Astronomy and Astrophysics – Astrophysics
Scientific paper
Jan 2012
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2012aas...21915428k&link_type=abstract
American Astronomical Society, AAS Meeting #219, #154.28
Astronomy and Astrophysics
Astrophysics
Scientific paper
We study the collimation of relativistic jets by the pressure of an ambient medium in the limit where the jet interior has lost causal contact with its surroundings. For a jet with an ultrarelativistic equation of state and external pressure that decreases as a power of distance, p r - n, the jet interior will lose causal contact when n > 2. The outer layers of the jet gradually collimate toward the jet axis as long as n < 4, however, leading to the formation of a shocked boundary layer. Supposing that pressure-matching across the shock front determines the shape of the shock, we study the resulting structure of a hydrodynamic jet in two ways: first by assuming that the pressure remains constant across the boundary layer and looking for solutions to the shock jump equations, and then by constructing self-similar boundary-layer solutions that allow for a pressure gradient across the shocked layer. We find that the constant-pressure solutions can be characterized by four initial parameters that determine the jet shape and whether the shock closes to the axis. Self-similar solutions for the boundary layer can be constructed for which the pressure monotonically decreases inward across the boundary layer, and the behavior of these solutions exhibit a strong dependence on the value of n. We also demonstrate that seeding a jet with a small toroidal magnetic field always results in the pressure becoming magnetically dominated at large radii, and we repeat our boundary-layer calculations taking this magnetic dominance into account. We discuss the insight our models provide into energy dissipation in relativistic astrophysical jets, such as those of AGN and blazars.
Begelman Mitchell C.
Kohler Susanna
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