# Nektarios Vlahakis - Department of Astrophysics, Astronomy and Mechanics, Faculty of Physics, University of Athens, Greece

## Contact Details

NameNektarios Vlahakis |
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AffiliationDepartment of Astrophysics, Astronomy and Mechanics, Faculty of Physics, University of Athens, Greece |
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CityAthens |
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CountryGreece |
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## Pubs By Year |
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## External Links |
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## Pub CategoriesAstrophysics (11) Solar and Stellar Astrophysics (4) High Energy Astrophysical Phenomena (4) Physics - Plasma Physics (1) Instrumentation and Methods for Astrophysics (1) Cosmology and Nongalactic Astrophysics (1) Astrophysics of Galaxies (1) |

## Publications Authored By Nektarios Vlahakis

**Authors:**Ivan Agudo, Markus Boettcher, Heino Falcke, Markos Georganopoulos, Gabriele Ghisellini, Gabriele Giovannini, Marcello Giroletti, Jose L. Gomez, Leonid Gurvits, Robert Laing, Matthew Lister, Jose-Maria Marti, Eileen T. Meyer, Yosuke Mizuno, Shane O'Sullivan, Paolo Padovani, Zsolt Paragi, Manel Perucho, Dominik Schleicher, Lukasz Stawarz, Nektarios Vlahakis, John Wardle

Relativistic jets in active galactic nuclei (AGN) are among the most powerful astrophysical objects discovered to date. Indeed, jetted AGN studies have been considered a prominent science case for SKA, and were included in several different chapters of the previous SKA Science Book (Carilli & Rawlings 2004). Most of the fundamental questions about the physics of relativistic jets still remain unanswered, and await high-sensitivity radio instruments such as SKA to solve them. Read More

Relativistic jets associated with long/soft gamma-ray bursts are formed and initially propagate in the interior of the progenitor star. Because of the subsequent loss of their external pressure support after they cross the stellar surface, these flows can be modeled as moving around a corner. A strong steady-state rarefaction wave is formed, and the sideways expansion is accompanied by a rarefaction acceleration. Read More

We construct and analyze a model of the relativistic steady-state magnetohydrodynamic (MHD) rarefaction that is induced when a planar symmetric flow (with one ignorable Cartesian coordinate) propagates under a steep drop of the external pressure profile. Using the method of self-similarity we derive a system of ordinary differential equations that describe the flow dynamics. In the specific limit of an initially homogeneous flow we also provide analytical results and accurate scaling laws. Read More

We investigate the differences between an outflow in a highly-resistive accretion disk corona, and the results with smaller or vanishing resistivity. For the first time, we determine conditions at the base of a two-dimensional radially self-similar outflow in the regime of very large resistivity. We performed simulations using the {\sc pluto} magnetohydrodynamics code, and found three modes of solutions. Read More

**Authors:**Matthias Stute

^{1}, Jose Gracia

^{2}, Nektarios Vlahakis

^{3}, Kanaris Tsinganos

^{4}, Andrea Mignone

^{5}, Silvano Massaglia

^{6}

**Affiliations:**

^{1}Institute of Astronomy and Astrophysics, Section Computational Physics, Eberhard Karls Universitaet Tuebingen, Germany,

^{2}High Performance Computing Center Stuttgart,

^{3}Department of Astrophysics, Astronomy and Mechanics, Faculty of Physics, University of Athens, Greece,

^{4}Department of Astrophysics, Astronomy and Mechanics, Faculty of Physics, University of Athens, Greece,

^{5}Dipartimento di Fisica, Universita degli Studi di Torino, Italy,

^{6}Dipartimento di Fisica, Universita degli Studi di Torino, Italy

Disc-winds originating from the inner parts of accretion discs are considered as the basic component of magnetically collimated outflows. The only available analytical MHD solutions to describe disc-driven jets are those characterized by the symmetry of radial self-similarity. However, radially self-similar MHD jet models, in general, have three geometrical shortcomings, (i) a singularity at the jet axis, (ii) the necessary assumption of axisymmetry, and (iii) the non-existence of an intrinsic radial scale, i. Read More

**Affiliations:**

^{1}Universita degli Studi di Torino, Italy,

^{2}High Performance Computing Center Stuttgart, Germany,

^{3}University of Athens, Greece,

^{4}University of Athens, Greece

**Category:**Solar and Stellar Astrophysics

(abridged) Significant progress has been made in the last years in the understanding of the jet formation mechanism through a combination of numerical simulations and analytical MHD models for outflows characterized by the symmetry of self-similarity. In a previous article we introduced models of truncated jets from disks, i.e. Read More

