SFB1491 - CIM

Project Description

Magnetic reconnection is a small-scale process in terms of astrophysical spatial and temporal scales, but it is expected to power some of the most energetic processes in the Universe. Magnetic reconnection converts magnetic energy into heating and non-thermal particle acceleration in most astrophysical plasmas. High-energy reconnection events are associated with astrophysical jets, from neutron star- beamed X-ray emission to the extended beam along the axis of rotation in active galactic nuclei (AGN). Powered by super-massive black holes, AGN jets pointing to Earth – called blazars – are the most luminous objects in the known Universe. Particles accelerated or pre-accelerated in magnetic reconnection are expected to produce the characteristic double-humped blazar jet energy spectrum; the fast reconnection scenario associated with tearing/plasmoid instability is compatible with the time scales of the flaring activity observed in these objects. In the last two decades, the understanding of magnetic reconnection in AGN jets has progressed at an astounding pace. Significant progress has been achieved theoretically, with a better understanding of fast reconnection (plasmoid instability) in relativistic regimes, and also computationally. Relativistic MHD simulations have shown plausible scenarios for the formation of reconnecting sites in AGN jets, while relativistic particle-in-cell (PIC) simulations have uncovered scaling laws that relate the reconnection regime (chiefly, magnetization levels) to the spectra of non-thermal particles, at least in certain geometries (e.g. 2D reconnection, starting from a relativistic Harris sheet or force-free configurations, with large simulation boxes).

 

These two lines of investigation have mostly proceeded in parallel, with limited feedback between studies at the "large, MHD" and "small, kinetic" scales. Still, understanding the coupling between system-scale and kinetic-scale processes is fundamental to understand up to which point magnetic reconnection can accelerate particles in astrophysical jets, and, if further energization is needed to explain observations, which secondary processes are the most likely candidates for it. PIC simulations can self-consistently describe the dynamics of particle acceleration, but fluid simulations are needed to understand how many reconnection sites can develop in a kinking, magnetized plasma column (the jet) and if they suffice to bring a significant fraction of the particles to the energies required to power the emissions, without involving further energization processes. In fact, information on reconnection in the idealized current sheets commonly simulated may fail to account for factors (e.g. development of instabilities, turbulence), that may either hinder or boost particle acceleration. For these reasons, a numerical approach that combines large-scale dynamics with a self-consistent description of particle acceleration processes is needed. The unique combination of skills within the RUB-TP1 chair and the close collaborations with the other projects in the proposal put us in the ideal position to develop this new approach, and quickly exploit the new scientific results we will obtain in the context of multimessenger astrophysics. The improved understanding of AGN reconnection that we will obtain from our innovative modeling approach will allow us to deliver accurate information (spectral index and cutoff energies for an eletron-positron and an electron-proton plasma) on particle energization to a number of other projects in the prosed CRC, where it will be used as initial conditions for cosmic-ray propagation models. This way, we can test the different signatures in the electromagnetic spectrum that arise from purely leptonic scenarios (electron-positron jet) and from (lepto)hadronic scenarios (electron-proton jet).