Global Seismic Oscillations in Soft Gamma Repeaters

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.