EURO_GRANOT · Exploring Ultra-Relativistic Outflows

Exploring Ultra-Relativistic Outflows

Project context and objectives This project studied relevant physics and observational implications of astrophysical ultra-relativistic outflows (moving at extremely close to the speed of light). Such outflows appear in some of the most violent and extreme objects in nature, from galactic micro-quasars to cosmological gamma-ray bursts (GRBs) and active galactic nuclei (AGN), which can probe strong field gravity, large densities and magnetic fields. Such astrophysical sources are thought to accelerate the highest energy cosmic rays, and serve as important sources of high-energy neutrinos and gravitational waves for upcoming detectors. Thus, a good understanding of their physics can address many important and fundamental questions. The new instruments built in order to study these sources help drive the development of new technology, with large potential economic impact. This primarily theoretical project studied different aspects of these outflows, which may help shed light on their underlying physics, and was divided into three parts. 1. Dynamics of impulsive relativistic outflows: We studied the dynamics of relativistic outflows that are impulsive, where short timescale large amplitude variations in the flow play a major role in their physics and observed properties, which start out highly magnetised near the central source. We discovered a new impulsive magnetic acceleration mechanism. Its properties were derived analytically and verified numerically. It can efficiently convert most of the initial magnetic energy into kinetic energy, thus potentially solving a long-standing important open question in this field (the sigma problem), making it is very relevant for many astrophysical sources. We studied the interaction of a single ejected shell of plasma with the external medium, and found new dynamical regimes relevant for GRBs and for the tidal disruption of a star by a super-massive black hole. Finally, we explored outflows that are initially divided into many well-separated sub-shells. Each sub-shell can first become kinetically dominated, leading to efficient dissipation in internal shocks that form as different sub-shells collide (very relevant for AGN and GRBs). In GRBs, the deceleration by the external medium can be important, and proceed through a low-magnetisation reverse shock. The initial division into sub-shells allows this shock to be relativistic and its emission to peak on the timescale of the GRB duration (which is not possible for a single shell), and enables the outflow to reach much higher Lorentz factors that help satisfy existing observational constraints on GRBs. 2. Time dependent opacity effects in impulsive relativistic sources: We developed a detailed, fully time-dependent self-consistent semi-analytic model that calculates the observed light curves and spectra taking into account intrinsic electron-positron pair production within an impulsive ultra-relativistic source. It has distinctive predictions that could be tested against observations. Later, we started a detailed follow-up work using a numerical code to study even more realistic configurations relevant to GRBs or AGN (it should be completed within a few months). 3. Stability properties of relativistic shocks: The stability of ultra-relativistic radiative magnetised shocks was studied. They were analytically found to be stable against global oscillations of the shock front, making the originally planned numerical part redundant. Additional work was carried out on topics related to astrophysical relativistic outflow sources, including numerical simulations and analytic models of GRB jet dynamics during the afterglow phase, numerical simulations of coalescing neutron stars, use of Fermi high-energy observations to study GRB physics, test Lorentz invariance and study the extragalactic background light, work on magnetars (highly magnetised neutron stars - their magnetic field decay and soft gamma repeater phenomenology), radio supernovae, and in other topics on GRBs.

Текст от CORDIS, на английски · Данни: CORDIS, © Европейски съюз