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Facultat de física

Relativistic collapse and explosion of rotating supermassive stars with thermonuclear effects

Pedro Montero
Max-Planck-Institut für Astrophysik, Garching (Alemanya)
We present results of general relativistic simulations of collapsing supermassive stars with and without rotation using the two-dimensional general relativistic numerical code Nada, which solves the Einstein equations written in the BSSN formalism and the general relativistic hydrodynamics equations with high resolution shock capturing schemes. These numerical simulations use an equation of state which includes effects of gas pressure, and in a tabulated form those associated with radiation and the electron-positron pairs. We also take into account the effect of thermonuclear energy released by hydrogen and helium burning. We find that objects with a mass of 5x10^{5} solar mass and an initial metallicity greater than Z_{CNO}~0.007 do explode if non-rotating, while the threshold metallicity for an explosion is reduced to Z_{CNO}~0.001 for objects uniformly rotating. The critical initial metallicity for a thermonuclear explosion increases for stars with mass ~10^{6} solar mass. For those stars that do not explode we follow the evolution beyond the phase of black hole formation. We compute the neutrino energy loss rates due to several processes that may be relevant during the gravitational collapse of these objects. The peak luminosities of neutrinos and antineutrinos of all flavors for models collapsing to a BH are ~10^{55} erg/s. The total radiated energy in neutrinos varies between ~10^{56} ergs for models collapsing to a BH, and ~10^{45}-10^{46} ergs for models exploding.

Stability of Iterative Algorithms for Rotating and Binary Neutron Stars

Charalampos Markakis
SFB/TR7 Gravitational Wave Astronomy, Theoretical Physics Institute, University of Jena (Alemanya)
Similar methods have been used to construct models of rapidly rotating or binary stars, in Newtonian and relativistic contexts. The choice of method has been based on numerical experiments, which indicate that particular methods converge quickly to a solution, while others diverge. The theory underlying these differences, however, has not been understood. In an attempt to provide a better theoretical understanding, we analytically examine the behavior of different iterative schemes near an exact static solution. We find the spectrum of the linearized iteration operator and show for self-consistent field methods that iterative instability corresponds to unstable modes of this operator. Minimizing the maximum eigenvalue accelerates convergence and allows computation of highly compact configurations that were previously inaccessible via self-consistent field methods.

Self-gravitating Tori around black holes and their instabilities

Nikolaos Stergioulas
Aristotle University of Thessaloniki (Grècia)
We give an overview of recent progress in constructing equilibrium models of self-gravitating tori around black holes, using a self-consistent field method. We focus on the properties of models with constant specific angular momentum that exactly fill their Roche lobe and show that their angular momentum is limited in the same way as in the case of a fixed background metric. Furthermore, we explore the stability of such disks against runaway and nonaxisymmetric instabilities using three-dimensional hydrodynamics simulations in full general relativity. All of our models develop unstable nonaxisymmetric modes on a dynamical time scale.We observe two distinct types of instabilities: the Papaloizou-Pringle and the so-called intermediate type instabilities. The development of the nonaxisymmetric mode with azimuthal number m=1 is accompanied by an outspiraling motion of the black hole, which significantly amplifies the growth rate of the m=1 mode in some cases. We comment on the generation of gravitational waves and on the development of the runaway instability.

Partially implicit high order Runge-Kutta methods for wave-like equations in spherical-type coordinates

Isabel Cordero-Carrión
Max Planck Institute for Astrophysics, Garching (Germany)
will present new partially implicit Runge-Kutta high order methods in order to numerically evolve in time a set of partial differential equations. These methods are based on the ideas used in Cordero-Carrión et al. (2011) to evolve numerically the hyperbolic sector of the metric in the so called Fully Constrained Formulation of Einstein equations, and are designed in order to overcome numerical instabilities, due, for example, to potential numerical unstable terms in the sources of the system or to effects of the chosen system of coordinates (as it was our case).

