Swift observations of SAX J1808.4-3658: monitoring the return to quiescence

TL;DR

This study uses Swift observations to monitor the decay of SAX J1808.4-3658 from outburst to quiescence, revealing unique luminosity variations and state switching in a magnetized neutron star.

Contribution

First detailed monitoring of the transition from outburst to quiescence in SAX J1808.4-3658, highlighting its unique luminosity behavior and accretion state changes.

Findings

Wide luminosity variations during decay

Switching between two states near quiescence

Interpretation in terms of neutron star accretion states

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.