Magnetohydrodynamics of Gamma-Ray Burst Outflows

Using relativistic, axisymmetric, ideal MHD, we examine the outflow from a disk around a compact object, taking into account the baryonic matter, the electron-positron/photon fluid, and the large-scale electromagnetic field. Focussing on the parameter regime appropriate to gamma-ray burst outflows, we demonstrate, through exact self-similar solutions, that the thermal force (which dominates the initial acceleration) and the Lorentz force (which dominates further out and contributes most of the acceleration) can convert up to ~50% of the initial total energy into asymptotic baryon kinetic energy. We examine how baryon loading and magnetic collimation affect the structure of the flow, including the regime where emission due to internal shocks could take place.Comment: To be published in ApJ Letters. 4 pages, 1 figur

Relativistic Magnetohydrodynamics with Application to Gamma-Ray Burst Outflows: II. Semianalytic Super-Alfvenic Solutions

We present exact radially self-similar solutions of special-relativistic magnetohydrodynamics representing ``hot'' super-Alfvenic outflows from strongly magnetized, rotating compact objects. We argue that such outflows can plausibly arise in gamma-ray burst (GRB) sources and demonstrate that, just as in the case of the trans-Alfvenic flows considered in the companion paper, they can attain Lorentz factors that correspond to a rough equipartition between the Poynting and kinetic-energy fluxes and become cylindrically collimated on scales compatible with GRB observations. As in the trans-Alfvenic case, the initial acceleration is thermal, but, in contrast to the solutions presented in Paper I, part of the enthalpy flux is transformed into Poynting flux during this phase. The subsequent, magnetically dominated acceleration can be significantly less rapid than in trans-Alfvenic flows.Comment: 8 pages, 4 figures, submitted to the Astrophysical Journal. The companion paper is astro-ph/030348

Observational Evidence for a Multiphase Outflow in QSO FIRST J1044+3656

Spectral absorption features in active galactic nuclei (AGNs) have traditionally been attributed to outflowing photoionized gas located at a distance of order a parsec from the central continuum source. However, recent observations of QSO FIRST J104459.6+365605 by de Kool and coworkers, when intepreted in the context of a single-phase gas model, imply that the absorption occurs much farther (approx 700 pc) from the center. We reinterpret these observations in terms of a shielded, multiphase gas, which we represent as a continuous low-density wind with embedded high-density clouds. Our model satisfies all the observational constraints with an absorbing gas that extends only out to about 4 pc from the central source. The different density components in this model coexist in the same region of space and have similar velocities, which makes it possible to account for the detection in this source of absorption features that correspond to different ionization parameters but have a similar velocity structure. This model also implies that only a small fraction of the gas along the line of sight to the center is outflowing at the observed speeds and that the clouds are dusty whereas the uniform gas component is dust free. We suggest that a similar picture may apply to other sources and discuss additional possible clues to the existence of multiphase outflows in AGNs.Comment: 6 pages, 2 figures, Accepted for publication in ApJ v569 n2, April 20, 200

Magnetic Driving of Relativistic Outflows in Active Galactic Nuclei. I. Interpretation of Parsec-Scale Accelerations

There is growing evidence that relativistic jets in active galactic nuclei undergo extended (parsec-scale) acceleration. We argue that, contrary to some suggestions in the literature, this acceleration cannot be purely hydrodynamic. Using exact semianalytic solutions of the relativistic MHD equations, we demonstrate that the parsec-scale acceleration to relativistic speeds inferred in sources like the radio galaxy NGC 6251 and the quasar 3C 345 can be attributed to magnetic driving. Additional observational implications of this model will be explored in future papers in this series.Comment: 6 pages, 2 figures, submitted to Ap

Wind-driving protostellar accretion discs – I. Formulation and parameter constraints

We study a model of weakly ionized, protostellar accretion discs that are threaded by a large-scale, ordered magnetic field and power a centrifugally driven wind. We consider the limiting case where the wind is the main repository of the excess disc angular momentum and generalize the radially localized disc model of Wardle & Königl, which focused on the ambipolar diffusion regime, to other field diffusivity regimes, notably Hall and Ohm. We present a general formulation of the problem for nearly Keplerian, vertically isothermal discs using both the conductivity-tensor and the multifluid approaches and simplify it to a normalized system of ordinary differential equations in the vertical space coordinate. We determine the relevant parameters of the problem and investigate, using the vertical-hydrostatic-equilibrium approximation and other simplifications, the parameter constraints on physically viable solutions for discs in which the neutral particles are dynamically well coupled to the field already at the mid-plane. When the charged particles constitute a two-component ion-electron plasma, one can identify four distinct sub-regimes in the parameter domain where the Hall diffusivity dominates and three sub-regimes in the Ohm-dominated domain. Two of the Hall sub-regimes can be characterized as being ambipolar diffusion-like and two as being Ohm-like: the properties of one member of the first pair of sub-regimes are identical to those of the ambipolar diffusion regime, whereas one member of the second pair has the same characteristics as one of the Ohm sub-regimes. All the Hall sub-regimes have Brb/|Bφb| (ratio of radial-to-azimuthal magnetic field amplitudes at the disc surface) >1, whereas in two Ohm sub-regimes this ratio is <1. When the two-component plasma consists, instead, of positively and negatively charged grains of equal mass, the entire Hall domain and one of the Ohm sub-regimes with Brb/|Bφb| < 1 disappear. All viable solutions require the mid-plane neutral-ion momentum exchange time to be shorter than the local orbital time. We also infer that vertical magnetic squeezing always dominates over gravitational tidal compression in this model. In a follow-up paper we will present exact solutions that test the results of this analysis in the Hall regime

Angular momentum transport in protostellar discs

Angular momentum transport in protostellar discs can take place either radially, through turbulence induced by the magnetorotational instability (MRI), or vertically, through the torque exerted by a large-scale magnetic field that threads the disc. Using semi-analytic and numerical results, we construct a model of steady-state discs that includes vertical transport by a centrifugally driven wind as well as MRI-induced turbulence. We present approximate criteria for the occurrence of either one of these mechanisms in an ambipolar diffusion-dominated disc. We derive 'strong field' solutions in which the angular momentum transport is purely vertical and 'weak field' solutions that are the stratified-disc analogues of the previously studied MRI channel modes; the latter are transformed into accretion solutions with predominantly radial angular momentum transport when we implement a turbulent-stress prescription based on published results of numerical simulations. We also analyse 'intermediate field strength' solutions in which both modes of transport operate at the same radial location; we conclude, however, that significant spatial overlap of these two mechanisms is unlikely to occur in practice. To further advance this study, we have developed a general scheme that incorporates also the Hall and Ohm conductivity regimes in discs with a realistic ionization structure