{
  "bibcode": "2010GCN.11195....1C",
  "body": "S. Campana (INAF-OAB), R. L. C. Starling (U Leicester), P. A. Evans  (U \nLeicester), and T. Sakamoto (NASA/UMBC) report on behalf of the Swift Team:\n\nSwift BAT triggered on the early activity of GRB 100902A (Sakamoto et \nal. 2010, GCN 11181). Swift XRT started observing 316 s after the \ntrigger observing the main event. By using the Swift/XRT spectrum \nrepository at http://www.swift.ac.uk/xrt_spectra/ (see Evans et al. \n2009, MNRAS, 397, 1177) we selected two time intervals with an almost \nconstant hardness ratio. These are at the end of the main event between \n500-550 s (in Window Timing, WT, mode) and the entire first orbit in \nPhoton Counting (PC) mode (between 600-3000 s). We then fit the two \nspectra with XSPEC using the model tbabs*ztbabs (cutoff), keeping the \nsame absorption pattern and leaving free the cutoff power-law model for \nthe two spectra (the cutoff power-law model provides much better results \nin terms of column density evaluation with respect to a simple power-law \nmodel when small spectral variations are present).\nWe assume a Galactic column density of 1.1x1021 cm-2 (Kalberla et al. \n2005, A&A 440 775) and we allow for a 30% uncertainty in the fit.  We \nfit the spectrum using C-statistics. In the intrinsic column density vs. \nredshift plane there is just one deep minimum hinting for a redshift \nz=4.5^+0.3_-0.2 (90% confidence level) and an intrinsic column density \nN_H(z)=(2.3^+0.5_-0.3)x1023 cm-2. The X-ray spectrum is very soft with \nGamma~3.9 at the end of the prompt phase (with a cut-off energy >17 keV) \nand with Gamma~1.9 and E_cut~5 keV in the PC spectrum. The two spectra \nindicate the same redshift even separately.\nThe high redshift and the large absorbing column density are in line \nwith the non-detection with UVOT and other ground based optical facilities.\n\nThe contour plot is available at\nhttp://www.brera.inaf.it/utenti/campana/100902.gif",
  "circularId": 11195,
  "createdOn": 1283521208000,
  "email": "sergio.campana@brera.inaf.it",
  "subject": "Tentative redshift of GRB100902A from Swift-XRT data",
  "submitter": "Sergio Campana at INAF-OAB  <sergio.campana@brera.inaf.it>",
  "eventId": "GRB 100902A"
}