We report on the March-April 1997 BeppoSAX observations of Aql X-1, the firstto monitor the evolution of the spectral and time variability properties of aneutron star soft X-ray transient from the outburst decay to quiescence. Weobserved a fast X-ray flux decay, which brought the source luminosity from~10^36 to ~10^{33} erg/s in less than 10 days. The X-ray spectrum showed apower law high energy tail with photon index Gamma~2 which hardened toGamma~1-1.5 as the source reached quiescence. These observations, together withthe detection by RossiXTE of a periodicity of a few milliseconds during anX-ray burst, likely indicate that the rapid flux decay is caused by the onsetof the propeller effect arising from the very fast rotation of the neutron starmagnetosphere. The X-ray luminosity and hard spectrum that characterise thequiescent emission can be consistently interpreted as shock emission by aturned-on rotation-powered pulsar.
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