MONTE-CARLO SIMULATION OF COMPTONIZATION IN PLASMA ACCRETING ONTO NEUTRON-STARS

It is shown by the Monte Carlo simulation that high-energy X-rays are produced through the Compton scattering in plasma accreting onto neutron stars. The high-energy X-rays are produced in the following situation. We consider an accreting neutron star where the gas infalls radially. The neutron star surface is heated by the gravitational energy release of the accreting gas and radiates soft X-ray photons. The X-rays from the surface scattered by electrons in the plasma accreting onto the star. Since the infalling velocity is nearly half of the light speed, the energy of the scattered X-ray changes by 50%. Some X-rays are scattered repeatedly between the star and the accreting gas and become high-energy X-rays. The scattered X-ray photons have a power-law spectrum in high-energy range. We apply this model to X-ray spectra of type II bursts from the rapid burster, MXB 1730 - 335.