TITLE: GCN CIRCULAR NUMBER: 10517 SUBJECT: Tentative redshift of GRB100316D from X-ray data DATE: 10/03/17 06:37:22 GMT FROM: Sergio Campana at INAF-OAB S. Campana (INAF-OAB) reports on behalf of the Swift-XRT team: Swift XRT observed GRB 100316D (Stamatikos et al. 2010, GCN10496) in Windowed Timing (WT) mode in the 144-737 s time interval. During this interval the light curve decays very slowly (decay index alpha=-0.13, Starling et al. 2010, GCN 10505). In addition, the 1.5-10 keV to 0.3-1.5 keV hardness ratio remains constant (reduced chi2=0.91 with 97 degrees of freedom, dof). Motivated by the high count rate (around 30 c s^-1, but far from the pile up limit) and by the constant spectral shape we tried to estimate the GRB redshift from the X-ray data. We assume a Galactic column density of 7x10^20 cm^-2 (Kalberla et al. 2005, A&A 440 775) and fit the WT spectrum taken from the Leicester pages (http://www.swift.ac.uk/xrt_spectra/00416135/) with an absorbed (phabs*zphabs) cutoff power law model (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). Given the high count rate we selected single pixel events only. The resulting fit is good (reduced chi2=1.2, 333 dof). The redshift is constrained to lie within the 90% confidence level (delta chi2=4.61) of 0.014