Untitled - Laboratoire d'Astrophysique de l'Observatoire de Grenoble

Chapter 16
Perspectives
The distinctive feature of our group lies in its focus on physical processes rather than on specific astrophysical
objects. This emphasis has proven to be very fruitful: it allows to make progress on problems that appear in very
different astrophysical contexts (e.g. turbulence in accretion disks, accretion-ejection, non-thermal radiation
from jets) as it relies on the strong theoretical background required to correctly interpret and understand
observations. This last goal is mainly achieved through collaborations with the FOST team and international
observational groups. We want therefore to maintain this specificity within the LAOG.
However, our common interest in specific processes, namely MHD flows (laminar and turbulent), anomalous
transport, kinetic particle acceleration and high energy radiative processes, naturally pinpoints to one astrophysical
context best suited for the development and testing of our theories. Our future activity will thus be
mainly devoted to the understanding of the physics of Black Hole environments.
The study of MHD accretion-ejection flows around a central object will follow two distinct directions. The
first one is the analysis of the timing properties of compact objects : quasars and microquasars. As said
previously, X-ray binaries provide the best available constraints on the time evolution of accretion-ejection
structures. This aspect involves first a strong collaboration between several members of our group. Indeed,
within our ”two-flow” paradigm, flares are produced by a catastrophic pair creation that must have a feedback
on the surrounding MHD jet. One long term goal is therefore to couple pair plasma creation and dynamics
with MHD. The understanding of these phenomena will then be extended to AGNs which exhibit also strong
variability. The hiring of a young scientist involved in this difficult topic would be very valuable to reinforce
this theoretical activity.
The second direction of study is the investigation of the magnetic interaction between a magnetized central
object and its accretion disk, with a focus on angular momentum transfer. This topic is naturally building
strong links with the FOST team. The best observationally studied objects are the young stars but our work
will also apply to cataclysmic variables and some neutron stars. Numerical simulations are most likely the only
efficient tool to attack this critical problem. This work has already begun with one PhD thesis (N. Bessolaz)
and a JETSET post-doc (C. Zanni) and will definitely be continued by using the most recent available codes.
The recent recruitment of P. Varnière within our group and her strong implication in the development of the
MHD version of the AstroBear code is an invaluable input. Although a simultaneous treatment of both the
local and global physics is still out of reach of the current large scale computer resources, we nevertheless hope
to incorporate future results from MHD local analyzes as sub-grid constraints. On the longer term, we will
simulate the environment of a rotating black hole, surrounded by an accretion-ejection structure.
The issue of turbulent transport in accretion disks is of major importance in astrophysics, with several open
issues and controversies. One of them, the role of hydrodynamic nonlinear instabilities, has recently been resolved
by our group, implying that purely hydrodynamic turbulent transport is most likely too inefficient. The
source of transport therefore lies either in (magneto)hydrodynamic, 2D, wavelike structures, or in a local MHD
instability driving turbulence, such as the magneto-rotational instability. In the latter case, the main difficulty
and unsolved problem is the role of magnetic reconnection on the overall efficiency of the induced turbulent
transport. No study has yet tackled this crucial issue, although numerical reconnection most probably largely
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