Swift Observations of SAX J1808.4-3658: Monitoring the Return to Quiescence

Abstract

The transient accreting millisecond pulsar SAX J1808.4-3658 has shown several outbursts to date but the transition from outburst to quiescence has never been investigated in detail. Thanks to the Swift observing flexibility, we monitored for the first time the decay to quiescence during the 2005 outburst. At variance with other transients, wide luminosity variations are observed. In addition, close to quiescence, SAX J1808.4-3658 seems to switch between two different states. We interpret them in terms of the accretion states accessible to a magnetized, fast-rotating neutron star.