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A. Bayliss, E. L. Sandquist, R. E. Taam (Northwestern U.)
The behavior of nuclear burning in the accreted layer of a neutron star is investigated for helium and hydrogen-helium mixtures. Attention is focused on the propagation of a thermal wave due to electron conduction or radiative diffusion in the lateral direction. The fully time-dependent calculations reveal that a steady state flame front is not necessarily applicable at high mass accretion rates (dM/dt > (dM/dt)Edd). In particular, there are parameter regimes in which a steady state structure is never attained within physically relevant timescales because the gas ahead of a front self-ignites. Hence, a thermonuclear flash may take place on a timescale unrelated to the timescale for a steady state front to propagate over a homogeneous region. The existence of irregular burst activity in highly luminous neutron star X-ray binary systems may provide some observational support for this theoretical picture.