Publications by Year: 2003

2003
Vlahakis N, Königl A. The Dynamics of Magnetized Outflows in GRBs. In: Vol. 662. ; 2003. pp. 166 - 168. WebsiteAbstract
Using relativistic, axisymmetric, ideal MHD, we examine the outflow from a debris disk around a newly formed stellar-mass black hole, taking into account the baryonic matter, the electron-positron/photon fluid, and the large-scale electromagnetic field. We clarify the relationship between the thermal (fireball) and magnetic (Poynting flux) acceleration mechanisms, identify the parameter regimes where qualitatively different behaviors are expected, and demonstrate that the observationally inferred properties of the GRB outflows can be attributed to magnetic driving. We show that the Lorentz force can convert up to 50% of the initial total energy into kinetic energy of a collimated flow of baryons. This energy, in turn, may be converted into radiation by internal shocks. We examine how baryon loading and magnetic collimation affect the structure of the flow.
Vlahakis N, Königl A. Analytical Modeling of Hydromagnetic Jets in AGNs. In: Vol. 290. ; 2003. pp. 219. WebsiteAbstract
We use exact self-similar solutions of the steady, axisymmetric, relativistic, hydromagnetic equations to study the formation of AGN jets. Our formalism allows us to examine the effects of the thermal, centrifugal, and electromagnetic forces on the flow acceleration and collimation. We apply our analysis to the jet in NGC 6251 and show that the puzzling sub-pc scale acceleration to mildly relativistic speeds recently inferred in this source from VLBI measurements can be attributed to magnetic driving.
Vlahakis N, Königl A. A model for GRB jets. [Internet]. 2003;287:249 - 252. WebsiteAbstract
By using relativistic, axisymmetric, ideal MHD, we examine the motion of the baryon/e+/-/ photon fluid that emanates from a stellar-mass compact object/debris-disk system (a common outcome of many progenitor models). We prove that the motion can be described as a frozen pulse, which permits the study of each shell of the pancake-shaped outflow using steady-state equations. The ejected energy flux is dominated by the electromagnetic (Poynting) contribution, but it can also have a nonnegligible e+/-/radiation (thermal fireball) component. We demonstrate, through exact self-similar solutions, that the flow is first thermally and subsequently magnetically accelerated up to equipartition between kinetic and Poynting fluxes, i.e., ~ 50% of the total energy is converted into baryonic kinetic energy. The electromagnetic forces also collimate the flow, reaching a cylindrical structure asymptotically.
Tsinganos K, Vlahakis N, Bogovalov S, Sauty C, Trussoni E, Lima JJG. Collimation of astrophysical MHD outflows. [Internet]. 2003;287:103 - 108. WebsiteAbstract
We explain in simple terms why a rotating and magnetized outflow forms a core with a jet and show numerical simulations which substantiate this argument. The outflow from a solar-type inefficient magnetic rotator is found to be very weakly collimated while the outflow from a ten times faster rotating YSO is shown to produce a tightly collimated jet. This gives rise to an evolutionary scenario for stellar outflows. We also propose a two-component model consisting of a wind outflow from a central object and a faster rotating outflow launched from a surrounding accretion disk which plays the role of the flow collimator.
Vlahakis N, Konigl A. Relativistic Outflows in Active Galactic Nuclei. [Internet]. 2003:astro-ph/0312254. WebsiteAbstract
There are observational indications that relativistic outflows in AGNs are accelerated over distances that far exceed the scale of the central engine. Examples include the radio galaxy NGC 6251, where knots in the radio jets were inferred to accelerate from ~0.13c at a distance of ~0.53 pc from the galactic nucleus to ~0.42c at r=1.0 pc, and the quasar 3C 345, where the Lorentz factor of the radio knot C7 was deduced to increase from ~5 to >10 as it moved from r=3 pc to r=20 pc. It is argued, using exact semianalytic solutions of the relativistic MHD equations, that this behavior is a signature of magnetic acceleration. The same basic driving mechanism may apply to the relativistic jets in AGNs, gamma-ray burst sources, and microquasars.