Axisymmetric resistive MHD simulations for radially self-similar initial conditions are performed, using the NIRVANA code. The magnetic diffusivity could occur in outflows above an accretion disk, being transferred from the underlying disk into the disk corona by MHD turbulence (anomalous turbulent diffusivity), or as a result of ambipolar diffusion in partially ionized flows. We introduce, in addition to the classical magnetic Reynolds number Rm, which measures the importance of resistive effects in the induction equation, a new number Rb, which measures the importance of the resistive effects in the energy equation. Read More

We present a relativistic-MHD numerical study of axisymmetric, magnetically driven jets with parameters applicable to gamma-ray burst (GRB) flows. We also present analytic expressions for the asymptotic jet shape and other flow parameters that agree very well with the numerical results. All current-carrying outflows exhibit self-collimation and consequent acceleration near the rotation axis, but unconfined outflows lose causal connectivity across the jet and therefore do not collimate or accelerate efficiently in their outer regions. Read More

**Authors:**Matthias Stute

^{1}, Kanaris Tsinganos

^{2}, Nektarios Vlahakis

^{3}, Titos Matsakos

^{4}, Jose Gracia

^{5}

**Affiliations:**

^{1}IASA and University of Athens,

^{2}IASA and University of Athens,

^{3}IASA and University of Athens,

^{4}Universita degli Studi di Torino,

^{5}DIAS

**Category:**Astrophysics

(Abridged) Finite radius accretion disks are a strong candidate for launching astrophysical jets from their inner parts and disk-winds are considered as the basic component of such magnetically collimated outflows. The only available analytical MHD solutions for describing disk-driven jets are those characterized by the symmetry of radial self-similarity. Radially self-similar MHD models, in general, have two geometrical shortcomings, a singularity at the jet axis and the non-existence of an intrinsic radial scale, i. Read More

Numerical simulations with self-similar initial and boundary conditions provide a link between theoretical and numerical investigations of jet dynamics. We perform axisymmetric resistive magnetohydrodynamic (MHD) simulations for a generalised solution of the Blandford & Payne type, and compare them with the corresponding analytical and numerical ideal-MHD solutions. We disentangle the effects of the numerical and physical diffusivity. Read More

Using steady, axisymmetric, ideal magnetohydrodynamics (MHD) we analyze relativistic outflows by means of examining the momentum equation along the flow and in the transfield direction. We argue that the asymptotic Lorentz factor is ~ mu-sigma_M, and the asymptotic value of the Poynting-to-matter energy flux ratio - the so-called sigma function - is given by sigma/(1+sigma) \~ sigma_M / mu, where sigma_M is the Michel's magnetization parameter and mu c^2 the total energy-to-mass flux ratio. We discuss how these values depend on the conditions near the origin of the flow. Read More

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. Read More

Using relativistic, steady, axisymmetric, ideal magnetohydrodynamics (MHD) we analyze the super-Alfvenic regime of a pulsar wind by means of solving the momentum equation along the flow as well as in the transfield direction. Employing a self-similar model, we demonstrate that ideal MHD can account for the full acceleration from high (>>1) to low (<<1) values of $\sigma$, the Poynting-to-matter energy flux ratio. The solutions also show a transition from a current-carrying to a return-current regime, partly satisfying the current-closure condition. Read More

We demonstrate that ``hot'' MHD outflows from neutron-rich black-hole debris disks can significantly alleviate the baryon-loading problem in gamma-ray burst (GRB) 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 gamma_d ~ 15 even as the protons continue to accelerate to Lorentz factor ~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 gamma_d > a few hundreds. Read More

**Affiliations:**

^{1}University of Chicago,

^{2}University of Chicago

**Category:**Astrophysics

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. Read More

**Affiliations:**

^{1}University of Chicago,

^{2}University of Chicago

**Category:**Astrophysics

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. Read More

**Affiliations:**

^{1}University of Athens,

^{2}University of Chicago,

^{3}University of Athens

**Category:**Astrophysics

We present a systematic method for constructing two-dimensional magnetohydrodynamic equilibria with compressible flow in Cartesian geometry. This systematic method has already been developed in spherical geometry and applied in modelling solar and stellar winds and outflows (Vlahakis & Tsinganos,1998) but is derived here in Cartesian geometry in the context of the solar atmosphere for the first time. Using the method we find several new classes of solutions, some of which generalise known solutions, including the Kippenhahn & Schl\"uter (1957) and Hood & Anzer (1990) solar prominence models and the Tsinganos, Surlantzis & Priest (1993) coronal loop model with flow, and some of which are completely new. Read More

**Affiliations:**

^{1}University of Chicago,

^{2}University of Chicago

**Category:**Astrophysics

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. Read More