The magnetosphere of oscillating neutron stars in general relativity |
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| Authors: | Ernazar B Abdikamalov Bobomurat J Ahmedov John C Miller |
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| Institution: | SISSA, International School for Advanced Studies, and INFN–Trieste, Via Beirut 2-4, 34014 Trieste, Italy;Institute of Nuclear Physics, Ulughbek, Tashkent 100214, Uzbekistan;Ulugh Beg Astronomical Institute, Astronomicheskaya 33, Tashkent 100052, Uzbekistan;Department of Physics (Astrophysics), University of Oxford, Keble Road, Oxford OX1 3RH |
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| Abstract: | Just as a rotating magnetized neutron star has material pulled away from its surface to populate a magnetosphere, a similar process can occur as a result of neutron-star pulsations rather than rotation. This is of interest in connection with the overall study of neutron star oscillation modes but with a particular focus on the situation for magnetars. Following a previous Newtonian analysis of the production of a force-free magnetosphere in this way Timokhin et al., we present here a corresponding general-relativistic analysis. We give a derivation of the general relativistic Maxwell equations for small-amplitude arbitrary oscillations of a non-rotating neutron star with a generic magnetic field and show that these can be solved analytically under the assumption of low current density in the magnetosphere. We apply our formalism to toroidal oscillations of a neutron star with a dipole magnetic field and find that the low current density approximation is valid for at least half of the oscillation modes, similarly to the Newtonian case. Using an improved formula for the determination of the last closed field line, we calculate the energy losses resulting from toroidal stellar oscillations for all of the modes for which the size of the polar cap is small. We find that general relativistic effects lead to shrinking of the size of the polar cap and an increase in the energy density of the outflowing plasma. These effects act in opposite directions but the net result is that the energy loss from the neutron star is significantly smaller than suggested by the Newtonian treatment. |
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| Keywords: | stars: magnetic field stars: neutron stars: oscillations pulsars: general |
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