TITLE: GCN CIRCULAR NUMBER: 11195 SUBJECT: Tentative redshift of GRB100902A from Swift-XRT data DATE: 10/09/03 13:40:08 GMT FROM: Sergio Campana at INAF-OAB S. Campana (INAF-OAB), R. L. C. Starling (U Leicester), P. A. Evans (U Leicester), and T. Sakamoto (NASA/UMBC) report on behalf of the Swift Team: Swift BAT triggered on the early activity of GRB 100902A (Sakamoto et al. 2010, GCN 11181). Swift XRT started observing 316 s after the trigger observing the main event. By using the Swift/XRT spectrum repository at http://www.swift.ac.uk/xrt_spectra/ (see Evans et al. 2009, MNRAS, 397, 1177) we selected two time intervals with an almost constant hardness ratio. These are at the end of the main event between 500-550 s (in Window Timing, WT, mode) and the entire first orbit in Photon Counting (PC) mode (between 600-3000 s). We then fit the two spectra with XSPEC using the model tbabs*ztbabs (cutoff), keeping the same absorption pattern and leaving free the cutoff power-law model for the two spectra (the cutoff power-law model provides much better results in terms of column density evaluation with respect to a simple power-law model when small spectral variations are present). We assume a Galactic column density of 1.1x1021 cm-2 (Kalberla et al. 2005, A&A 440 775) and we allow for a 30% uncertainty in the fit. We fit the spectrum using C-statistics. In the intrinsic column density vs. redshift plane there is just one deep minimum hinting for a redshift z=4.5^+0.3_-0.2 (90% confidence level) and an intrinsic column density N_H(z)=(2.3^+0.5_-0.3)x1023 cm-2. The X-ray spectrum is very soft with Gamma~3.9 at the end of the prompt phase (with a cut-off energy >17 keV) and with Gamma~1.9 and E_cut~5 keV in the PC spectrum. The two spectra indicate the same redshift even separately. The high redshift and the large absorbing column density are in line with the non-detection with UVOT and other ground based optical facilities. The contour plot is available at http://www.brera.inaf.it/utenti/campana/100902.gif