Populating pulsar magnetospheres via QED cascades

Pulsars are astronomical objects that gather a wealth of extreme physical conditions, making them extraordinary physics laboratories (for fields as diverse as general relativity, quantum mechanics, and plasma astrophysics). They are surrounded by strong dipolar magnetic fields (of the order of 1012 G) and support very active, exotic magnetospheres.

The ultra-intense magnetic field existent in these regions is close to the Schwinger field for vacuum breakdown, and hence the dynamics of charged particles must take into account the self-consistent interaction with these fields, as well as radiation reaction and QED mechanisms such as hard photon emission and their subsequent decay that results in electron-positron pairs. Unravelling the dynamics of astrophysical plasmas in these environments is a critical step towards a complete understanding of a multitude of phenomenology, including the radiation signatures and the spin down of pulsars, the formation and the quasi-steady state of their magnetospheres, and the acceleration of the most energetic particles in the Universe.

This movie shows a PIC simulation of the formation process of an electron-positron cascade in an ultra intense (~1012 G), curved magnetic field, taking into account hard photon emission and pairs production from first principles. For each initial electron (red spheres), initialized in adjacent magnetic field lines, we observe the formation of ~103 pairs and ~105 photons. The QED cascade develops as follows: curvature photons are emitted from the initial electrons, and propagate in a straight line until their momentum vector makes an angle with the curved magnetic field that is sufficiently large for the emission of an electron-positron pair. This pair is then accelerated by an electric field anti-parallel to the magnetic field shown in the movie, producing more curvature photons and closing the QED cascade loop. Electrons and positrons follow the magnetic field lines due to the extreme intensity of the field. The fact that photons travel in straight lines in these fields, however, leads to a development of the cascade in the direction transverse to the background magnetic field.

T. Grismayer, M. Vranic, R.A. Fonseca, L. O. Silva, Electromagnetic QED cascades From the laboratory to astrophysics, oral communication @ 11th International Conference on High Energy Density Laboratory Astrophysics, SLAC National Accelerator Laboratory, CA, USA