Very Low Luminosities from the Accretion‐driven Millisecond X‐Ray Pulsar SAX J1808.4−3658 during Quiescence

We have observed the millisecond X-ray pulsar SAX J1808.4-3658 on threeoccasions during its 2000 outburst with the BeppoSAX satellite. The source washighly variable and erratic during this outburst, and by coincidence weobtained data only during times when the source had very low luminosities.During our observations, we detected four faint sources. The source closest tothe position of SAX J1808.4-3658 is still ~1.6' away. This source can only beidentified with SAX J1808.4-3658 if we assume that the BeppoSAX positionalreconstruction is not completely understood. We also reanalyzed a BeppoSAXobservation taken in March 1999 when the source was in quiescence and duringwhich the source was thought to have been detected (Stella et al. 2000). Basedon the similarities (position and luminosity) of this source with the abovementioned source ~1.6' away from SAX J1808.4-3658, it is possible that they arethe same source. If this source is not the millisecond pulsar, then during allBeppoSAX observations of SAX J1808.4-3658 (the 2000 outburst ones and the 1999quiescent one), the millisecond pulsar was not detected. A reanalysis of theASCA quiescent data of SAX J1808.4-3658 (Dotani, Asai, & Wijnands 2000)confirms that during this observation the source was securely detected inquiescence. We discuss our results for SAX J1808.4-3658 in the context of thequiescent properties of low-mass X-ray binary transients.Comment: Accepted for publication by ApJ Main Journal, 30 January 200