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Gravitational waves and electromagnetic plasmas (F. Pegoraro - Universit'a di Pisa, Italy)

The recent announcement of the detection of a gravitational wave event, and the debate that followed on whether or not it was, or should have been, accompanied by an electromagnetic signal, is likely to bring renewed interest to the field of the interaction of an electromagnetic plasma with a time varying gravitational field. Such an interaction can take place in the superstrong field of a system of compact gravitational objects surrounded by a relativistic plasma: for this case there is ample recent literature, mostly within a general relativistic magnetohydrodynamic theory. It can also take place in the field of a gravitational wave, for which there is a string of papers dating back at least from the '70s. These papers are mainly aimed at studying the conversion of gravitational waves into electromagnetic or longitudinal plasma waves from the point of view of the gravitational waves damping and/or detection, or even possibly of their generation under laboratory conditions. On the contrary, in the astrophysical community interest has been focussed mainly on discrete processes, such as photongraviton conversion and viceversa, more than on collective processes in plasmas. There is an enormous imbalance in strength between electromagnetic and gravitational interactions. Thus it would appear natural to think that, if the huge objects that are supposed to be involved in the collapse that generated the gravitational waves are in contact with, or through their ultrastrong fields can interact with, an electromagnetic medium a fraction of the energy released must take the form of electromagnetic energy. A possible conversion mechanism can be identified by observing that, while the gravitational fields per se tend to make all matter move (free fall) along the geodesics of the deformed space geometry independently of matter being charged or not, they can induce charge and current separation in the presence of electromagnetic fields. The focus within the present meeting will be on the possible effects of oscillatory gravitational fields on a Vlasov plasma. In my talk I will present a somewhat schematic review of the literature on this subject (mostly linear theory in the search for resonances plus parametric conversion) and will try to discuss possible regimes of physical interest and identify effects where the gravitationalelectromagnetic coupling in a plasma may play an observationally significant role. A goal of this rather exploratory presentation is to see whether the tools that have been recently developed in the context of purely electromagnetic plasmas can be put to use in order to study kinetic plasma phenomena induced by fast oscillating, strong gravitational fields such as those that can be expected to occur not too far from the source of the recent gravitational wave event.

Salle 415 - LULI/Jussieu 23-33 4ème étage - 14h