Mass Accretion onto a Black Hole

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We have studied the mass accretion onto a black hole, comparing the X-ray transients(black hole candidates) with the low mass X-ray binaries(LMXB). The main results obtained in this study are summarized as follows.1. There are two components in the X-ray spectra of both X-ray transients and low mass X-ray binaries. One of the two components is common in both X-ray sources and is well fitted by the model spectrum expected from the accretion disk. The time variability of this components is very low in both sources. These evidence indicates that the accretion disks with similar properties exist around both black holes and neutron stars.2. another component is fitted by a power low spectrum in X-ray transients, while by a blackbody spectrum in LMXBs. This component may be emitted from the region close to the black hole and the neutron star.3. The second component in both sources exhibits a large time variation independentry of the disk component. This evidence indicates that there exist another channel for mass accretion onto the central star besides the ordinary accretion disk(optically thick and geometriaclly thin disk).We propose that the optically thick and geometrically thin disk is sandwiched in the optically thin and geometrically thick disk, which givers rise to the intense time variation.