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GIUV2013-002
In a broad sense, this group will be aimed at obtaining a deeper insight into the physical processes taking place in astrophysical magnetized plasmas, which involve a broad range of length and time scales. To study these scenarios we will employ different numerical codes as virtual tools that enable me to experiment on virtual laboratories (computers) with distinct initial and boundary conditions, in a fully analogous way to the experiments that can be done in an actual laboratory. Among the kind of sources I am interested to consider, I outline the following: Gamma-Ray Bursts (GRBs), extragalactic jets from Active Galactic Nuclei (AGN), magnetars and collapsing stellar cores. A number of important questions are still open regarding the fundamental properties of these astrophysical sources. The complete description of the collimation and acceleration of astrophysical jets is still being elucidated. The composition, high-energy emission, and the mechanisms by which jets propagate from their formation sites to the locations where they are observed is a subject of active scientific debate. Predicting source dynamics and gravitational waveforms is important to understand hoped-for...In a broad sense, this group will be aimed at obtaining a deeper insight into the physical processes taking place in astrophysical magnetized plasmas, which involve a broad range of length and time scales. To study these scenarios we will employ different numerical codes as virtual tools that enable me to experiment on virtual laboratories (computers) with distinct initial and boundary conditions, in a fully analogous way to the experiments that can be done in an actual laboratory. Among the kind of sources I am interested to consider, I outline the following: Gamma-Ray Bursts (GRBs), extragalactic jets from Active Galactic Nuclei (AGN), magnetars and collapsing stellar cores. A number of important questions are still open regarding the fundamental properties of these astrophysical sources. The complete description of the collimation and acceleration of astrophysical jets is still being elucidated. The composition, high-energy emission, and the mechanisms by which jets propagate from their formation sites to the locations where they are observed is a subject of active scientific debate. Predicting source dynamics and gravitational waveforms is important to understand hoped-for observations in the current generation of ground-based, gravitational-wave detectors, and essential to achieve design sensitivity in future space-based detectors. Additionally, there are analytical issues on the formalism in relativistic dynamics that are not completely resolved, particularly in the covariant extension of resistive magnetohydrodynamics. All these problems are so complex that only a computational approach is feasible. I plan to study them by means of (Magneto-)Hydrodynamics (MHD) numerical simulations with a suitable coupling with the dynamics of populations of non-thermal emitting particles. Most of these astrophysical plasmas are relativistic (e.g., GRBs, AGN jets). Thus, they must be treated with a suitable Special or General Relativity approach. The virtual laboratory I plan to develop will therefore be fully equipped with the most modern algorithms to cope with Special Relativistic MHD (SRMHD) or General Relativistic MHD (GRMHD) fluids. Other scenarios can be appropriately described by a classical or Newtonian MHD approach; hence the virtual lab will also be prepared for that.A principal focus of the project will be to assess the relevance of magnetic fields in the generation, collimation and ulterior propagation of relativistic jets from the GRB progenitors and from AGNs. Research lines: 1. - Magnetic field amplification in proto-neutron stars (PNS). Inferring the mechanism by which the magnetic field is amplified from seed values in extraordinarily dense plasma to dynamically relevant figures, and predicting which are the preferred field topologies as well as the possible effects of the field onto the dynamics of GRBs (e.g., the formation of jets) is a long standing issue, whose solution can be figured out by means of a combination of local and global (GR)MHD simulations. Since most supernova progenitors are expected to be slow rotators (Heger et al 2000) convection and dynamo effects in the PNS are, most probably, the main magnetic field amplification mechanism in these objects. However a subclass of rapidly rotating progenitors is expected to exist (Woosley & Heger 2006; Yoon et al. 2008) which would explain the observed correlation of some type Ic supernovae (SNe) and long GRBs. The most promising mechanism to account for the rapid growth of magnetic field in the collapse of a rapidly rotating stellar core, conducive to a PNS, is the Magneto-Rotational-Instability (MRI). The extremely small scales at which the fastest growing field modes develop, challenge any numerical approach, even direct (local) numerical simulations of small representative boxes of a PNSs. The disparity of time and length scales over which the field amplification takes place makes it necessary to perform also a global numerical modeling of the system, which includes the whole PNSs and its environment. I plan to develop new computational strategies to feedback the results of local numerical simulations on global ones. One of the ground-braking outcomes of this work shall be sub-grid models for global numerical simulations, which will be able to account properly for the growth of the magnetic field (because of MRI) from unresolved grid scales. Such models will allow us to bridge the existing gap between microscopic and macroscopic