  Aql X-1 is a soft X-ray transient source and emits type I X-ray bursts. A
spectral state transition was observed with RXTE during its outburst decay in
February and March 1997. Its 10-30 keV and 5-10 keV count rate ratio increased
suddenly when its luminosity was between (4-12) E35 ergs/s, assuming a 2.5 kpc
distance. Spectral fitting with a model composed of a blackbody and a power-law
components showed that its blackbody component decreased and the power-law
became much harder significantly and simultaneously. We interpret this
transition as caused by the centrifugal barrier, or commonly known as the
`propeller' effect. We thus infer that the magnetic field strength of the
neutron 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 soft
X-ray transient Cen X-4 is about 2E9 Gauss. We also propose a unified scheme
for spectral state transitions in soft X-ray transients, from soft high state
to hard low state and further to quiescent state. With this scheme accretion
onto neutron star may take place even during the propeller regime.
