Description du sujet de thèse

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. 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), in the framework the ANR funded HENBoS (High Energy Neutrinos from Black hOles Systems) project. APC provides an ideal environment for this type of studies, as it brings together experts in experimental, theoretical and numerical multimessenger astrophysics.

Contexte de travail

The PhD work will take place in the framework of the ANR funded HENBoS project (2025 – 2029), which seeks to describe with unprecedented accuracy and scope massive black hole systems as cosmic accelerators. Our vision and our understanding of supermassive black hole systems have indeed known dramatic progress over the last decade. In particular, they have shown that these objects behave as accelerators of very high energy particles (VHE) and that they are likely sources of (VHE) neutrinos. Understanding where and how particles are accelerated in these extreme environments has thus become a key question, with far-reaching consequences for multi-messenger astrophysics and black hole astronomy. The HENBoS project is part of this rapidly developing context. The strategy is based on high-performance numerical simulations using a GRMHD-PIC technique, which allows to track VHE ions on the fly. These simulations will be supplemented by cutting-edge analytical methods for injecting particles onto the grid. Their results will be convolved with advanced radiative codes to calculate fluxes and multi-messenger spectra. These simulations will also be supplemented by kinetic simulations of particle acceleration in turbulent plasmas and by phenomenological models. The overarching goal of this ambitious programme is to describe the acceleration process across length scales, in the turbulent flow, in the inner jet and in the corona, for different accretion regimes and types of sources, to carry out specific studies for certain sources, and establish detection forecasts for future multi-messenger detectors.

Contraintes et risques

No particular risks.
Travel within France and abroad is to be expected.