Hydrodynamical Simulations of Recollimation Shocks within Relativistic Astrophysical Jets

Christopher Kaye,Timothy Cawthorne,Philip Hughes

doi:10.3390/galaxies6020053

Abstract

Astrophysical jets launched from active galactic nuclei can remain tightly collimated over large distances due, in part, to recollimation shocks. Formed within the jets due to their supersonic nature, recollimation shocks are predicted to leave signatures in the observed radio emission due to magnetic flux freezing and the geometric relationship between magnetic fields and the polarization of synchrotron radiation. In the course of this work, we will compare how predictions of emission from recollimation shocks change when the flow is modelled using a hydrodynamical code, as opposed to semi-dynamical and magnetohydrodynamical codes. Jets generally exhibit low levels of polarization, which implies a substantially disordered magnetic field. It is difficult to model such fields using magnetohydrodynamics, hence this work uses hydrodynamical code and a statistical treatment of the magnetic field (c.f. Scheuer and Matthews, 1990). It should then be possible to assess whether certain radio jet phenomena, such as knots and radio-cores, may be modelled as singular or multiple recollimation shocks. To date, the hydrodynamical code has been successfully built and executed on UCLan’s supercomputer cluster, and parallelepiped vector triads have been included to monitor the fluid deformation within the simulation, so that the emergent flux and polarization may be calculated. The parallelepiped advection is currently being verified and some results are discussed. Code for radiative transfer throughout the jet is also being implemented, in order to simulate images for comparison with previous works and observations.

Highlights

Astrophysical jets are produced by a variety of sources, but the biggest and most luminous examples originate from active galactic nuclei (AGNs)
Semi-dynamical models represent the action of a shock through jump conditions in the parameters, which gives a sharp edge to the shock
Conical recollimation shocks with a randomly tangled magnetic field upstream were simulated [12], and Cawthorne [18] built upon by this model by adding a parallel upstream magnetic field component, and convolving the results with a circular beam, for comparison with observations. These models strongly suggest that knots in radio jets may result from conical shocks; later simulations of stationary shocks in jets 3C 120 [19] and 1803+784 [20] replicated characteristics observed in these sources

Summary

Introduction

Astrophysical jets are produced by a variety of sources, but the biggest and most luminous examples originate from active galactic nuclei (AGNs) These jets have relativistic bulk motion (with Lorentz factors γ ∼ 10), and are likely composed of an electron–positron dominant plasma [1]. It has been shown that moderate compression of a randomly tangled magnetic field can drastically increase the degree of order of the field, as seen from angles close to the plane of compression e.g., [5,6,7] This compression increases the plasma emissivity [8] (Section 3), meaning that the observed synchrotron radiation has a higher intensity and degree of polarization e.g., [9,10]. Other phenomena may have greater influence on the overall jet structure e.g., [11], recollimation shocks should obviously leave polarization signatures in the emission of jets [12], so it may be possible to use recollimation shocks as a diagnostic in determining the some properties of their host AGN [13,14]

Fluid Dynamics and Simulations

Semi-Dynamical Simulations

Numerical Simulations

Methodology

Results