NASA, George C Marshall Space Flight Ctr, Huntsville, AL 35812 USA
; Inst High Energy Phys, LCRHEA, Beijing 100039, Peoples R China
; NASA, Goddard Space Flight Ctr, Greenbelt, MD 20771 USA
Aquila X-l 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 1997 February and March. Its 10-30 keV and 5-10 keV count rate ratio increased suddenly when its luminosity was between 4 and 12 x 10(35) ergs s(-1), assuming a 2.5 kpc distance. Spectral fitting with a model composed of a blackbody and a power-law component showed that its blackbody component decreased and that the power-law component became much harder significantly and simultaneously. We interpret this transition to be caused by the centrifugal barrier, more commonly known as the "propeller" effect. We thus infer that the magnetic field strength of the neutron star is around 1 x 10(8) G, if the neutron star spin period is 1.8 ms. Similarly, we infer the neutron star magnetic field strength in another soft Xray transient Cen X-4 is about 2 x 10(9) G. 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.