Observing Aql X-1 during the transition to quiescence

Aql X1 is a transient source hosting a fastly spinning weakly magnetic neutron star, showing recurrent outbursts at a rate of 12 per year. During one of these, BeppoSAX allowed to monitoring the return to quiescence: the Xray luminosity decreased very rapidly with an efolding time of about 1 day. This rapid decay was accompanied by a steep increase in the hardness ratio, interpreted as the onset of the propeller regime. Moreover, the neutron star parameters suggests that the quiescent luminosity is powered by shock emission from an underlying (enshrouded) radio pulsar. We propose to observe these transitions with XMM in order to gain crutial insight in the propeller physics as well as in the transition to quiescence of Aql X1. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]