GRB 101225A: a stellar murder on Christmas Day

Miguel Ángel Aloy
Departament d'Astronomia i Astrofísica, Universitat de València
Long Gamma-Ray Bursts (GRBs) are the most dramatic examples of massive stellar deaths, usually associated with supernovae (Woosley et al. 2006). They release ultra-relativistic jets producing non-thermal emission through synchrotron radiation as they interact with the surrounding medium (Zhang et al. 2004). Here we report observations of the peculiar GRB 101225A (the 'Christmas burst'). Its gamma-ray emission was exceptionally long and followed by a bright X-ray transient with a hot thermal component and an unusual optical counterpart. During the first 10 days, the optical emission evolved as an expanding, cooling blackbody after which an additional component, consistent with a faint supernova, emerged. We determine its distance to 1.6 Gpc by fitting the spectral-energy distribution and light curve of the optical emission with a GRB-supernova template. Deep optical observations may have revealed a faint, unresolved host galaxy. Our proposed progenitor is a helium star-neutron star merger that underwent a common envelope phase expelling its hydrogen envelope. The resulting explosion created a GRB-like jet which gets thermalized by interacting with the dense, previously ejected material and thus creating the observed black-body, until finally the emission from the supernova dominated. An alternative explanation is a minor body falling onto a neutron star in the Galaxy (Campana et al. 2011).

High-energy emission from relativistic outflows in compact environments

Valentí Bosch-Ramon
Dublin Institute for Advanced Studies, Irlanda
Relativistic outflows produced in binary systems, or the SMBH in the center of AGN, are subject to violent disturbances due to the crowded media in which they are embedded. The direct impact or the entrainment of external matter by these outflows lead to strong hydrodynamical perturbations and energy dissipation. This energy can be partially channelled into non-thermal particles that will produce emission from radio to very high energies via different processes. In this talk, I will briefly discuss some interaction scenarios in the context of relativistic outflows in compact environments, focusing on the evolution of the flows involved, and on the high-energy non-thermal processes.

El Centro de Estudios de Física del Cosmos de Aragón y el Observatorio Astrofísico de Javalambre

Mariano Moles
CEFCA, Centro de Estudios de Física del Cosmos de Aragón

Photospheric Thermal Radiation From GRB Collapsar Jets

Akira Mizuta
Theory Center, Institute of Particle and Nuclear Studies, KEK (High Energy Accelerator Research Organization), Japan
The gamma-ray bursts (GRBs) are one of the most luminous events in the universe. The radiation mechanism of GRB prompt emission is still underdebate. If a central engine makes a fireball, we can expect strong thermal emission from the photosphere. We have performed numerical hydrodynamic simulations of GRB jet propagation from collapsars and have derived photospheric thermal radiation by the post process. The light curves and spectra are derived for the observers at different viewing angles. We observe a few seconds time variability in the light curves caused by the internal mass density and Lorentz factor discontinuities in the jets. When we plot E_p-E_{iso} or E_p-L_{iso}_p, we can see similar dependence with empirical GRB cosmological relations such as Amati relation and Yonetoku relation.

Radio transients from stellar tidal disruption by supermassive black holes

Dimitrios Giannios
Department of Astrophysical Sciences, Peyton Hall, Princeton University, Princeton, USA
The tidal disruption of a star by a supermassive black hole provides us with a rare glimpse of these otherwise dormant beasts. It has long been predicted that the disruption will be accompanied by a thermal 'flare', powered by the accretion of bound stellar debris. Several candidate disruptions have been discovered in this manner at optical, UV and X-ray wavelengths. Here we explore the observational consequences if a modest fraction of the accretion power is channeled into an ultra relativistic outflow. We show that a relativistic jet decelerates due to its interaction with the interstellar medium at sub-parsec distances from the black hole. Synchrotron radiation from electrons accelerated by the reverse shock powers a bright radio-infrared transient that peaks on a timescale ~1 yr after disruption. Emission from the forward shock may be detectable for several years after the peak. Deep radio follow-up observations of tidal disruption candidates at late times can test for the presence of relativistic ejecta. Upcoming radio transient surveys may independently discover tens to hundreds of tidal disruptions per year, complimenting searches at other wavelengths. Non-thermal emission from tidal disruption probes the physics of jet formation under relatively clean conditions, in which the flow parameters are independently constrained.

Relativistic plasma dynamics in Pulsar Wind Nebulae and in long GRB projenitors

Prof. Luca Del Zanna
Università degli Studi di Firenze, Itàlia
In this talk I will briefly describe our numerical models for two different classes of Astrophysical sources of high-energy radiation: namely Pulsar Wind Nebulae and (long) GRB projenitors in the magnetar scenario. Under some respects, in spite of many differences, these two objects may share some common physics, especially the magnetic mechanism needed to collimate the polar relativistic jets. I will also discuss the plasma diagnostic techniques that we use to infer non-thermal emission (in the case of PWNe) and some recent important observations in the gamma rays. Finally, and only under request!, I will briefly summarize the main features and the latest developments of the GRMHD code ECHO employed for the simulations.

