Magnetic Acceleration and Collimation of Gamma-Ray Burst Jets

Exact semianalytic solutions for GRB outflows were recently derived using the equations of special-relativistic ideal MHD (see Vlahakis & Konigl in these Proceedings for a summary). This contribution focuses on the implications of these results to various modeling and observational issues in GRB sources, including the baryon loading problem, polarization measurements of the prompt and reverse-shock emission, and the possible existence of a two-component outflow.Comment: 5 pages, 1 figure, to appear in proceedings of 2003 Gamma-Ray Burst Conference (Santa Fe, New Mexico, September 8-12, 2003), to be published by AI

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 analyze ``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.Comment: 8 pages, 4 figures, 1 table; accepted for publication in MNRA

Are Magnetic Wind-Driving Disks Inherently Unstable?

There have been claims in the literature that accretion disks in which a centrifugally driven wind is the dominant mode of angular momentum transport are inherently unstable. This issue is considered here by applying an equilibrium-curve analysis to the wind-driving, ambipolar diffusion-dominated, magnetic disk model of Wardle & Konigl (1993). The equilibrium solution curves for this class of models typically exhibit two distinct branches. It is argued that only one of these branches represents unstable equilibria and that a real disk/wind system likely corresponds to a stable solution.Comment: 5 pages, 2 figures, to be published in ApJ, vol. 617 (2004 Dec 20). Uses emulateapj.cl

The Evolutionary Status of SS433

We consider possible evolutionary models for SS 433. We assume that common-envelope evolution is avoided if radiation pressure is able to expel most of a super-Eddington accretion flow from a region smaller than the accretor's Roche lobe. This condition is satisfied, at least initially, for largely radiative donors with masses in the range 4-12 solar masses. For donors more massive than about 5 solar masses, moderate mass ratios q = M_2/M_1 > 1 are indicated, thus tending to favor black-hole accretors. For lower mass donors, evolutionary considerations do not distinguish between a neutron star or black hole accretor. In all cases the mass transfer (and mass loss) rates are much larger than the likely mass-loss rate in the precessing jets. Almost all of the transferred mass is expelled at radii considerably larger than the jet acceleration region, producing the "stationary" H-alpha line, the infrared luminosity, and accounting for the low X-ray luminosity.Comment: 13 pages, Astrophysical Journal Letters, accepte

A Comparison of the Morphology and Stability of Relativistic and Nonrelativistic Jets

We compare results from a relativistic and a nonrelativistic set of 2D axisymmetric jet simulations. For a set of five relativistic simulations that either increase the Lorentz factor or decrease the adiabatic index we compute nonrelativistic simulations with equal useful power or thrust. We examine these simulations for morphological and dynamical differences, focusing on the velocity field, the width of the cocoon, the age of the jets, and the internal structure of the jet itself. The primary result of these comparisons is that the velocity field of nonrelativistic jet simulations cannot be scaled up to give the spatial distribution of Lorentz factors seen in relativistic simulations. Since the local Lorentz factor plays a major role in determining the total intensity for parsec scale extragalactic jets, this suggests that a nonrelativistic simulation cannot yield the proper intensity distribution for a relativistic jet. Another general result is that each relativistic jet and its nonrelativistic equivalents have similar ages (in dynamical time units, = R/a_a, where R is the initial radius of a cylindrical jet and a_a is the sound speed in the ambient medium). In addition to these comparisons, we have completed four new relativistic simulations to investigate the effect of varying thermal pressure on relativistic jets. The simulations generally confirm that faster (larger Lorentz factor) and colder jets are more stable, with smaller amplitude and longer wavelength internal variations. The apparent stability of these jets does not follow from linear normal mode analysis, which suggests that there are available growing Kelvin-Helmholtz modes. (Abridged.)Comment: 32 pages, AASTEX, to appear in May 10, 1999 issue of ApJ, better versions of Figures 1 and 6 are available at http://crux.astr.ua.edu/~rosen/rel/rhdh.htm

The prompt X-ray emission of GRB011211: possible evidence of a transient absorption feature

We report on observation results of the prompt X- and gamma-ray emission from GRB011211. This event was detected with the Gamma-Ray Burst Monitor and one of the Wide Field Cameras aboard the BeppoSAX satellite. The optical counterpart to the GRB was soon identified and its redshift determined (z = 2.140), while with the XMM-Newton satellite, the X-ray afterglow emission was detected. Evidence of soft X-ray emission lines was reported by Reeves et al. (2002), but not confirmed by other authors. In investigating the spectral evolution of the prompt emission we find the possible evidence of a transient absorption feature at 6.9^{+0.6}_{-0.5} keV during the rise of the primary event. The significance of the feature is derived with non parametric tests and numerical simulations, finding a chance probability which ranges from 3x10^{-3} down to 4x10^{-4}. The feature shows a Gaussian profile and an equivalent width of 1.2^{+0.5}_{-0.6} keV. We discuss our results and their possible interpretation.Comment: 23 pages, 3 Tables, 6 Figures. Accepted for publication in Astrophysical Journa

Force-Free Models of Magnetically Linked Star-Disk Systems

Disk accretion onto a magnetized star occurs in a variety of astrophysical contexts, from young stars to X-ray pulsars. The magnetohydrodynamic interaction between the stellar field and the accreting matter can have a strong effect on the disk structure, the transfer of mass and angular momentum between the disk and the star, and the production of bipolar outflows, e.g., plasma jets. We study a key element of this interaction - the time evolution of the magnetic field configuration brought about by the relative rotation between the disk and the star - using simplified, largely semianalytic, models. We first discuss the rapid inflation and opening up of the magnetic field lines in the corona above the accretion disk, which is caused by the differential rotation twisting. Then we consider additional physical effects that tend to limit this expansion, such as the effect of plasma inertia and the possibility of reconnection in the disk's corona, the latter possibly leading to repeated cycles in the evolution. We also derive the condition for the existence of a steady state for a resistive disk and conclude that a steady state configuration is not realistically possible. Finally, we generalize our analysis of the opening of magnetic field lines by using a non-self-similar numerical model that applies to an arbitrarily rotating (e.g. keplerian) disk.Comment: 75 pages, 22 figures, 2 tables. Submitted to Astrophysical Journa