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Swift/XRT follow-up observations of IGR J17473-2721

ATel #1459; D. Altamirano (UvA), N. Degenaar (UvA), J. in 't Zand (SRON), C. Markwardt (U. Md./CRESST/NASA GSFC), R. Wijnands (UvA)
on 3 Apr 2008; 16:34 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Rudy Wijnands (rudy@space.mit.edu)

Subjects: X-ray, Binary, Black Hole, Neutron Star, Transient

Referred to by ATel #: 1460, 1461, 1468, 1651

During an AGILE pointing at the Galactic Center region, a type-I X-ray burst was detected by SuperAGILE (ATEL #1445) from a position that is consistent with that of IGR J17473-2721 (ATEL #498 & #500, also known as XTE J1747-274, ATEL #467) suggesting that this source exhibit renewed activity. To conclusively identify the burst source with the XTE/IGR transient, we obtained a Swift/XRT observation of ~ 4.1 ksec on Monday 31st of March, 2008. Within the SuperAGILE error circle, there is only one source detected at a position (J2000) of R.A. = 17h47m18.12s, Dec. =-27:20:38.66, with an uncertainty of ~3.5 arcseconds. This is consistent with the Chandra position of IGR J17473-2721 (ATEL #521). Therefore, we confirm that the SuperAGILE X-ray burst has originated from this source, establishing that this is transiently accreting neutron star. The X-ray energy spectrum was extracted from pc-mode data. It can be fitted with an absorbed power law model with Nh=(3.8+0.48-0.14) x 10^22 cm^-2, photon index 1.68+0.21-0.07 and a final reduced Chi2 of 1.26. The 2-10 keV absorbed and unabsorbed fluxes are 2.27E-10 erg/cm2/s and 3.01E-10 erg/cm2/s, respectively. We have detected a ~100 seconds long X-ray burst (in one of the observations recorded in wt -non-imaging- mode). We have extracted the energy spectrum of the burst peak (first ~2 seconds) and fitted it with an absorbed black body model. By fixing Nh to the value obtained above, we find that the temperature is kT~2.29+0.22-0.18 keV, and the unabsorbed bolometric flux (as estimated in the 0.01-100 keV range) 1.1E-7 erg/cm2/s. We find no evidence for photospheric radius expansion, implying that the luminosity remained below the Eddington Limit. From this we infer upper limits to the distance between 3.9 and 5.4 kpc, depending on whether the neutron star photosphere is hydrogen-rich or poor, respectively (e.g., Kuulkers et al. 2003, A&A, 399, 663). With these upper limits on the distance, we estimate in 1x10^36 erg/s the upper limit of the 2-10 keV Luminosity (persistent emission). We note that the spectral analysis mentioned above was complicated by instrumental pile-up issues. We have tried to correct for this by excluding the inner source pixels from the data-analysis and applying a psf-correction. We thank the Swift team for scheduling the observation.