Abstract

Type II burst properties of the Rapid Burster have been studied with the X-ray astronomy satellite ASCA. To study spectral evolutions throughout a burst cycle from the onset of a burst to the next burst, we constructed six composite burst-cycles in terms of the burst fluence, and divided each of them into seven phases. We then fitted the 42 spectra with a two-component model consisting of a blackbody (BB) component expected from the boundary layer, and a multi-color-disk (MCD) component expected from the standard disk, systematically. The results show that the luminosity of the BB component is much larger than that of the MCD component during a burst, while the two luminosities become comparable during a persistent phase. They also indicate that the innermost radius of the standard disk tends to recede outwards in the burst decay. These results suggest that although the standard disk extends close to the neutron star surface during a persistent phase, another accretion flow onto the neutron star is needed during a burst phase. We argue that an advection-dominated accretion flow (ADAF) would be a dominant flow onto the neutron star during a burst, and that a limit cycle of the accretion flow between an ADAF and a standard disk can explain the overall properties of the type II bursts.

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