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Session 26 - X-ray Emission Flow, Neutron Stars & White Dwarfs.
Oral session, Monday, January 15
Salon del Rey South, Hilton

[26.05] Vibrating neutron stars, quakes, and gamma-ray bursts

S. E. Woosley (UCSC)

In order to explore the observational consequences a sudden adjustments in neutron star structure, a number of numerical simulations of sound wave propagation in the neutron star crust have been carried out using a one dimensional implicit hydrodynamics code with appropriate, though simplified physics. A neutron star is constructed in hydrostatic equilibrium and its natural oscillation frequency determined empirically, 0.34 ms. All but the outer 0.01 solar masses of the star is then removed and replaced by a sinusoidally oscillating inner boundary of arbitrary amplitude. For amplitudes less than about 1 meter, very little dissipation occurs. For larger amplitudes, sound waves steepen into shocks near the surface. The largest dissipation occurs during the first oscillation, during the fall back from the expanded state. For an amplitude of 10 meters, the vibrational energy is about 10^47 ergs (\sim 10^53 (\Delta R/R)^2), about 1% of which (2 \pi R^2 \rho v_s (\Delta R)^2 \tau^-1 with \rho \sim 10^13 and v_s \sim 10^9) propagates as a sound wave and gets dissipated by shocks in the envelope. Vastly super-Eddington luminosities are possible, up to 10^45 erg s^-1 because cooling from the magnetically confined surface creates a superadiabatic temperature gradient which drives convection. Temperatures of roughly 5 \times 10^9 K are maintained in the envelope so long as vibrations continue and roughly 10^45 erg resides in the outer 10^26 gm which can be carried out by convection over a longer period. These conditions may match well the March 5 event. Quakes (single adjustments of the inner boundary occuring on a free fall time scale) having smaller amplitudes but faster time scales (e.g., 25 cm, 10^-6 s) can explain soft repeaters. However a larger version - a simulated phase transition with an amplitude of 1 km, does not yield significant relativistic mass ejection (\sim 5 \times 10^-9 solar masses, mostly \Gamma < 2) and is not a good cosmological model for gamma-ray bursts.

Program listing for Monday