This Thursday, 20:10 GMT, astronomy journal club will look at a possible explanation for the mysterious gamma-ray flares in the Crab Nebula – Extreme particle acceleration in magnetic reconnection layers. Application to the gamma-ray flares in the Crab Nebula (Cerutti et al. 2012). The meeting will be hosted by Chris Arridge, who also suggested the paper. He explains more about it below.
Recently AGILE and Fermi have found short (4 and 16 day) and bright gamma-ray flares from the Crab Nebula. The short duration of these flares suggests that they were emitted via synchrotron radiation from 10^15 eV electrons in a very small region of the nebula <0.014 pc across. These characteristics pose serious challenges for particle acceleration theory.
Magnetic reconnection is a fundamental process in plasmas for converting magnetic to kinetic energy and is observationally seen in the solar corona, the solar wind, planetary magnetospheres, and laboratory thermonuclear fusion devices. It is a process that is commonly studied in the context of solar system plasmas, from reconnection in the solar corona, to its importance in space weather at Earth, to dynamics in the magnetospheres of the outer planets. These studies use observations, both in situ and using remote sensing, simulations using MHD, Hybrid, test particle, and PIC simulations.
In this paper the authors use relativistic test particle simulations and a magnetic reconnection model to explain the characteristics of gamma-ray flares in the Crab Nebula. The mathematical developemnt has much in common with fundamental studies of magnetic reconnection in solar system plasmas but with important differences. They find that the emission is highly collimated and that the synchrotron spectrum peaks above 100 MeV. The mechanism is a plausible explanation for the flares in nebula and may be important at other astrophysical objects.
The paper is fairly weighty and mathematical in places, but the majority of the paper is quite accessible and has some nice clear explanations of the basic physical effects at work in the acceleration of the particles.