Global Seismic Oscillations in Soft Gamma Repeaters

There is evidence that soft gamma repeaters (SGRs) are neutron stars whichexperience frequent starquakes, possibly driven by an evolving, ultra-strongmagnetic field. The empirical power-law distribution of SGR burst energies,analogous to the Gutenberg-Richter law for earthquakes, exhibits a turn-over athigh energies consistent with a global limit on the crust fracture size. Withsuch large starquakes occurring, the significant excitation of global seismicoscillations (GSOs) seems likely. Moreover, GSOs may be self-exciting in astellar crust that is strained by many, randomly-oriented stresses. We explainwhy low-order toroidal modes, which preserve the shape of the star and haveobservable frequencies as low as ~ 30 Hz, may be especially susceptible toexcitation. We estimate the eigenfrequencies as a function of stellar mass andradius, and their magnetic and rotational shiftings/splittings. We alsodescribes ways in which these modes might be detected and damped. There ismarginal evidence for 23 ms oscillations in the hard initial pulse of the 1979March 5th event. This could be due to the $_3t_0$ mode in a neutron star with B~ 10^{14} G or less; or it could be the fundamental toroidal mode if the fieldin the deep crust of SGR 0526-66 is ~ 4 X 10^{15} G, in agreement with otherevidence. If confirmed, GSOs would give corroborating evidence forcrust-fracturing magnetic fields in SGRs: B >~ 10^{14} G.Comment: 12 pages, AASTeX, no figures. Accepted for Astrophysical Journal Letter