Spectral State Transitions in Aql X-1: Evidence for `Propeller' Effects

摘要

Aql X-1 is a soft X-ray transient source and emits type I X-ray bursts. Aspectral state transition was observed with RXTE during its outburst decay inFebruary and March 1997. Its 10-30 keV and 5-10 keV count rate ratio increasedsuddenly when its luminosity was between (4-12) E35 ergs/s, assuming a 2.5 kpcdistance. Spectral fitting with a model composed of a blackbody and a power-lawcomponents showed that its blackbody component decreased and the power-lawbecame much harder significantly and simultaneously. We interpret thistransition as caused by the centrifugal barrier, or commonly known as the`propeller' effect. We thus infer that the magnetic field strength of theneutron star is around 1E8 Gauss, if the neutron star spin period is 1.8 ms.Similarly we infer the neutron star magnetic field strength in another softX-ray transient Cen X-4 is about 2E9 Gauss. We also propose a unified schemefor spectral state transitions in soft X-ray transients, from soft high stateto hard low state and further to quiescent state. With this scheme accretiononto neutron star may take place even during the propeller regime.