Swift Observations of SAX J1808.4-3658
ATel #527; Albert K. H. Kong (MIT)
on 19 Jun 2005; 06:32 UT
Credential Certification: Albert Kong (akong@cfa.harvard.edu)
Subjects: Optical, X-ray, Binary, Neutron Star, Transient, Pulsar
We report Swift observations of the transient millisecond pulsar SAX J1808.4-3658 which is currently in outburst. SAX J1808.4-3658 was observed with Swift on 2005 June 16. The source was clearly detected in the X-ray Telescope images and it is bright enough to show significant pile-up in the 787s Photon Counting mode data. We extracted the spectrum from the 119s Windowed Timing data and performed spectral fit. The spectrum can be fitted with an absorbed power-law model with N_H=2.3e21 cm^-2 and photon index of 2.2. The absorbed 2-10 keV flux was 2.8e-10 ergs/cm^2/s which is about a factor of
2.7 fainter than the Chandra observation taken on June 10 (ATEL #517). No X-ray bursts were detected.
The optical counterpart of SAX J1808.4-3658 was also clearly detected in the Swift UV/Optical Telescope V-band image. The exposure through the V filter is 907s and the source was detected with V=18.5+/-0.1. This is about 1 magnitude fainter than previous observations taken in early June (ATEL #511 and #515). The simultaneous X-ray/optical observations indicate that SAX J1808.4-3658 is in the decay stage of the outburst.