Parallel, grid-adaptive simulations of relativistic jets

Zakaria Meliani
Katholieke Universiteit Leuven, Bèlgica
The two types of Fanaroff-Riley radio loud galaxies, FRI and FRII, exhibit strong jets but with different properties. These differences may be associated to the central engine and/or the external medium. In this seminar I will present two models. I will start to discuss the dichotomy FRI/FRII accroding to the properties of the central engine, In this model we linked the AGN classification FRI and FRII to the transverse stratification of the jet. Indeed, theoretical arguments support this transverse stratification of jets with two components induced by intrinsic features of the central engine (accretion disk + black hole). We demonstrate that two-component jets with a relatively low kinetic energy flux contribution from the outer disk-jet are subject to the development of a relativistically enhanced, rotation-induced Rayleigh-Taylor type non-axisymmetric instability. This instability induces strong mixing between both components, decelerating the inner jet and leading to overall jet decollimation. In the second part, I will present the model that we proposed to explain HYbrid MOrphology Radio Sources (HYMORS), thereby invoking density discontinuities and variation in the initial open angle of jet. We explore how one-sided jet deceleration and a transition to FR I type can occur in HYMORS, which start as FR II (and remain so on the other side). The result of these work is that the FRII/FRI transition, can be associated to the central engine that induces transverse stratification of the jet and/or can be associated to the central engine (jet open angle) + external medium (density jump).

The distribution of stars around massive black holes in galactic nuclei - Implications for extreme-mass ratio inspirals and LISA

Pau Amaro-Seoane
Max-Planck Institut für Gravitationsphysik (Albert Einstein-Institut), Potsdam, Alemanya
Mass segregated stellar cusps are the natural configuration for nuclear stellar clusters in steady state with a central massive black hole of mass Mbh <~ few x 10^6 Msun. Detailed N-body and Fokker-Planck studies agree quite well with each other in the description of the bulk properties of the stellar distribution of such systems. For typical initial mass functions, the cluster is in the regime of strong mass segregation and therefore stellar-mass black holes dominate the spatial density in the innermost regions close to the massive black hole. As a natural consequence, the rates of extreme mass ratio inspirals detectable by LISA will be dominated by the inspiral of stellar-mass black holes.

Solar Wind Interaction with the Local Interstellar Medium: Achievements and Challenges s

N. V. Pogorelov
Physics Department and Center for Space Plasma and Aeronomic Research University of Alabama in Huntsville, USA
From the astrophysical perspective, the solar wind (SW) interaction with the local interstellar medium (LISM) is a particular case of the flow around an ejecting star placed into surrounding medium. I will describe the mathematical and physical statements of this problem and demonstrate how the interstellar magnetic field shapes the astropause. While the Voyager 1 and 2 spacecraft are exploring the boundaries of the heliosphere, our newest spacecraft, the Interstellar Boundary Explorer (IBEX) has started exploring the outermost reaches of the heliosphere, but from an orbit at 1 AU measuring the fluxes of energetic neutral atoms (ENAs) created in the boundary regions separating the heliosphere from the LISM. The first IBEX results revealed a sky-spanning “ribbon” of unexpectedly intense emissions of ENAs that had not been predicted previously by any physical model. For the next 5-10 years, heliophysics research is faced with an extraordinary opportunity that cannot be soon repeated. This is to make in situ measurements of the SW from the Sun to the heliospheric boundaries and, at the same time, extract information about the global behavior of the evolving heliosphere through ENA observations by the IBEX. In this talk, the effects are described of unsteady SW and interstellar magnetic field (ISMF) pressure of the shape of the heliopause and the terminations shock. We analyze the ISMF influence on the deflection of the neutral hydrogen flow in the inner heliosphere from its original direction in the unperturbed LISM and on the position of the ENA ribbon. It is shown that the ISMF direction strongly correlates with the ribbon location, which is useful for constraining the properties of the interstellar medium in the immediate vicinity of the heliospheric boundary.

