Swift Observations of SAX J1808.4-3658
ATel #529; S. Campana (INAF-OAB), L. Stella (INAF-OAR), G. L. Israel (INAF-OAR), T. Belloni (INAF-OAB), C. Pagani (INAF-OAB/PSU), D. N. Burrows (PSU), N. Gehrels (GSFC)
on 21 Jun 2005; 15:13 UT
Credential Certification: Sergio Campana (campana@merate.mi.astro.it)
Subjects: X-ray, Binary, Neutron Star, Transient
Swift observed SAX J1808.4-3658 during its outburst phase.
The observation started on June 16 2005 UT00:42:51. A first report was presented by Kong (ATEL 527). Here we report on Photon Counting (PC) data and Window Timing (WT) data together. The source was observed for 784 s in PC and 119 s in WT. The source is bright and appears well off-axis on the PC image (and at the border of the WT strip). The count rate in WT is 12.26+/-0.32 c/s (0.5-10 keV). In PC the source is heavily piled-up (with a hole at the image center). We extracted photons from an annular region to avoid pile-up and generate the appropiate response matrix. We adopted the same absorbed black body + power law spectrum as in ATEL (517, Chandra HETG). The fit is good and the black body component is highly significant (F-Test probability of 1.5E-05). The column density is (1.1+/-0.3)x10^20 cm-2 (consistent with the Galactic value). The black body temperature is 0.63+/-0.11 keV and the power law photon index 2.1+/-0.2 (all errors are 90% confidence level for one parameter of interest). The WT fluxes (less affected by pile-up correction and better calibrated) are:
2-10 absorbed : (2.5+/-0.2)E-10 erg/cm2/s
0.5-10 unabsorbed: (5.4+/-0.3)E-10 erg/cm2/s
