Powerful astrophysical objects such as gamma-ray bursts, blazars, and supernovae
provide ideal environments for efficient particle acceleration. These bursts of highenergy particles result from an efficient conversion of kinetic or Poynting flux innonthermal distributions through collective plasma processes. Astrophysical shockwaves are one of the most outstanding and extensively studied representatives of suchcomplex many-body phenomena. They cover a large variety of physical conditions inmagnetization, velocity, and composition, and are intrinsically nonlinear across very
disparate scales. The study of these systems is a key challenge to unveiling the physics
underlying observed multimessenger spectra. This nonlinear problem is tackled usinglarge-scale kinetic simulations in combination with linear and nonlinear analyticalmodels. In this context, my research focuses on exploring the physics of shock waves inthree different regimes.
The afterglow emission of gamma-ray burst is interpreted as a synchrotron selfCompton (SSC) emission from electrons accelerated on a forward relativistic blast wave.
The shock propagates at the interface between the interstellar medium and therelativistic ejecta. Their dynamics are led by the formation of kinetic scale magnetic
structures, coherent over hundreds of kilometers, which compress and heat theambient plasma in the shock upstream. These structures then cross the shock front andscatter nonthermal particles to sustain a Fermi-type acceleration and SSC emission.
Some current issues in this regard are the nonlinear evolution of these electromagnetic
instabilities in the shock upstream, their decay rate in the shock downstream, and theeffect of pair creation on the shock dynamics.
Subrelativistic shocks of high alfvénic Mach numbers are particularly relevant tosupernovae remnants. While their overall dynamics bear similarities to theaforementioned relativistic shocks, the nature of the instabilities may differ. One of theoutstanding questions emerges from the difference in inertia between electrons andions of the interstellar medium. While the mean kinetic energy pool is dominated by theions in the far upstream, a substantial fraction is transferred to the electrons as they
cross the shock. Modeling this energy transfer is a key challenge to understand theobserved downstream temperature ratio. Another problem of interest is of course themechanism at the origin of electron injection of particles from the thermal bulk to thenonthermal distribution of accelerated particles in these shock waves.
The first light detected from almost explosive stellar objects is shaped by its propagationin an optically thick environment. The shock wave propagating in such an opaqueplasma is said to be radiation mediated and the transition from the optically thick to theinterstellar medium is called the shock breakout. In other words, while compression andheating of the background plasma result from coherent electromagnetic structures inthe above-stated shocks, radiation-mediated shocks result from the compression by aphoton beam through Compton scattering and pair production. A priori subdominant
and often neglected, plasma instabilities are present and may impact the shock
dynamics. An important problem is thus to model the feedback of plasma instabilities on
the spectra at shock breakout.
Modeling these phenomena requires understanding the micro-physics of collisionlessshocks, where particles are accelerated through their interaction with turbulent
electromagnetic fields that are themselves generated by micro-instabilities. We study
these kinetic processes using large-scale Particle-In-Cell (PIC)simulations. The PIC
method solves self-consistently the evolution of the particle distribution coupled to
Maxwell's equations and consists of advancing a collection of particles through a grid on
which the electromagnetic fields are discretized. In our studies, we use the state-of-theart PIC massively parallelized code Tristan-MP [1] and Osiris [2] to include QED andradiation effects. We also develop reduced Monte Carlo-Poisson solvers to study thedynamics of multispecies plasma with microturbulence.
[1] https://ntoles.github.io/tristan-wiki[2] http://epp.tecnico.ulisboa.pt/osiris/