[vsnet-grb-info 9727] Tentative redshift of GRB100902A from Swift-XRT data

[GCN Circulars]

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  <sergio.campana at brera.inaf.it>

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