Speaker
Description
From observations, we see non-thermal radio emission with emissivity ranging from 1 to 3, extended over a large region (∼ megaparsec) in galaxy clusters, which contain largely collisionless plasma. To explain this, the electrons should be energized or accelerated. One explanation is that the energy released during a merger event could be channeled to accelerate electrons through turbulence or shock. However, the efficiency of these mechanisms are very low.
We, therefore, will look for another particle acceleration mechanism: magnetic reconnection in collisionless plasma. Reconnection sites form naturally due to fluctuation dynamo operating in the intra-cluster medium (ICM), producing strong magnetic fields that reverse on short length scales. The plasma in the ICM being non-relativistic (temperature ∼ 10^8 K ), we aim to explore particle acceleration due to reconnection using PIC simulation for non-relativistic, electron-positron plasma. We use the publicly available PIC code WarpX for that.
Driven by the tearing instability, magnetic reconnection begins by giving rise to magnetic islands and x-points, where the inductive electric field gets generated and accelerate the particles. Thus, magnetic energy gets converted to particles’ kinetic energy. As reconnection proceeds, secondary current sheets appear and they give rise to plasmoids, which grow in size, coalesce to form one big island and eventually shuts down the reconnection. In the due process, the particles, which were thermal initially, end up having a non-thermal distribution with a power law index ~-4, which explains the observed radio emission in galaxy clusters.
