TITLE: GCN CIRCULAR NUMBER: 10940 SUBJECT: Tentative redshift of GRB100704A from Swift-XRT data DATE: 10/07/06 15:45:20 GMT FROM: Sergio Campana at INAF-OAB S. Campana (INAF-OAB) and D. Grupe (PSU) report on behalf of the Swift-XRT team: Swift XRT observed the bright GRB 100704A (Grupe et al. 2010, GCN 10929) starting 86 s after the BAT trigger. After strong X-ray flares the light curve started decaying following a power law (Grupe 2010, GCN 10935). We selected the time interval 4,510-28,896 s after the burst onset to collect the Photon Counting (PC) spectrum. During this interval the spectrum is almost constant as testified by a constant power law photon index (chi2=102 with 123 degrees of freedom). We fit the PC spectrum (comprising 2002 counts) with a (Galactic plus intrinsic) absorbed cutoff power-law model (tbabs*ztbabs*cutoff) 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). We assume a Galactic column density of 1.1x1021 cm-2 (Kalberla et al. 2005, A&A 440 775) and allowed for a 30% variability given its high value (and sampling all the values obtained within 1 degree from the source). We fit the spectrum using C-statistics. 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=3.6^+1.1 _-1.2 and N_H (z)=(9.7^+9.9 _-5.6 )x10^22 cm^-2 . Thus, even if we cannot exclude that GRB100704A is at low redshift, a high redshift (z~3.5), highly absorbed (N_H (z)~1x10^23 cm^-2 ) solution is preferred. This compares well with the non-detection with UVOT (Kuin and Grupe 2010, GCN 10934) and MOA robotic telescope (Omori et al. 2010, GCN 10931). We also note that if no interviening systems are present along the line of sight a 90% lower limit on the metallicity of 0.03 solar can be set (based on the assumption that the absorbing medium is not Thomson thick). The contour plot is available at http://www.brera.inaf.it/utenti/campana/100704.gif