Torque and energy exchange between an accretion disk and a rapidly spinning neutron star

Torque-coupled energy flows are proposed to explain the bimodal behavior of the quasi-periodic oscillations, spectrum, radio outbursts, and optical flux in Sco X-1. Accretion torques move energy in or out a flywheel (stored rotational energy). The material torque from accreting matter reduces the luminosity available at the neutron star surface, thought to be the source of the blackbody component of low-mass X-ray binary spectra. In equilibrium, the material spinup torque must be balanced by a spindown torque against the disk or expelled matter. In spindown work is dissipated in the disk, it becomes brighter. This explains the large excesses of disk over blackbody flux. The disk flux changes only gradually with accretion rate, while the neutron star surface flux increases rapidly, as for the two-component spectral behavior observed in low-mass X-ray binaries. For lower accretion rates, the spindown luminosity of the neutron star dominates, and total luminosity increases with decreasing mass accretion rate.