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 March-April 1997 BeppoSAX observations of Aql X-1, the first to monitor the evolution of the spectral and time variability properties of a neutron star soft X–ray transient from the outburst decay to quiescence. We observed a fast X–ray flux decay, which brought the source luminosity from ∼ 1036 to ∼ 1033 ergs−1 in less than 10 days. The X–ray spectrum showed a power law high energy tail with photon index Γ∼ 2 which hardened to Γ∼ 1 − 1.5 as the source reached quiescence. These observations, together with the detection by RossiXTE of a periodicity of a few milliseconds during an X–ray burst, likely indicate that the rapid flux decay is caused by the onset of the propeller effect arising from the very fast rotation of the neutron star magnetosphere. The X–ray luminosity and hard spectrum that characterise the quiescent emission can be consistently interpreted as shock emission by a turned-on rotation-powered pulsar. Subject headings: stars: neutron — stars: individual (Aql X-1) — pulsars: general — X–ray: stars