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ATEL # 896; G.L. Israel (INAF-OAR), S. Campana (INAF-OAB)
on 22 Sep 2006; 14:13 UT
Distributed as an Instant Email Notice (Transients)
Password Certification: GianLuca Israel (firstname.lastname@example.org)
Subjects: X-ray, Neutron Stars, Pulsars, Soft Gamma-ray Repeaters, Transients
We performed a detailed spectral analysis of the whole Swift TOO observation of the anomalous X-ray pulsar CXO J164710.2-455216 in the cluster Westerlund 1 (Muno et al. 2006, ApJ 636 L41, see ATel #893). The final XRT effective exposure time is 7.4 ks in PC mode and 1.3 ks in WT and about 3700+1900 photons are extracted from the source (BG level of about 2.5%). The source is clearly fading with a power law index of about -1.3 in the 0.5-1day time interval from the BAT event. The single black body (BB) model used in ATel #893 is not anymore sufficient to well fit the spectrum (reduced chi^2 of 1.4). A BB plus a power-law (kT=0.65+/-0.17keV and Gamma=2.7+/-1.2) or two BBs (kTsoft=0.57+/-0.12keV and kThard=1.3+1.5-0.3keV) give a rather better estimate of the incident spectrum (reduced chi^2 of 1.1 for both). The 0.5-10 keV observed and unabsorbed fluxes are 3.5x10^-11 erg cm^-2 s^-1 and 1.2x10^-10 erg cm^-2 s^-1, respectively, for the BB+power law model.
Due to the relatively long sampling time of the XRT CCD (2.5s) only rough estimates of the timing properties are obtained. We find a low significance signal at approximatively P=10.611(2) with a pulsed fraction of 10%+\-2%, significantly lower than that detected in the 2005 Chandra datasets by Muno and Collaborators.