AN INTERPRETATION OF X-RAY-SPECTRA OF TYPE-II BURSTS FROM THE RAPID BURSTER - COMPTONIZATION DUE TO ACCRETION FLOW

A new model is proposed for the spectrum of a type II burst from the "rapid burster." The spectrum of a type II burst has been considered to be well fitted with a black body spectrum with temperature T ∼ 1.5 keV. Recent observations with Tenma and EXOSAT, however, have found an excess from the blackbody spectrum in the high-energy range (E > 10 keV). The high-energy component is approximated to be a power-law spectrum of f(E) ∝ E-4, where E and f(E) are the X-ray energy and observed counts per unit energy, respectively. Our model explains this high-energy component as follows. During a type II burst gas is accreted onto the neutron star of the "rapid burster" (MXB 1730-335). The neutron star surface is heated by the gravitational energy release and radiates thermal X-ray photons. A part of the X-ray photons are scattered by electrons in the accretion flow. Since the electrons have a high velocity (∼0.3c), the energy of the scattered X-ray photon increases owing to the Compton effect. Some X-ray photons are scattered repeatedly and become high-energy photons. The scattered X-ray photons are observed as the high-energy power-law component. It is also shown that the spectrum observed with Tenma can be fitted with our model. Our model gives a method to evaluate the velocity and the optical depth of the accretion flow from observations of the high-energy component.