A MODEL FOR THE HIGH-ENERGY POWER-LAW COMPONENT OBSERVED IN THE X-RAY-SPECTRA OF LOW-MASS X-RAY BINARIES

According to recent observations, an X-ray spectrum of an LMXB (low-mass X-ray binary) is well approximated by a power-law for E > 10 keV when the luminosity is very low. We propose the following model for this high-energy component. In an LMXB, a neutron star accretes gas from its companion through an accretion disk. The gas accreted from the inner edge of the accretion disk has angular momentum and rotates with roughly the Keplerian velocity on the neutron star surface. X-ray photons from the neutron star are scattered in the rapidly rotating surface layer named the boundary layer. The scattered X-ray photons change their energy by the Compton effect. Some of the X-ray photons are scattered repeatedly between the boundary layer and the neutron star and observed as high-energy X-ray photons. Our model reproduces the observational feature of the high-energy power-law component. The photon index of our model spectrum is expressed as a function of the model parameters.