2006ApJ...652..597P

Matter accreting onto the magnetic poles of a neutron star spreads under gravity toward the magnetic equator, burying the polar magnetic field and compressing it into a narrow equatorial belt. Steady state, Grad-Shafranov calculations with a self-consistent mass-flux distribution (and a semiquantitative treatment of Ohmic diffusion) show that, for M_a≳10^–5 M_☉, the maximum field strength and latitudinal half-width of the equatorial magnetic belt are B_max=5.6x10¹⁵(M_a/10^–4 M_☉)^0.32 G and Δθ=max[3°(M_a/10^–4M_☉)^–1.5,3°(M_a/10^–4 M_☉)^0.5(M{dot}_a/10^–8 M_☉yr^–1)^–0.5], respectively, where M_ais the total accreted mass and M{dot}_ais the accretion rate. It is shown that the belt prevents north-south heat transport by conduction, convection, radiation, and ageostrophic shear. This may explain why millisecond oscillations observed in the tails of thermonuclear (type I) X-ray bursts in low-mass X-ray binaries are highly sinusoidal; the thermonuclear flame is sequestered in the magnetic hemisphere that ignites first. The model is also consistent with the occasional occurrence of closely spaced pairs of bursts. Time-dependent, ideal-magnetohydrodynamic simulations confirm that the equatorial belt is not disrupted by Parker and interchange instabilities.