Non-Axisymmetric Precession of Magnetars and Fast Radio Bursts. (arXiv:2107.12911v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Wasserman_I/0/1/0/all/0/1">Ira Wasserman</a> (1 and 2), <a href="http://arxiv.org/find/astro-ph/1/au:+Cordes_J/0/1/0/all/0/1">James M. Cordes</a> (1), <a href="http://arxiv.org/find/astro-ph/1/au:+Chatterjee_S/0/1/0/all/0/1">Shami Chatterjee</a> (1), <a href="http://arxiv.org/find/astro-ph/1/au:+Batra_G/0/1/0/all/0/1">Gauri Batra</a> (2) ((1) Cornell Center for Astrophysics and Planetary Sciences, Cornell University, (2) Laboratory for Elementary Particle Physics, Cornell University)
The repeating FRBs 180916.J0158 and 121102 are visible during
periodically-occuring windows in time. We consider the constraints on internal
magnetic fields and geometry if the cyclical behavior observed for
FRB~180916.J0158 and FRB 121102 is due to precession of magnetars. In order to
frustrate vortex line pinning we argue that internal magnetic fields must be
stronger than about $10^{16}$ Gauss, which is large enough to prevent
superconductivity in the core and destroy the crustal lattice structure. We
conjecture that the magnetic field inside precessing magnetars has three
components, (1) a dipole component with characteristic strength $sim 10^{14}$
Gauss; (2) a toroidal component with characteristic strength $sim
10^{15}-10^{16}$ Gauss which only occupies a modest fraction of the stellar
volume; and (3) a disordered field with characteristic strength $sim 10^{16}$
Gauss. The disordered field is primarily responsible for permitting precession,
which stops once this field component decays away, which we conjecture happens
after $sim 1000$ years. Conceivably, as the disordered component damps
bursting activity diminishes and eventually ceases. We model the quadrupolar
magnetic distortion of the star, which is due to its ordered components
primarily, as triaxial and very likely prolate. We address the question of
whether or not the spin frequency ought to be detectable for precessing,
bursting magnetars by constructing a specific model in which bursts happen
randomly in time with random directions distributed in or between cones
relative to a single symmetry axis. Within the context of these specific
models, we find that there are precession geometries for which detecting the
spin frequency is very unlikely.
The repeating FRBs 180916.J0158 and 121102 are visible during
periodically-occuring windows in time. We consider the constraints on internal
magnetic fields and geometry if the cyclical behavior observed for
FRB~180916.J0158 and FRB 121102 is due to precession of magnetars. In order to
frustrate vortex line pinning we argue that internal magnetic fields must be
stronger than about $10^{16}$ Gauss, which is large enough to prevent
superconductivity in the core and destroy the crustal lattice structure. We
conjecture that the magnetic field inside precessing magnetars has three
components, (1) a dipole component with characteristic strength $sim 10^{14}$
Gauss; (2) a toroidal component with characteristic strength $sim
10^{15}-10^{16}$ Gauss which only occupies a modest fraction of the stellar
volume; and (3) a disordered field with characteristic strength $sim 10^{16}$
Gauss. The disordered field is primarily responsible for permitting precession,
which stops once this field component decays away, which we conjecture happens
after $sim 1000$ years. Conceivably, as the disordered component damps
bursting activity diminishes and eventually ceases. We model the quadrupolar
magnetic distortion of the star, which is due to its ordered components
primarily, as triaxial and very likely prolate. We address the question of
whether or not the spin frequency ought to be detectable for precessing,
bursting magnetars by constructing a specific model in which bursts happen
randomly in time with random directions distributed in or between cones
relative to a single symmetry axis. Within the context of these specific
models, we find that there are precession geometries for which detecting the
spin frequency is very unlikely.
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