MAGNETIC BURIAL AND THE HARMONIC CONTENT OF MILLISECOND OSCILLATIONS IN THERMONUCLEAR X-RAY BURSTS

摘要

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 approx> 10~(-5) solar mass, the maximum field strength and latitudinal half-width of the equatorial magnetic belt are B_(max) = 5.6 x 10~(15)(M_a/10~(-4) solar mass)~(0.32) G and Δθ = max [3°(M_a/10~(-4)solar mass)~(-1.5),3°(M_a/10~(-4) solar mass)~(0.5)(M_a/10~(-8) solar mass yr~(-1))~(-0.5)], respectively, where M_a is the total accreted mass and M_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 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.