scales in this field. Furthermore, in this context we will pay special attention to non-ideal MHD effects, which can be decisive to set the levels at which the growth of the magnetic field saturates (Simon & Hawley 2009). Additional field amplification can be mediated by instabilities crucial for core-collapse SNe, viz. convection and the stationary accretion shock instability (SASI). While their appeal for standard core-collapse SNe lies in the fact that these instabilities do not rely on rapid rotation, they may also be important in intermediate steps of GRBs, e.g., between the formation of a hypermassive proto-neutron star (convection) and its subsequent collapse to a black hole, or in the accretion flow onto the black hole (SASI). I plan to study the corresponding growth of magnetic fields and of the dynamic backreaction onto the flow using models employing simplified microphysics (e.g., replacing detailed neutrino transport by cooling functions) as well as detailed radiation-MHD simulations. 2.- GRMHD jet generation. We will try to understand the relevance of the magnetic field in the generation, collimation and ulterior propagation of a relativistic jet from the progenitor of a GRB and from AGNs. We will work under the assumption that the mechanisms of formation and collimation are similar in both astrophysical scenarios and, indeed, we will pursue the objective of finding similarities and universalities in relativistic flows. There is an obvious connection between this goal and objective 1, since progenitors of long GRBs are, most probably, collapsars (see, Woosley 1993; MacFadyen & Woosley 1999), whose central engine -a solar-mass BH girded by a geometrically thick accretion disk- is likely threaded by huge magnetic fields, which originate via MRI from the collapse of the core of the progenitor star. One of the deficiencies of the current numerical approaches is the artificial set up of the central engine and of the magnetic field strength and topology. Typically, a quasi-equilibrium torus pierced by poloidal field lines is placed orbiting around a rotating BH. Perturbations of the initial torus matter trigger the accretion that fuels bipolar outflows. Both the initial accretion-torus configuration and the field topology are set up ad-hoc. I plan to use the outcomes of the global simulations to be performed in point 1 as initial models for GRMHD simulations that, consistently, account for the collapse of the PNS to a BH and the generation of jets in collapsars. 3.- Radiative transport and microphysics. Close to the central engine, the accretion disk and jet radiative physics are keys to understand the evolution of the jet and why different systems have different terminal velocity. Through annihilation of photons in AGNs, the radiative physics may illuminate the origin of jet composition by determining the electron-positron mass-loading of the jet, and so its Lorentz factor. For GRBs, the radiative annihilation of neutrinos and the effect of Fick diffusion (Levinson & Eichler 2003) may give an understanding of the Lorentz factor of the jet and the origin of baryon contamination. Furthermore, neutrino-driven winds may originate from the accretion disk. They may change the collimation, stability and baryon pollution of the ultrarelativistic GRB-jet, as well as being of extraordinary relevance to the synthesis of r-process nuclei, which may explain the observed abundances of such elements and yield a radioactivity signal accompanying short GRBs. Therefore, as applied to the field of progenitors of GRBs, a realistic equation of state, photodisintegration of nuclei, general relativistic neutrino transport (ray-tracing similar to Birkl et al. 2006 or two-moment transport as in Obergaulinger 2008), and neutrino cooling (similar to, e.g. Kohri et al. 2005) are missing in the state-of-the-art work in this field and will be implemented in the numerical experiments I am planning for this proposal. In case of AGN jets, simplified photon transport, and photon Comptonization may be included as new elements in our numerical models in order to obtain a more consistent picture. Finally, I plan to estimate the gravitational wave emission associated to the birth of relativistic jets using the tools developed by both my former group at MPA (Obergaulinger et al. 2006) and my current host (Cordero, et al., in preparation). The close relation of non-GRB SNe with collapsars will allow me to apply the methods outlined above also to these systems to study, e.g., the interplay of hydromagnetic instabilities and neutrino transport. Inclusion of many of the former elements is an interdisciplinary task that may involve the common work with computer scientists in order to design numerically efficient algorithms. 4.- Radiative processes. The observed differences in the radiative properties of jets in AGNs and GRBs suggest that the environment likely plays a significant role in the emission at large distances from the central engine. Both blazars and GRBs exhibit non-thermal emission. But, emission of long duration GRBs becomes harder with increasing luminosity, while in blazars the opposite happens (Ghirlanda et al. 2004, 2005). Also, GRBs emit most of the energy in ?-rays and less than 10% to the lower frequency afterglow (Piran 2005), while blazars release only 10% in ?-rays, the rest being produced in the radio lobe (Ghisellini & Celotti 2002). On the other hand, a worthy byproduct of the comparison of synthetic spectra and light curves with actual observations can be the determination of the amount of thermal matter present in extragalactic jets. This fact constitutes a proxy to determine their composition (in particular of blazar jets). The radiative physics of jets in AGNs and GRBs at large distances from the source will be subject of a specific work following the approach developed in Mimica, Aloy & Müller (2007) and Mimica et al (2009). 