Vlahakis N. Hydromagnetic acceleration in relativistic outflows. [Internet]. 2003;47:701 - 704. WebsiteAbstract
We demonstrate that hydromagnetic acceleration can be the driving mechanism of relativistic outflows in AGNs as well as in Gamma-ray burst sources and Crab-like pulsars. Using semianalytical solutions of the full set of the steady, axisymmetric, ideal hydromagnetic equations in flat spacetime—i.e., solving the momentum equation along the flow as well as in the transfield direction—we model the acceleration of relativistic outflows. We find that a Poynting-flux dominated flow near the source reaches equipartition between matter and magnetic energy-fluxes, or even becomes completely matter-dominated (as in the case of Crab-like pulsar winds).
Vlahakis N, Königl A. Relativistic Magnetohydrodynamics with Application to Gamma-Ray Burst Outflows. II. Semianalytic Super-Alfvénic Solutions. [Internet]. 2003;596:1104 - 1112. WebsiteAbstract
We present exact radially self-similar solutions of special relativistic magnetohydrodynamics representing ``hot'' super-Alfvénic 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-Alfvénic 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-Alfvénic case, the initial acceleration is thermal, but, in contrast to the solutions presented in the companion paper, 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-Alfvénic flows.
Vlahakis N, Königl A. Relativistic Magnetohydrodynamics with Application to Gamma-Ray Burst Outflows. I. Theory and Semianalytic Trans-Alfvénic Solutions. [Internet]. 2003;596:1080 - 1103. WebsiteAbstract
We present a general formulation of special relativistic magnetohydrodynamics and derive exact radially self-similar solutions for axisymmetric outflows from strongly magnetized, rotating compact objects. We generalize previous work by including thermal effects and analyze in detail the various forces that guide, accelerate, and collimate the flow. We demonstrate that, under the assumptions of a quasi-steady poloidal magnetic field and of a highly relativistic poloidal velocity, the equations become effectively time independent and the motion can be described as a frozen pulse. We concentrate on trans-Alfvénic solutions and consider outflows that are super-Alfvénic throughout in the companion paper. Our results are applicable to relativistic jets in gamma-ray burst (GRB) sources, active galactic nuclei, and microquasars, but our discussion focuses on GRBs. We envision the outflows in this case to initially consist of a hot and optically thick mixture of baryons, electron-positron pairs, and photons. We show that the flow is at first accelerated thermally but that the bulk of the acceleration is magnetic, with the asymptotic Lorentz factor corresponding to a rough equipartition between the Poynting and kinetic energy fluxes (i.e., ~50% of the injected total energy is converted into baryonic kinetic energy). The electromagnetic forces also strongly collimate the flow, giving rise to an asymptotically cylindrical structure.
Vlahakis N, Peng F, Königl A. Neutron-rich Hydromagnetic Outflows in Gamma-Ray Burst Sources. [Internet]. 2003;594:L23 - L26. WebsiteAbstract
We demonstrate that ``hot'' MHD outflows from neutron-rich black hole debris disks can significantly alleviate the baryon-loading problem in gamma-ray burst sources. We argue that the neutron-to-proton ratio in disk-fed outflows might be as high as ~30 and show, with the help of an exact semianalytic relativistic-MHD solution, that the neutrons can decouple at a Lorentz factor γd~15 even as the protons continue to accelerate to γ∞~200 and end up acquiring ~30% of the injected energy. We clarify the crucial role that the magnetic field plays in this process and prove that purely hydrodynamic outflows must have γd>~few×102. The motion of the decoupled neutrons is not collinear with that of the decoupled protons, so, in contrast to previous suggestions based on purely hydrodynamic models, the two particle groups most likely do not collide after decoupling. The decoupled neutron flow might nevertheless contribute to the observed afterglow emission.