Publications by Year: 2013

Vlahakis N, Sauty C, Cayatte V, Matsakos T, Tsinganos K, Lima J. Are counter-rotating jets possible?. In: ; 2013. pp. 31 - 31. WebsiteAbstract
Young stellar object observations suggest that some jets rotate in the opposite direction with respect to their disk. In a recent study, we have shown that this can be well in agreement with the magnetocentrifugal mechanism that is believed to launch such outflows. Here, we extend this analytical derivation to relativistic jets demonstrating that under rather general conditions counterrotation can indeed take place. We also illustrate the involved mechanism by performing relativistic magnetohydrodynamic jet simulations.
Karampelas K, Millas D, Katsoulakos G, Lingri D, Vlahakis N. Solving the wind equation for relativistic magnetized jets. In: ; 2013. pp. 30 - 30. WebsiteAbstract
We approach the problem of bulk acceleration in relativistic, cold, magnetized outflows, by solving the momentum equation along the flow, a.k.a. the wind equation, under the assumptions of steady-state and axisymmetry. The bulk Lorentz factor of the flow depends on the geometry of the field/streamlines and by extension, on the form of the "bunching function" S=r^2 B_p/ A, where r is the cylindrical distance, B_p the poloidal magnetic field, and A the magnetic flux function. We investigate the general characteristics of the S function and how its choice affects the terminal Lorentz factor gamma_f and the acceleration efficiency gamma_f/mu, where mu is the total energy to mass flux ratio (which equals the maximum possible Lorentz factor of the outflow). Various fast-rise, slow-decay examples are selected for S, each one with a corresponding field/streamlines geometry, with a global maximum near the fast magnetosonic critical point, as required from the regularity condition. As it is proved, proper choices of S can lead to efficiencies greater than 50%. Last, we apply our results to the momentum equation across the flow, in an effort to estimate their validity, as well as identifying the factors that lead to an accurate full-problem solution. The results of this work, depending on the choices of the flow integral mu, can be applied to relativistic GRB or AGN jets.
Stute M, Gracia J, Vlahakis N, Tsinganos K. Launching protostellar jets from finite-radius accretion disks. In: ; 2013. WebsiteAbstract
Analytical radially self-similar models are the best available solutions describing disk-winds, but they need several improvements. We introduce models of jets from truncated disks, i.e. numerical simulations based on a radially self-similar MHD solution but including the effects of a finite radius of the jet-emitting disk, hence the outflow. We compare these models with available observational data, by varying the jet density and velocity, the mass of the protostar, the radius of this truncation and the inclination. In order to our models with observed jet widths inferred from recent optical images taken with HST and ground-based AO observations, we create emission maps in different forbidden lines, and from such emission maps, determine the jet width as the full-width half-maximum of the emission. We can reproduce the jet widths of several examples and its variations very well. We conclude that truncation - i.e. taking the finite radius of the jet launching region into account - is needed to reproduce the observed jet widths, and our simulations limit the possible range of truncation radii. The effects of inclination are important for modeling the intrinsic variations seen in observed jet widths. Our models can be used to infer the inclinations in the observed sample independently.
Sapountzis K, Vlahakis N. Acceleration of Magnetized Collapsar Jets After Breakout. In: Vol. 61. ; 2013. pp. 181 - 184. WebsiteAbstract
In the collapsar model of long GRBs the jet is formed at the center of the progenitor star, propagates in its interior, and produces the observed gamma rays much after its breakout from the star. The loss of pressure support during breakout induces a strong rarefaction wave that propagates inside the jet and causes its bulk acceleration. This mechanism has been already studied using axisymmetric magnetohydrodynamic (MHD) simulations assuming a prescribed shape for the surface between the jet and its environment, as well as using simple rarefaction waves in planar geometry. Trying to improve over these works, we solve the steady-state, axisymmetric, relativistic MHD equations using the method of characteristics. In this way the jet boundary is found self-consistently and the rarefaction wave is studied in the axisymmetric geometry. In this poster we present our first results and a comparison with previous works.
CTA Consortium T, :, Abril O, Acharya BS, Actis M, Agnetta G, Aguilar JA, Aharonian F, Ajello M, Akhperjanian A, et al. CTA contributions to the 33rd International Cosmic Ray Conference (ICRC2013). [Internet]. 2013:arXiv:1307.2232. WebsiteAbstract
Compilation of CTA contributions to the proceedings of the 33rd International Cosmic Ray Conference (ICRC2013), which took place in 2-9 July, 2013, in Rio de Janeiro, Brazil
Acharya BS, Actis M, Aghajani T, Agnetta G, Aguilar J, Aharonian F, Ajello M, Akhperjanian A, Alcubierre M, Aleksić J, et al. Introducing the CTA concept. [Internet]. 2013;43:3 - 18. WebsiteAbstract
The Cherenkov Telescope Array (CTA) is a new observatory for very high-energy (VHE) gamma rays. CTA has ambitions science goals, for which it is necessary to achieve full-sky coverage, to improve the sensitivity by about an order of magnitude, to span about four decades of energy, from a few tens of GeV to above 100 TeV with enhanced angular and energy resolutions over existing VHE gamma-ray observatories. An international collaboration has formed with more than 1000 members from 27 countries in Europe, Asia, Africa and North and South America. In 2010 the CTA Consortium completed a Design Study and started a three-year Preparatory Phase which leads to production readiness of CTA in 2014. In this paper we introduce the science goals and the concept of CTA, and provide an overview of the project.
Sapountzis K, Vlahakis N. Rarefaction acceleration in magnetized gamma-ray burst jets. [Internet]. 2013;434:1779 - 1788. WebsiteAbstract
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 modelled as moving around a corner. A strong steady-state rarefaction wave is formed, and the sideways expansion is accompanied by a rarefaction acceleration. We investigate the efficiency and the general characteristics of this mechanism by integrating the steady-state, special relativistic, magnetohydrodynamic equations, using a special set of partial exact solutions in planar geometry (r self-similar with respect to the `corner'). We also derive analytical approximate scalings in the ultrarelativistic cold/magnetized, and hydrodynamic limits. The mechanism is more effective in magnetized than in purely hydrodynamic flows. It substantially increases the Lorentz factor without much affecting the opening of the jet; the resulting values of their product can be much greater than unity, allowing for possible breaks in the afterglow light curves. These findings are similar to the ones from numerical simulations of axisymmetric jets by Komissarov et al. and Tchekhovskoy et al., although in our approach we describe the rarefaction as a steady-state simple wave and self-consistently calculate the opening of the jet that corresponds to zero external pressure.
Tsinganos K, Matsakos T, Tesileanu O, Vlahakis N, Massaglia S, Mignone A, Trussoni E, Cayatte V, Stehle C. YSO jet simulations: from theory to synthetic observations. In: ; 2013. pp. 38 - 39. WebsiteAbstract
A plethora of analytical studies have addressed the physical mechanisms of jet launching and propagation in young stellar objects. However, their link to observations is still missing due to the complexity of the emission processes involved. In this work we address this issue, by presenting MHD simulations of two-component YSO jet models that are based on analytical disk and stellar outflow solutions. We include ionization and optically thin radiation losses during the temporal evolution of the flow and we post process the output files to generate synthetic emission maps. Our results are confronted to observational data and we find that our models predict the correct range of values for the density, temperature and velocity of YSO jets. Moreover, the synthetic emission maps of the - 39 - doublets [OI], [N II] and [S II] outline a well collimated and knot-structured jet, which is surrounded by a less dense and slower wind, not observable in these lines. The jet is found to have a small opening angle and a radius that is also comparable to observations.