SGR 1822-1606 (Swift J1822.3-1606): X-ray spectrum and refined spin period from Swift XRT analysis
ATel #3490; P. Esposito (INAF-OAC), N. Rea (CSIC-IEEC), G. L. Israel (INAF-OAR) report on behalf of a larger collaboration
on 16 Jul 2011; 14:49 UT
Distributed as an Instant Email Notice Transients
Credential Certification: GianLuca Israel (gianluca@mporzio.astro.it)
Subjects: X-ray, Neutron Star, Soft Gamma-ray Repeater, Transient, Pulsar
We have analysed 1.6 ks s of Photon Counting (PC) XRT data of the new SGR/magnetar candidate Swift J1822.3-1606 (Cummings et al. GCN #12159), including the first 0.6 ks on which Pagani et al. reported in GCN #12163.
We found that the source spectrum is well described by a power-law plus blackbody model, modified for the interstellar absorption (reduced chi-squared = 1.04 for 97 degrees of freedom). The best fitting parameters are: nH = 3e+21 cm^-2, photon index Gamma = 0.6, and blackbody temperature equivalent to 0.7 keV. The 0.5-10 kev observed (not corrected for the absorption) flux is approximately 2e-10 ergs/cm^2/s.
With a standard folding analysis of the solar system barycentered light curves, we measured a period of 8.434(5) s (1-sigma uncertainty), consistent with the early value by Pagani et al. (GCN 12163). The pulsed fraction defined as (Fmax - Fmin)/(Fmax + Fmin), where Fmax and Fmin are the observed background-subtracted count rates at the peak and at the minimum, is (31 +/- 5)% in the 0.2-10 keV band.
Finally we note that Chandra observed the field of Swift J1822.3-1606 three times, on 2006-08-21, 2006-08-27, and 2010-05-27. No source was detected at the XRT position of the SGR (Pagani et al. GCN #12163) in any observation, with 3-sigma upper limits on the 0.5-10 keV X-ray flux below 1e-14 ergs/cm^2/s (assuming nH = 3e+21 cm^-2 and a blackbody with kT = 0.5 keV as the quiescent spectrum).
