Phase Speed Of Electrostatic Waves: The Critical Parameter For Efficient Electron Surfing Acceleration
2006 (English)In: IAU XXVIth General Assembly,2006, Prague: International Astronomical Union , 2006Conference paper (Other academic)
The emission of energetic radiation by astrophysical sources, e.g. the accretion disks of micro-quasars and active galactic nuclei, and gamma ray bursts, requires efficient particle acceleration mechanisms. Previously, electron surfing acceleration (ESA) has been introduced as a process that may accelerate electrons to unlimited energies. The ESA mechanism requires a quasi-electrostatic wave that can, for example, be driven by a two-stream instability. The wave traps electrons and transports them across an ambient magnetic field that is oriented perpendicularly to the wave vector. For sufficiently strong waves, the electrons cannot de-trap, which is opening up the prospect of unlimited electron acceleration. Particle-in-cell simulations have, however, revealed an important limitation of ESA. The saturated wave is not stable and its collapse releases the electrons. For non-relativistic phase speeds, the electron energy is limited to keV-regimes. We have found with the help of particle-in-cell simulations that relativistic phase speeds of the wave stabilize the system. We discuss how the electrons can be trapped for a longer time while the relativistic phase speed implies a rapid electron acceleration. We show that the peak electron speed accessible to the ESA mechanism increases into relativistic regimes. Quasi-electrostatic waves with a phase speed of 0.6c, which we may find in the upstream region of mildly relativistic shocks in micro-quasar accretion disks, can accelerate electrons to gamma factors of a few. Increasing the phase speed of the waves to values between 0.9c and 0.99c, which we may find at the most energetic astrophysical objects, e.g. active galactic nuclei and gamma ray bursts, yields peak electron gamma factors above 100. We show how a new limit emerges,in form of a peak electron gamma factor that equals the proton-to electron mass ratio. Such electron flow speeds are comparable to the fastest observed plasma flow speeds associated with gamma ray burst jets and the collimated jets of active galactic nuclei. ESA may thus contribute to their generation.
Place, publisher, year, edition, pages
Prague: International Astronomical Union , 2006.
National CategoryEngineering and Technology
IdentifiersURN: urn:nbn:se:liu:diva-35711Local ID: 28245OAI: oai:DiVA.org:liu-35711DiVA: diva2:256559