Global Seismic Oscillations in Soft Gamma Repeaters

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Abstract
There is evidence that soft gamma repeaters (SGRs) are neutron stars that experience frequent starquakes, possibly driven by an evolving, ultrastrong magnetic field. The empirical power-law distribution of SGR burst energies, analogous to the Gutenberg-Richter law for earthquakes, exhibits a turnover at high energies, consistent with a global limit on the crust fracture size. With such large starquakes occurring, the significant excitation of global seismic oscillations (GSOs) seems likely. Moreover, GSOs may be self-exciting in a stellar crust that is strained by many randomly oriented stresses. We explain why low-order toroidal modes, which preserve the shape of the star and have observable frequencies as low as ~30 Hz, may be especially susceptible to excitation. We estimate the eigenfrequencies as a function of stellar mass and radius, as well as their magnetic and rotational shiftings/splittings. We also describe ways in which these modes might be detected and damped. There is marginal evidence for 23 ms oscillations in the hard initial pulse of the 1979 March 5 event. This could be due to the 3t0 mode in a neutron star with B ~1014 G or less, or it could be the fundamental toroidal mode if the field in the deep crust of SGR 0526-66 is ~4 × 1015 G, in agreement with other evidence. If confirmed, GSOs would give corroborating evidence for crust-fracturing magnetic fields in SGRs: B 1014 G.
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