Understanding the ultrafast TeV Variability from blazars

Dimitrios Giannios
Department of Astrophysical Sciences, Peyton Hall, Princeton University, Princeton, USA
Recently two blazars (PKS 2155-304 and Mrk 501) have exhibited TeV flaring on timescales 1-2 orders of magnitude shorter that the light-crossing time of the host black holes. I discuss how these observations challenge our current paradigm for blazar emission and indicate that jet instabilities are the source of the ultrafast variability. A model of reconnection minijets forming inside the jet will be presented for the blazar variability.

Cosmic-Ray driven dynamo in galactic disks

Michal Hanasz
Centre for Astronomy, Nicolaus Copernicus University, Torun, Poland
I am going to present recent developments of local and global, galactic-scale numerical models, of the Cosmic-Ray driven dynamo, which was originally proposed by Parker (1992). We conduct global, galactic-scale CR-MHD numerical simulations of the dynamics of interstellar medium, composed of gas, magnetic-field, and cosmic-ray components. We take into account cosmic rays, accelerated in randomly distributed supernova remnants, and assume that supernovae deposit small-scale, randomly oriented, dipolar magnetic-fields into the ISM. We find that, the magnetization of galactic disks by exploding magnetized stars forms a favorable initial condition for the galactic dynamo process. The amplification timescale of the large-scale magnetic field, resulting from the CR-driven dynamo, is comparable to the galactic rotation period. The process converts efficiently small-scale magnetic fields of SN-remnants into the galactic-scale magnetic fields. The resulting magnetic-field structure resembles the observed X-shaped magnetic fields in edge-on galaxies.

Interfaces in Numerical Relativistic Hydrodynamics

Dr. Stephen Millmore
School of Mathematics, University of Southampton, England.
Current non-linear numerical simulations of neutron stars use single component models. However, evidence suggests that there are several different regions present within a neutron star. Qualitatively different behaviour is expected to occur within these regions and can be characterised by the equation of state. Problems arise when modelling a single component neutron star with a spatially varying equation of state, due to instabilities in the numerical methods. An alternative method of dealing with these regions is to use a multicomponent model. In such a case, level set methods can be used to track the interface between components. Boundary conditions must then be chosen and imposed at any interfaces to model the interaction between the fluid components correctly. Such techniques have been used with success in Newtonian computational fluid dynamics. Here I shall present an extension of these techniques to relativistic situations, and construct a simple multicomponent star model in general relativity by applying this extension. I then will provide some insight into the extension of these techniques for multidimensional situations.

Influence of self-gravity on the runaway instability of black hole-torus systems

Pedro Montero
Max-Planck-Institut für Astrophysik, Garching, Germany
Results from the first fully general relativistic numerical simulations in axisymmetry of a system formed by a black hole surrounded by a self-gravitating torus in equilibrium are presented, aiming to assess the influence of the torus self-gravity on the onset of the runaway instability. We consider several models with varying torus-to-black hole mass ratio and angular momentum distribution orbiting in equilibrium around a non-rotating black hole. The tori are perturbed to induce the mass transfer towards the black hole. Our numerical simulations show that all models exhibit a persistent phase of axisymmetric oscillations around their equilibria for several dynamical timescales without the appearance of the runaway instability, indicating that the self-gravity of the torus does not play a critical role favoring the onset of the instability, at least during the first few dynamical timescales.

Numerical Relativity: Solving the issues in general relativity

Prof. Masaru Shibata
Yukawa Institute for Theoretical Physics, Kyoto University, Japan
The next decade will be an exciting epoch in the field of general relativity: Gravitational-wave detectors will directly detect gravitational waves for the first time, and subsequently clarify coalescing mechanism of binary compact objects such as binary neutron stars and formation mechanism of black holes. Simultaneous detection of gravitational waves and electromagentic waves may determine the central engine of gamma-ray bursts (GRBs). Mechanism of supernova may be constrained if gravitational waves from it are detected. Numerical relativity plays an important role for theoretically predicting gravitational waveforms and mechanism of general relativistic astrophysical phenomena such as GRBs. I will review the perspective of the field of gravitational waves, and also talk the current activity of numerical relativity..

