XMM Study of the 401Hz Accreting Pulsar SAX J1808.4-3658 in Quiescence

We propose to carry out the first detailed study of the quiescent state of 401Hz pulsar in the soft Xray transient SAX J1808.43658. A recent BeppoSAX observation has detected the source at a quiescent level of 10+33 ergs s1. The proposed observations will address a number of crucial issues that this unique source can help answering with unprecedented accuracy. These include: (i) the characteristics and origin of the quiescent Xray emission; (ii) the presence of coherent pulsations in the quiescent state; (iii) the physical regime that characterises the quiescent state, i.e. the propeller vs radio pulsar regime, the possible r^ance of the neutron star cooling, the presence of an ADAF. The XMM instrumentation is ideally suited for this a study. truncated!, Please see actual data for full text [truncated!, Please see actual data for full text]