discussed with reference to the relativistic jet in the quasar 3C 273.