## Particle acceleration in the polar cap region of an oscillating neutron star

^{1} Laboratory of Applied Mathematics, University of Trento, via Mesiano 77, 38100 Trento, Italy

e-mail: olindo.zanotti@gmail.com
^{2} Max-Planck-Institut für Gravitationsphysik, Albert Einstein Institut, 14476 Golm, Germany
^{3} Institute of Nuclear Physics, Ulughbek, 100214 Tashkent, Uzbekistan
^{4} Ulugh Beg Astronomical Institute, Astronomicheskaya 33, 100052 Tashkent, Uzbekistan
^{5} ZARM, University Bremen, Am Fallturm, 28359 Bremen, Germany

Received: 2 November 2011

Accepted: 8 February 2012

*Aims.* We revisit particle acceleration in the polar cap region of a neutron star by taking into account both general relativistic effects and the presence of toroidal oscillations at the star surface. In particular, we address the question of whether toroidal oscillations at the stellar surface can affect the acceleration properties in the polar cap.

*Methods.* We solve numerically the relativistic electrodynamics equations in the stationary regime, focusing on the computation of the Lorentz factor of a space-charge-limited electron flow accelerated in the polar cap region of a rotating and oscillating pulsar. To this extent, we adopt the correct expression of the general relativistic Goldreich-Julian charge density in the presence of toroidal oscillations.

*Results.* Depending on the ratio of the actual charge density of the pulsar magnetosphere to the Goldreich-Julian charge density, we distinguish two different regimes of the Lorentz factor of the particle flow, namely an oscillatory regime produced for sub-GJ current density configurations, which does not produce an efficient acceleration, and a true accelerating regime for super-GJ current density configurations. We find that star oscillations may be responsible for a significant asymmetry in the pulse profile that depends on the orientation of the oscillations with respect to the pulsar magnetic field. In particular, significant enhancements of the Lorentz factor are produced by stellar oscillations in the super-GJ current density regime.

Key words: magnetic fields / relativistic processes / stars: neutron / pulsars: general

*© ESO, 2012*