Recollimation and Radiative Focusing of Relativistic Jets: Applications to Blazars and M87

TL;DR

This paper models how radiative cooling can focus relativistic jets, explaining rapid variability in blazars and M87, with implications for understanding jet collimation and emission features at large distances.

Contribution

It introduces a semi-analytical model demonstrating radiative cooling-induced jet focusing and applies it to explain observed phenomena in blazars and M87.

Findings

Radiative cooling can cause jet recollimation and focusing.

Focusing explains rapid variability in blazars.

Good collimation in M87 accounts for X-ray flux variability.

Abstract

Recent observations of M87 and some blazars reveal violent activity in small regions located at relatively large distances from the central engine. Motivated by these considerations, we study the hydrodynamic collimation of a relativistic cooling outflow using a semi-analytical model developed earlier. We first demonstrate that radiative cooling of the shocked outflow layer can lead to a focusing of the outflow and its reconfinement in a region having a very small cross-sectional radius. Such a configuration can produce rapid variability at large distances from the central engine via reflections of the converging recollimation shock. Possible applications of this model to TeV blazars are discussed. We then apply our model to M87. The low radiative efficiency of the M87 jet renders focusing unlikely. However, the shallow profile of the ambient medium pressure inferred from observations, results in extremely good collimation that can explain the reported variability of the X-ray flux emitted from the HST-1 knot.