The Erratic Luminosity Behavior of SAX J1808.4-3658 during Its 2000 Outburst

We report on the highly variable and erratic long-term X-ray luminosity behavior of the only known accretion-driven millisecond X-ray pulsar SAX J1808.4-3658 during its 2000 outburst, as observed with the Rossi X-ray Timing Explorer (RXTE) satellite. The maximum observed luminosity is similar to2.5-10(35) ergs s(-1) (3-25 keV; for a distance of 2.5 kpc), which is approximately a factor of 10 lower than that observed during the 1996 and 1998 outbursts. Due to solar constraints, the source could not be observed for several months with RXTE before 2000 January 21. Therefore, the exact moment of the outburst onset is unknown, and the peak luminosity could have been significantly higher. On some occasions SAX J1808.4-3658 was observed with luminosities of similar to 10(35) ergs s(-1), but on other occasions it could not be detected with RXTE, resulting in typical upper limits of a few times 10(33) ergs s(-1) (3-25 keV). The nondetections of the source during its 2000 outburst obtained with the BeppoSAX satellite demonstrate that its luminosity was at times less than 10(32) ergs s(-1) (0.5-10 keV). However, only a few days after these BeppoSAX observations, we detected the source again with RXTE at high luminosities, giving a factor of greater than 1000 of luminosity swings in this systems on timescales of days. The last detection of SAX J1808.4-3658 with RXTE was on 2000 May 13, almost 4 months after the first detection during this outburst. Due to the lack of sensitivity and observations during the 1996 and 1998 outbursts, it cannot be excluded that after those outbursts the source remained active for months and that the source behavior during the 2000 outburst is not unique. Long duration activity at low luminosities has been observed in other transients (both neutron stars and black holes), although not with such extreme variability, which might point to a different origin for this behavior for the millisecond X-ray pulsar.