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R. Wijnands (CSR, MIT)
More than two years after the discovery of the accretion driven millisecond X-ray pulsar (SAX J1808.4-3658), it is still very unclear why this system is the only X-ray binary that exhibits coherent millisecond X-ray pulsations. I will present an update of the most recent observational properties of this source and how they have advanced our understanding of this unique system. In early 2000, the source exhibited a very long (several months) episode of low-level activity, during which it showed extremely violent quasi-periodic X-ray flaring with a repetition frequency of about one Hertz. Such violent flaring behavior has never been observed in any other neutron star X-ray binary. This behavior of SAX J1808.4-3658 could be related to the fact that it is a millisecond X-ray pulsar and thus give us a clue to the pulsating mechanism. Alternatively, it could be a property of all X-ray binaries which has only been observed in SAX J1808.4-3658 because of the intense observing campaign on this source at very low accretion rates. In the latter case similar behavior might also be observable in the non-pulsating systems when they are observed at similarly low accretion rates. From reanalysis of the X-ray bursts observed with BeppoSAX, an improved distance estimate of 2.5 kpc was obtained (in 't Zand et al. 2000, in prep.). This new distance has profound consequences for the estimates of the mass of the companion star (very low) and the inclination of the system (relatively high inclination). Furthermore, from recent X-ray and optical observations during the quiescent state of this source there is now strong evidence that residual accretion is taking place in quiescence.
This work was supported by NASA through the Chandra Postdoctoral Fellowship (PF9-10010) awarded by CXC, which is operated by SAO for NASA (NAS8-39073).
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