Magnetohydrodynamic Tidal Waves on a Spinning Magnetic Compact Star

Abstract

In an X-ray binary system, thecompanion star feeds the compact neutron star with plasma material viaaccretion. The spinning neutron star is likely covered with a thin``magnetized ocean'' and may support magnetohydrodynamic (MHD) tidal waves.While modulating the thermal properties of the ocean, MHD tidal wavesperiodically shake the base of the stellar magnetosphere that trapsenergetic particles, including radiating relativistic electrons. Fora radio pulsar, MHD tidal waves in the stellar surface layer maymodulate radio emission processes and leave indelible signatures ontimescales different from the spin period. Accretion activities arecapable of exciting these waves, but may also meanwhile obstruct orobscure their detection. Under fortuitous conditions, MHD tidal wavesmight be detectable and offer valuable means to probe properties ofthe underlying neutron star. Similar situations may also occur fora cataclysmic variable-an accretion binary system that contains arotating magnetic white dwarf. This Letter presents the theory for MHDtidal waves in the magnetized ocean of a rotating degenerate star andemphasizes their potential diagnostics in X-ray and radio emissions.