Collisionless shock physics: from non-relativistic to relativistic shocks
Abstract: Astrophysical shock waves are ubiquitous in sources of high energy particles such as supernovae remnants, active galactic nuclei or gamma-ray bursts. In the first case, the shock waves are non-
relativistic, while in the latter, they can be ultra-relativistic, yet a common point is the emission of
non-thermal power-law spectra of high energy radiation, which is usually observed as synchrotron
or Inverse Compton photons emitted by the accelerated electrons. Such radiation require the
amplification of the magnetic field as turbulent fluctuations. One possibility is that the turbulent
fluctuations are self-generated by the relativistic particles themselves.
Theoretical studies as well as observational findings have brought to light the complex relationship
that exists between the accelerated (non-thermal) particles, the dynamics and the structure of the
shock wave, the surrounding magnetised turbulence and the efficiency of particle injection, actually
the very nature of the acceleration process. This result is quite remarkable, as it means that the
interpretation of current and forthcoming high resolution astrophysical data will teach us on the
microphysics of collisionless shock waves, whether non-relativistic or ultra-relativistic. This is also
highly challenging, because that relationship implies that (i) the process of acceleration is highly
non-linear and must be considered in its full generality; that, furthermore, (ii) its comprehension
must borrow knowledge from several disciplines (high energy astrophysics, particles physics and
space plasma physics); and (iii) its study consequently requires the development of new numerical
tools. This is the main objective of this working team, i.e., to assemble a trans-disciplinary community
of physicists from these disciplines in order to study the physics of high energy radiation from
shock waves, starting from the microphysics of the shock wave itself, hence discuss the
development of numerical tools dedicated to these studies. The project will be mostly dedicated to
the Fermi acceleration process at collisionless shocks. A special attention will be paid to the
evolution of the performance of the process with respect to the shock velocity and to the physical
properties of the environment (magnetization, magnetic field obliquity, degree of ionization).
The following tasks will be considered during this project: 1/ effects of the turbulence properties
over the Fermi acceleration process efficiency in relativistic shocks 2/ dominant plasma instabilities
in various shock velocity and upstream magnetisation regimes 3/ connection between
magnetospherical and astrophysical shock acceleration studies 4/ critical discussions on the
available multiwavelength observations and prospects for future experiments.
