Speaker
Mr
Simone Giacche`
(Max-Planck Institute for Nuclear Physics)
Description
The Pulsar Wind Nebulae (PWNe) PSR B1259-63 has been
observed to emit periodic GeV flares, whose power can be
comparable to the total pulsar spin-down luminosity.
Because of the short timescale involved,
these photons are likely to be
produced via inverse Compton scattering of stellar photons or
Synchrotron radiation by a population of very energetic electrons
(from GeV to TeV energies) in proximity of the wind termination shock (TS).
This perpendicular shock is created by
the interaction between the magnetised,
relativistic, electron-positron wind launched by the
pulsar with the companion star outflow.
When the rotational frequency of the pulsar is greater than the local
plasma frequency in the wind, a shock precursor forms ahead of the TS,
where the Poynting flux is dissipated. This condition is satisfied
at the TS in a gamma-ray binary when the system is far from
periastron, but not necessarily when the stars are in
close proximity to each other (Mochol & Kirk 2013).
It is stll unclear whether and how this structure can
accelerate electrons to high energies.
We investigate this in a two-step procedure.
Firstly, a 1-dimensional, relativistic,
2-fluid code is used to reproduce the turbulent fields
in the equatorial plane at the location of the TS.
We numerically integrate test particle trajectories in
the background fields of a steady configuration
of the precursor realised for an upstream Lorentz factor
$\Gamma=40$ and a magnetisation parameter $\sigma=10$.
We follow each particle until it
either escapes downstream after transmission or upstream
after reflection. We find that $\sim 50\%$ of the incoming particles
are reflected upstream by the turbulent fields for these parameters. Secondly, we simulate Fermi-like
acceleration by supplementing magnetic fluctuations with
prescribed statistical properties both in the pulsar wind upstream of the shock,
and in the nebula downstream of the shock,
where the field is assumed to have been dissipated.
The resulting stochastic trajectories are numerically integrated (Achterberg & Kruells 1992).
We compare the power-law index and the angular distribution
of accelerated particles with the same quantities obtained
with a numerical simulation where the average magnetic field is null
on both sides of the shock and the only source of deflection
for energetic particles is the scattering off magnetic irregularities
(Achterberg et al. 2001).
We argue that the proposed scenario is relevant for PWNe in $\gamma$-ray
binaries such as PSR B1259-63.
Primary author
Mr
Simone Giacche`
(Max-Planck Institute for Nuclear Physics)
Co-authors
Prof.
John Kirk
(Max-Planck Institute for Nucear Physics)
Dr
Takanobu Amano
(Department of Earth and Planetary Science, University of Tokyo)