ALMA: Observing the Cold Universe

Robert Lainga
Yukawa Institute for Theoretical Physics, Kyoto University, Japan
ALMA, the Atacama Large Millimetre/Sub-millimetre Array, is the largest current project in ground-based astronomy. It is an aperture synthesis array, optimized for high frequencies and currently under construction at a high, dry site in the Atacama desert of Northern Chile. When complete, it will provide unprecedented sensitivity, spatial resolution and imaging fidelity over the frequency range from 30 - 950 GHz. I will describe the key scientific goals of the project and outline its technical design. Interferometric observations with the first two ALMA antennas have recently been made, and construction is progressing rapidly. I will give an update on the current status and preparations for Early Science.

Understanding extragalactic jets from theory and observations

Manel Perucho Pla
Departament d`Astronomia i Astrofisica, Universitat de Valencia
I will review some aspects of my three years of research in the Max-Planck-Institut. These include modelling of extragalactic jets using numerical codes, stability theory and VLBI observations. In particular, I will report about a) results on evolution of components in the jet of the radio-galaxy 3C111 from the point of view of relativistic hydrodynamics, b) the observable effects of growing instabilities in jets as shown by detailed analysis of VLBI images of the jet in the quasar 0836+710, and c) I will present a new, fully parallelised, 3D relativistic hydrodynamics numerical code, along with the first results it has produced. These include the confirmation of the growth of resonant Kelvin-Helmholtz modes in sheared relativistic flows that can contribute to long term jet stabilization, as it was predicted from 2D simulations and KH stability theory.

Numerical simulations of radiation from blazar jets

Dr. Petar Mimica
Departament d`Astronomia i Astrofisica, Universitat de Valencia
Blazars are a class of AGN which exhibit a rapid variability at radio, infrared, visual and X-ray wavelengths. They are characterized by a non-thermal spectral continuum and a strong and rapidly varying polarization. Probable cause of the variability are collisions of the parts of the inner jet moving at different velocities, while the jet is oriented close to the line of sight. Results of relativistic hydrodynamic (RHD) and relativistictic magnetohydrodynamic (RMHD) simulations of collisions of dense shells within blazar jets are presented, as well as the synthetic light curves resulting from them. Basic characteristics of the dynamics and the efficiencies of two-shell collisions are shown, and differences between the RHD and the RMHD scenarios are discussed. Influence of the initial shell properties and the resulting light curves is studied, and an analytic model is outlined, which aids in interpreting the observed blazar flares.

Jets in AGN & More...

Dr. Eduardo Ros
Max-Planck-Institut fuer Radioastronomie, Bonn, Germany
We present some observational results on relativistic jets in Active Galactic Nuclei (QSOs, BL Lacs and Radio Galaxies). The Very Long Baseline Array is observing more than 170 sources at 15 GHz since 1994. More than 1000 images have been obtained to date. This database allows us to study in detail the kinematics in jets at parsec-scales. We present some results on particular sources, especially the findings in the nearby galaxy NGC1052. To close our talk, an overview on future radio astronomical projects such as the Square Kilometer Array is also being presented.

The influence of the plasma composition on the long-term evolution of relativistic jet

Dr. Miguel Angel Aloy Toras
Max-Planck-Institut für Astrophysik Garching Germany
We study the influence of the matter content of extragalactic jets on their morphology, dynamics and emission properties. For this purpose we consider jets of extremely different compositions including pure leptonic and baryonic plasmas. Our work is based on two-dimensional relativistic hydrodynamic simulations of the long-term evolution of powerful extragalactic jets propagating into a homogeneous environment. The equation of state used in the simulations accounts for an arbitrary mixture of electrons, protons and electron-positron pairs. Using the hydrodynamic models we have also computed synthetic radio maps and the thermal Bremsstrahlung X-ray emission from their cavities. Although there is a difference of about three orders of magnitude in the temperatures of the cavities inflated by the simulated jets, we find that both the morphology and the dynamic behaviour are almost independent on the assumed composition of the jets. Their evolution proceeds in two distinct epochs. During the first one multidimensional effects are unimportant and the jets propagate ballistically. The second epoch starts when the first larger vortices are produced near the jet head causing the beam cross section to increase and the jet to decelerate. The evolution of the cocoon and cavity is in agreement with a simple theoretical model. The beam velocities are relativistic ($\Gamma \simeq 4$) at kiloparsec scales supporting the idea that the X-ray emission of several extragalactic jets may be due to relativistically boosted CMB photons. The radio emission of all models is dominated by the contribution of the hot spots. All models exhibit a depression in the X-rays surface brightness of the cavity interior in agreement with recent observations.