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We present numerical simulations of axisymmetric, magnetically driven outflows that reproduce the inferred properties of ultrarelativistic gamma‐ray burst (GRB) jets. These results extend our previous simulations of outflows accelerated to moderately relativistic speeds, which are applicable to jets of active galactic nuclei. In contrast to several recent investigations, which have employed the magnetodynamics approximation, our numerical scheme solves the full set of equations of special relativistic, ideal magnetohydrodynamics, which enables us to explicitly calculate the jet velocity and magnetic‐to‐kinetic energy conversion efficiency – key parameters of interest for astrophysical applications. We confirm that the magnetic acceleration scheme remains robust into the ultrarelativistic regime, as previously indicated by semi‐analytic self‐similar solutions. We find that all current‐carrying outflows exhibit self‐collimation and consequent acceleration near the rotation axis, but that unconfined outflows lose causal connectivity across the jet and therefore do not collimate or accelerate efficiently in their outer regions. We show that magnetically accelerated jets confined by an external pressure that varies as z−α (0 < α≤ 2) assume a paraboloidal shape z∝ra (where r, z are cylindrical coordinates and a > 1), and we obtain analytic expressions for the one‐to‐one correspondence between the pressure distribution and the asymptotic jet shape. We demonstrate that the acceleration efficiency of jets with paraboloidal streamlines is ≳50 per cent, with the numerical value being higher the lower the initial magnetization. We derive asymptotic analytic expressions for the acceleration of initially cold outflows along paraboloidal streamlines and verify that they provide good descriptions of the simulated flows. Our modelled jets (corresponding to 3/2 < a < 3) attain Lorentz factors Γ≳ 102 on scales ∼ 1010–1012 cm, consistent with the possibility that long/soft GRB jets are accelerated within envelopes of collapsing massive stars, and Γ≳ 30 on scales ∼9 × 108–3 × 1010 cm, consistent with the possibility that short/hard GRB jets are accelerated on scales where they can be confined by moderately relativistic winds from accretion discs. We also find that Γθv∼ 1 for outflows that undergo an efficient magnetic‐to‐kinetic energy conversion, where θv is the opening half‐angle of the poloidal streamlines. This relation implies that the γ‐ray emitting components of GRB outflows accelerated in this way are very narrow, with θv≲ 1° in regions where Γ≳ 100, and that the afterglow light curves of these components would either exhibit a very early jet break or show no jet break at all.

We present numerical simulations of axisymmetric, magnetically driven relativistic jets. Our special‐relativistic, ideal‐magnetohydrodynamics numerical scheme is specifically designed to optimize accuracy and resolution and to minimize numerical dissipation. In addition, we implement a grid‐extension method that reduces the computation time by up to three orders of magnitude and makes it possible to follow the flow up to six decades in spatial scale. To eliminate the dissipative effects induced by a free boundary with an ambient medium we assume that the flow is confined by a rigid wall of a prescribed shape, which we take to be z∝ra (in cylindrical coordinates, with a ranging from 1 to 3). We also prescribe, through the rotation profile at the inlet boundary, the injected poloidal current distribution: we explore cases where the return current flows either within the volume of the jet or on the outer boundary. The outflows are initially cold, sub‐Alfvénic and Poynting flux‐dominated, with a total‐to‐rest‐mass energy flux ratio μ∼ 15. We find that in all cases they converge to a steady state characterized by a spatially extended acceleration region. The acceleration process is very efficient: on the outermost scale of the simulation as much as ∼ 77  per cent of the Poynting flux has been converted into kinetic energy flux, and the terminal Lorentz factor approaches its maximum possible value (Γ∞≃μ). We also find a high collimation efficiency: all our simulated jets (including the limiting case of an unconfined flow) develop a cylindrical core. We argue that this could be the rule for current‐carrying outflows that start with a low initial Lorentz factor (Γ0∼ 1). Our conclusions on the high acceleration and collimation efficiencies are not sensitive to the particular shape of the confining boundary or to the details of the injected current distribution, and they are qualitatively consistent with the semi‐analytic self‐similar solutions derived by Vlahakis and Königl. We apply our results to the interpretation of relativistic jets in active galactic nuclei: we argue that they naturally account for the spatially extended accelerations inferred in these sources (Γ∞≳ 10 attained on radial scales R≳ 1017 cm) and are consistent with the transition to the matter‐dominated regime occurring already at R≳ 1016 cm.

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Linear polarization at the level of ∼ 1 − 3% has by now been measured in several GRB afterglows. Whereas the degree of polarization, P, was found to vary in some sources, the position angle, θ p , was roughly constant in all cases. Until now, the polarization has been commonly attributed to synchrotron radiation from a jet with a tangled magnetic field that is viewed somewhat off axis. However, this model predicts either a peak in P or a 90 • change in θ p around the "jet break" time in the lightcurve, for which there has so far been no observational confirmation. We propose an alternative interpretation, wherein the polarization is attributed, at least in part, to a large-scale, ordered magnetic field in the ambient medium. The ordered component may dominate the polarization even if the total emissivity is dominated by a tangled field generated by postshock turbulence. In this picture, θ p is roughly constant because of the uniformity of the field, whereas P varies as a result of changes in the ratio of the ordered-to-random mean-squared field amplitudes. We point out that variable afterglow light curves should be accompanied by a variable polarization. The radiation from the original ejecta, which includes the prompt γ-ray emission and the emission from the reverse shock (the 'optical flash' and 'radio flare'), could potentially exhibit a high degree of polarization (up to ∼ 60%) induced by an ordered transverse magnetic field advected from the central source. 3 See Covino et al. 2003a for references to the above observations as well as to subsequent polarization measurements.

We present self-similar solutions that describe the gravitational collapse of rotating, isothermal, magnetic molecular-cloud cores. These solutions make it possible, for the first time, to study the formation of rotationally supported protostellar disks of the type detected around many young stellar objects in the context of a realistic scenario of star formation in magnetically supported, weakly ionized, molecular cloud cores. This work focuses on the evolution after a point mass first forms at the center and generalizes previous results by Contopoulos, Ciolek, & Königl that did not include rotation. Our semianalytic scheme incorporates ambipolar diffusion and magnetic braking and allows us to examine the full range of expected behaviors and their dependence on the physical parameters. We find that, for typical parameter values, the inflow first passes through an ambipolar-diffusion shock (at a radius r a ), where the magnetic flux decouples from the matter, and subsequently through a centrifugal shock (at r c ), inward of which a rotationally supported disk (of mass M d ) is established. By the time (∼ 10 5 yr) that the central mass M c grows to ∼ 1 M ⊙ , r a 10 3 AU, r c 10 2 AU, and M d /M c 0.1. The derived disk properties are consistent with data on T Tauri systems, and our results imply that protostellar disks may well be Keplerian also during earlier phases of their evolution. We demonstrate that the disk is likely to drive centrifugal outflows that transport angular momentum and mass, and we show how the radially self-similar wind solution of Blandford & Payne can be naturally incorporated into the disk model. We further verify that gravitational torques and magnetorotational instability-induced turbulence typically do not play an important role in the angular momentum transport. For completeness, we also present solutions for the limiting cases of fast rotation (where the collapse results in a massive disk with such a large outer radius that it traps the ambipolar-diffusion front) and strong braking (where no disk is formed and the collapse resembles that of a nonrotating core at small radii), as well as solutions describing the rotational collapse of ideal-MHD and of nonmagnetic model cores.

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 such as 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.