AQUILA X-1 FROM OUTBURST TO QUIESCENCE - THE ONSET OF THE PROPELLER EFFECT AND SIGNS OF A TURNED-ON ROTATION-POWERED PULSAR

We report on the 1997 March-April BeppoSAX observations of Aquila X-1, which were the first to monitor the evolution of the spectral and tim e-variability properties of a neutron star soft X-ray transient from t he outburst decay to quiescence. We observed a fast X-ray flux decay, which brought the source luminosity from similar to 10(36) to similar to 10(33) ergs s(-1) in less than 10 days. The X-ray spectrum showed a power-law high-energy tail with a photon index Gamma - 2, which harde ned to Gamma - 1-1.5 as the source reached quiescence. These observati ons, together with the detection by the Rossi X-Ray Timing Explorer of a periodicity of a few milliseconds during an X-ray burst, likely ind icate that the rapid flux decay is caused by the onset of the propelle r effect, which arises from the very fast rotation of the neutron star magnetosphere. The X-ray luminosity and hard spectrum that characteri ze the quiescent emission can be consistently interpreted as shock emi ssion by a turned-on rotation-powered pulsar.