Acceleration and Collimation of Relativistic Magnetohydrodynamic Disk Winds

Abstract

We perform axisymmetric relativistic magnetohydrodynamic simulations to investigate the acceleration and collimation of jets and outflows from disks around compact objects. Newtonian gravity is added to the relativistic treatment in order to establish the physical boundary condition of an underlying accretion disk in centrifugal and pressure equilibrium. The fiducial disk surface (respectively a slow disk wind) is prescribed as boundary condition for the outflow. We apply this technique for the first time in the context of relativistic jets. The strength of this approach is that it allows us to run a parameter study in order to investigate how the accretion disk conditions govern the outflow formation. Substantial effort has been made to implement a current-free, numerical outflow boundary condition in order to avoid artificial collimation present in the standard outflow conditions. Our simulations using the PLUTO code run for 500 inner disk rotations and on a physical grid size of 100 × 200 inner disk radii. The simulations evolve from an initial state in hydrostatic equilibrium and an initially force-free magnetic field configuration. Two options for the initial field geometries are applied-an hourglass-shaped potential magnetic field and a split monopole field. Most of our parameter runs evolve into a steady state solution which can be further analyzed concerning the physical mechanism at work. In general, we obtain collimated beams of mildly relativistic speed with Lorentz factors up to 6 and mass-weighted half-opening angles of 3-7 deg. The split-monopole initial setup usually results in less collimated outflows. The light surface of the outflow magnetosphere tends to align vertically-implying three relativistically distinct regimes in the flow-an inner subrelativistic domain close to the jet axis, a (rather narrow) relativistic jet and a surrounding subrelativistic outflow launched from the outer disk surface-similar to the spine-sheath structure currently discussed for asymptotic jet propagation and stability. The outer subrelativistic disk-wind is a promising candidate for the X-ray absorption winds that are observed in many radio-quiet active galactic nuclei. The hot winds under investigation acquire only low Lorentz factors due to the rather high plasma-β we have applied in order to provide an initial force-balance in the disk corona. When we increase the outflow Poynting flux by injecting an additional disk toroidal field into the outflow, the jet velocities achieved are higher. These flows gain super-magnetosonic speed and remain Poynting flux dominated.

Two component relativistic acceleration and polarized radiation of the parsec-scale AGN jet

“…Sauty et al 2004 and N. Globus in this volume) and an outer disk wind similar to Porth and Fendt (2010). Beyond the acceleration region, Meliani and Keppens (2009) showed that the interaction between the two components can give rise to a Raleigh-Taylor-type instability and ultimately cause jet disruption.…”

Magnetic Fields in Astrophysical Jets: From Launch to Termination

“…The presence of non-monotonic and/or complex transverse Faraday-rotation gradients in the core region have been directly demonstrated by the simulations of Broderick and McKinney (2010) and Porth and Fendt (2010). These simulations indicate the difficulty of accurately deriving parameters of the jet and the helical field it is believed to carry based on observed transverse Faraday-rotation gradients.…”

“…• (Porth and Fendt 2010). These authors set up a relativistic analogue of the simulations, and emphasized flows far enough away from the central black hole that general relativistic effects could be ignored.…”

“…Examples include the studies carried out by Zakamska et al (2008), Gracia et al (2009), Mimica et al (2009), Broderick and McKinney (2010, Porth and Fendt (2010), Porth et al (2011) and Clausen-Brown et al (2011). Mimica et al (2009 and Mimica andAloy (2010, 2012) have considered various observational signatures of the presence of shocks and shock-heated gas in relativistic jets, including jets carrying helical fields.…”

Relativistic spine jets from Schwarzschild black holes

“…The availability of more and more powerful computing facilities and sophisticated numerical codes allows a quite complete description and understanding of the jet acceleration/collimation (Komissarov et al 2007;Porth & Fendt 2010) and the accretion/ejection process (Koide et al 1998(Koide et al , 1999McKinney 2006;McKinney & Blandford 2009;Gracia et al 2006Gracia et al , 2009. Recent numerical simulations have progressed to general relativistic magnetohydrodynamic (GRMHD) jet launching, as in McKinney (2006) and Hardee et al (2007), also suggesting the formation of jets with two components.…”