TITLE: GCN CIRCULAR NUMBER: 10588 SUBJECT: Tentative redshift of GRB100413A from X-ray data DATE: 10/04/14 10:39:40 GMT FROM: Sergio Campana at INAF-OAB S. Campana (INAF-OAB), P.A. Evans (U. Leicester) and S. T. Holland (CRESST/USRA/GSFC) report on behalf of the Swift-XRT team: Swift XRT observed GRB 100413A (Holland et al. 2010, GCN 10581) in Windowed Timing (WT) mode in the 146-550 s time interval. During this interval the 1.5-10 keV to 0.3-1.5 keV hardness ratio slightly decayed, testifying for small spectral variations. We assume a Galactic column density of 7.3x10^20 cm^-2 (Kalberla et al. 2005, A&A 440 775) and fit the WT spectrum with an absorbed (tbabs*ztbabs) cutoff power law model within XSPEC (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). Data were grouped to have 30 counts per spectral bin. The resulting fit is good (reduced chi2=0.9, 194 dof). Filtering the data for single pixel events does not change the results. In the intrinsic column density vs. redshift plane there is a low significance (3 sigma) solution at low redshifts but the minimum lies at higher redshifts with a 2 sigma confidence level solution of z=3.9+1.7-0.7 and N_H(z)=(5.5+9.8-2.1)x1022 cm-2. Thus, even if we cannot exclude that GRB100413A is at low redshift, a high redshift (z~4), highly absorbed (N_H(z)~6x1022 cm-2) solution is preferred. This compares well with the non-detection with UVOT (Holland et al. 2010) and other robotic telescopes. The contour plot is available at http://www.brera.inaf.it/utenti/campana/100413.gif