Matter accreting onto the magnetic poles of a neutron star spreads under gravity towards 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 semi-quantitative treatment of Ohmic diffusion) show that, for $\Ma \gtrsim 10^{ -5}\Msun$, the maximum field strength and latitudinal half-width of the equatorial magnetic belt are $B_{ \rm max} = 5.6\times 10^{ 15} (\Ma/10^{ -4}\Msun)^{ 0.32}$ G and $\Delta\theta = \max[3^{ \circ} (\Ma/10^{ -4}\Msun)^{ -1.5},3^{ \circ} (\Ma/10^{ -4}\Msun)^{ 0.5}(\dot{ M}_{ \rm a}/10^{ -8}\Msun { \rm yr}^{ -1})^{ -0.5}]$ respectively, where $\Ma$ is the total accreted mass and $\dot{ M}_{ \rm a}$ is 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 which 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.
Categories: astro-ph