Highly magnetized environments around compact astrophysical sources (black holes, neutron stars) and their relativistic outflows provide exquisite conditions for accelerating charged particles to very high energies (TeV to PeV and beyond; VHE) and producing multimessenger signals (e.g. photons and neutrinos). Indeed, the pervasive turbulence can ensure efficient stochastic particle acceleration, while the ambient backgrounds provide ideal targets for radiative and hadronic interactions. In recent years, progress in this field has benefited from the development of high-performance computing, in particular magnetohydrodynamic (MHD) simulations to model the large-scale flow and particle-in-cell (PIC) simulations to study particle kinetics on microscopic plasma scales. In addition, recent detections of electromagnetic flares in various wavebands, and especially the recent association of VHE neutrinos with active massive black hole systems, have both established these systems as VHE emitters and provided unprecedented observational probes of their physics. Accordingly, understanding how particles are accelerated in such conditions and through which channel neutrinos and photons are produced have become hot topics in multimessenger astrophysics.

The goal of this PhD is to contribute to the theoretical efforts in this field, to advance our understanding of the acceleration processes from first principles, and to model their phenomenological signatures in relativistic sources. The PhD work will therefore include analytical developments as well as high performance numerical computing. The PhD student will use (and perform) PIC simulations to study the physics of particle acceleration in highly magnetized (relativistic) turbulence under conditions similar to those of the above sources. Particular attention will be paid to the nonlinear back-reaction of accelerated particles on their turbulent environment, since VHE particles add viscosity and resistivity to the flow, and thus damp the turbulence that energizes them. This process is gaining some attention because it can determine the final shape of the energy distribution of the accelerated particles. These simulations will then be used to improve existing theoretical models of particle acceleration, in order to extrapolate the results of the simulations to astrophysical scales of interest, and to calculate multimessenger yields. The PhD student will thus also work on phenomenological applications of these studies to multimessenger astrophysics.

The thesis will be carried out under the supervision of Martin Lemoine at Astroparticule & Cosmologie (APC, CNRS – Université Paris-Cité), which provides an ideal environment for this type of studies, bringing together experts in experimental, theoretical and numerical multimessenger astrophysics.