5.- Improving previous work. I plan to improve my previous results in two ways: (i) by increasing the number of dimensions in which the models are computed and (ii) by including dynamically important magnetic fields. In case of progenitors of GRBs, 2D axisymmetric models have already been computed. Future simulations will be three-dimensional in order to assess the stability of the generated outflows as well as to account for the proper mass entrainment in the jet. Furthermore, the addition of magnetic fields in either 2D or 3D will extend the range of applicability of the results of Aloy, Janka & Müller (2005) and Mizuno & Aloy (2009). Extending my previous work in the field of internal shocks in relativistic jets from one to two spatial dimensions is needed to account for the lateral expansion of the outflows. This is a key question, e.g., in the transition regime between the prompt GRB emission and the early afterglow. It is also important to have a reliable estimate of the efficiency of the model of internal shocks in converting kinetic into radiated energy. On the other hand, I plan to compute the evolution of ultra-relativistic, magnetized outflows in the GRB context, starting from the end of the acceleration phase, through the internal shocks phase (prompt emission) all the way to the end of the afterglow phase. If successful, even one-dimensional simulations would provide the first consistent prediction for the dependence of GRB dynamics, and both prompt and afterglow emission, on the magnetization of the flow, equation of state and, possibly, presence of non-ideal effects (magnetic dissipation). The relativistic Rayleigh-Taylor instability of a decelerating shell (Levinson 200), and its implications for GRBs will be addressed by means of multidimensional R(M)HD simulations. Together with Dr. Cerdá-Durán, I plan to extend the recent results of Cerdá-Duran et al. (2009) on quasi- periodic oscillations in the tail of giant flares of SGRs. The precise mechanism by which the oscillation spectrum of the magnetar interior modulates the emission in the magnetosphere will be studied by adding realistic magnetosphere models to the present simulations. The emission properties of the flares, including spectra and X-ray maps, can be computed using similar techniques as in points 3 and 4. 6.- Beyond ideal MHD. Although an ideal RMHD modeling of the sites where relativistic jets are produced has already proven to be very fruitful, non-ideal effects (particularly, viscosity and resistivity) are important (1) when the flow develops current sheets; (2) where pair creation contributes a non-negligible amount of rest-mass, internal energy, or momentum density; and (3) if the rest-mass flux due to ambipolar and Fick diffusion is not negligible. I plan to develop new algorithms to account for most of these effects. I will address my first efforts to develop a resistive RMHD code following the lines shown by Komissarov (2007). Scattered in the previous objectives, I have sketched a number of astrophysical scenarios where non-ideal effects might be potentially important. To these sources, one may also add solar flares, where non-ideal MHD, even beyond Ohmic resistivity, could be extremely exciting. Let me stress that, even at the theoretical level, the development of a fully covariant theory for the reconnection of magnetic field is, in its own right, a ground-breaking challenge. Finally, I point out that non-ideal effects are also potentially important in some of the MHD applications we are planning (see point 1). Capabities of the group: Our group develops a basic, non-oriented research in the field of Relativistic Plasma Astrophysics. Most of our activities are relatied with the numerical modeling of (magnetized) fluids. Thus, beyond our obvious Astrophysical capabilities, we have exepertise in High-Performance Computing.
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- I will pursue the goal of understanding the process of amplification of seed magnetic fields until they become dynamically relevant, e.g., using semi-global and local simulations of representative boxes of collapsed stellar cores. A big emphasis will be put on including all the relevant microphysics (e.g. neutrino physics), non-ideal effects (particularly, reconnection physics) and energy transport due to neutrinos and photons to account for the relevant processes in the former systems. A milestone of this group will be to end up with a numerical tool that enables us to deal with General Relativistic Radiation Magnetohydrodynamics problems in Astrophysics
- Modelado de plasmas astrofísicos asistido por computador. Modelado de Plasmas Astrofísicos Asistido por Computador.
| Nombre | Carácter de la participación | Entidad | Descripción |
|---|---|---|---|
| Miguel Ángel Aloy Toras | Director-a | UVEG-Valencia | Catedràtic-a d'Universitat |
| Equip d'investigació | |||
| Michael Gabler | Membre | UVEG-Valencia | Investigador-a contratactat-da doctor-a distinguit-da de excelència Cv |
| Beatrice Giudici | Membre | UVEG-Valencia | Investigador-a en Formació Prometeo |
| Adam Griffiths | Membre | UVEG-Valencia | Investigador-a en Formació Prometeo |
| Martín Franz Obergaulinger | Membre | UVEG-Valencia | Investigador-a doctor-a UVEG Senior |
| Jens Florian Mahlmann | Col·laborador-a | USA-Princeton | Investigador-a |
| José Antonio Pons Botella | Col·laborador-a | UA-Alicante | Catedràtic-a d'Universitat |
| Equip de Treball | |||
| Georges Meynet | Equip de Treball | CHE-UGenève | Catedràtic-a d'Universitat |
