Magnetic field structure of relativistic jets in AGN

1. Magnetic field structure of relativistic jets in AGN M. Roca-Sogorb1, M. Perucho2, J.L. Gómez1, J.M. Martí3, L. Antón3, M.A. Aloy3 & I. Agudo1 1Instituto de Astrofísica de Andalucía (CSIC). Granada, Spain 2Max-Planck Institut für Radioastronomie. Bonn, Germany 3Departament d’Astronomía i Astrofisica. Universitat de València. Valencia, Spain INTRODUCTION SET UP FOR THE NUMERICAL SIMULATIONS In order to study the role played by the magnetic field in the emission and dynamics of relativistic jets in Active Galactic Nuclei, we have performed simulations of relativistic over-pressured jets with helical magnetic fields and we have generated synthetic total and polarized intensity maps that can be directly compared with observations. The model is characterized by the injection velocity into the external medium vb,( i.e, Lorentz factor γ), and the specific internal energy, εb. Two more parameters, the beam to external medium ratios of pressure ()and rest-mass density at the injection position, are used to fix the properties of the atmosphere through which the jet propagates. An ideal gas equation is used, characterized by the adiabatic exponent, Г. We have considered over-pressured relativistic jets and we have performed simulations with different values of the two parameters that control the magnetic field in the jet, the pitch angle and the magnetization parameter . eRMHD SIMULATIONS RELATIVISTIC MAGNETOHYDRODYNAMICAL CODE: The underlying relativistic magnetohydrodynamical (RMHD) simulations have been performed using a numerical code that solves the RMHD equations in conservative form and cylindrical coordinates with axial symmetry. Details of the numerical code can be found in Leismann et al. 2005 and references therein. Numerical fluxes between contiguous zones are computed using an approximate Riemann solver. Spatial second order accuracy is achieved by means of piecewise linear, monotonic reconstruction of the fluid variables. Averaged values of the conserved variables are advanced in time by means of a Runge-Kutta algorithm of third order. The magnetic field configuration is kept divergence-free thanks to the implementation of a constrained transport method. HELICAL MAGNETIC FIELD STRUCTURE We are assuming that the magnetic field of the jet consists of two components. This structure corresponds to an axial current that goes inside the jet and a return axial current that goes in the surface of the jet (see Lind et al,1989 and Komissarov,1999) Toroidal Magnetic Field: Axial Magnetic Field: COMPUTING THE EMISSION: Using the RMHD results as inputs we compute synthetic radio continuum emission (total and polarized) maps that can be directly compared with observations. In order to do that, we integrate the transfer equation for the synchrotron radiation (for an optically thin frequency and in arbitrary units) taking into account all the appropriate relativistic effects (see Gómez,2002 and references therein, for a complete description). THE INFLUENCE OF THE MAGNETIC FIELD PITCH ANGLE On the dynamics: On the emission: θ= 30º We have computed the synthetic emission maps for a viewing angle of θ= 30º . Weobserve symmetric emission for model A while model B shows a stratification in the emission across the jet width, due to the different angle between the internal pitch angle of the helical magnetic field and the line of sight. The polarization in the northern side of the jet is always either orthogonal or parallel to the jet axis, due to geometrical effects because the helical magnetic field (see also Aloy et al,2000) Figure 3: From top to bottom panels show total intensity for a viewing angle of θ=30º for models A and B. The jet shows a pattern of recollimation shocks due to the initial over-pressure factor between the jet and the external medium Figure 1: From top to bottom, panels show the distribution of the logarithm of the pressure, lorentz factor and distribution of pitch angle for model A. θ= 70º Figure 2: Panel shows the distribution of pitch angle for model B. Figure 4: From top to bottom panels show polarized intensity for a viewing angle of θ=70º for models A and B. The bars show the direction of the electric vector.We have limited the emission to the inner 0.75 radii of the jet THE INFLUENCE OF THE MAGNETIZATION PARAMETER In order to study the influence of the magnetic field we have introduced an increase of the magnetization parameter at the jet inlet of the previous model B, from =1 to =3. A higher value of  means that the jet is more over-pressured relative to the external medium, leading in the formation of a strong plane perpendicular shock that modifies the previous structure of recollimation shocks. θ= 90º Figure 5: On the left, from top to bottom panels show the logarithm of the pressure and distribution of the poloidal magnetic field for model C. On the right, panels show total and polarized intensities for a viewing angle of θ=90º. The bars show the direction of the electric vector. EVOLUTION OF A SHOCK FROM CONICAL TO PLANE PERPENDICULAR The evolution of the shock in Model C shows how the Electric Vector Position Angle (EVPA) rotate from their initial oblique configuration to a perpendicular distribution, as the shock evolves from conical to plane perpendicular. 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