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GCN Circular 18483

Subject
Tentative redshift for GRB 151027A from Swift-XRT data
Date
2015-10-27T10:03:40Z (9 years ago)
From
Sergio Campana at INAF-OAB <sergio.campana@brera.inaf.it>
S. Campana (INAF-OAB) report on behalf of the Swift Team:

Swift BAT triggered on the early activity of GRB 151027A (Maselli et
al. 2015, GCN 18478). Swift XRT started observing 87 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 a time interval with an almost
constant hardness ratio, starting at 150 s after the burst onset
and lasting 281 s. We then fit the Window Timing (WT) spectrum with
XSPEC using the model tbabs*ztbabs*pow.
We assume a Galactic column density of 3.75x10^20 cm^-2 (Willingale et
al. 2013, MNRAS, 431, 394) and we allow for a 30% uncertainty in the fit.
We fit the spectrum using C-statistics (C-stat=593.74 with 637 d.o.f.).
In the intrinsic column density vs. redshift plane there is just one 
deep minimum
hinting for a redshift z=0.38+/-0.17 (90% confidence level) and an
intrinsic column density N_H(z)=(3.6+/-0.7)x10^21 cm^-2.
The X-ray spectrum is soft with Gamma=2.6, (assuming a cut-off power
law spectrum the allowed redshift range expands to 0.27-1.38).

The contour plot is available at
http://www.brera.inaf.it/utenti/campana/151027.gif
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