Magnetic burial and the harmonic content of millisecond oscillations in thermonuclear X-ray bursts

Abstract

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 Ma ≳ 10-5 M⊙, the maximum field strength and latitudinal half-width of the equatorial magnetic belt are Bmax = 5.6 × 1015(Ma/10-4 M⊙) 0.32 G and Δθ = max [3°(Ma/10 -4M⊙)-1.5, 3°(Ma/10 -4M⊙)0.5 (Ma/10-8 M⊙ yr-1)-0.5], respectively, where M a is the total accreted mass and Ma is the accretion rate. It is shown that the belt prevents north-south heat ..