TITLE:   GCN CIRCULAR
NUMBER:  11501
SUBJECT: Tentative redshift of GRB101225A (Christmas's burst) from Swift-XRT data
DATE:    10/12/27 16:29:52 GMT
FROM:    Sergio Campana at INAF-OAB  <sergio.campana@brera.inaf.it>

S. Campana (INAF-OAB), S. Covino (INAF-OAB),
J.L. Racusin (NASA/GSFC) and K. L. Page (U Leicester)
report on behalf of the Swift Team:

GRB 101225A was detected by Swift BAT as an image trigger (Racusin
al. 2010, GCN 11493). Swift XRT started observing 1383 s after the
trigger observing the main event. The afterglow is very bright and
the XRT remained in the Windowed Timing (WT) mode for two consecutive
orbits. Within each orbit the spectrum remain constant as judged from
the hardness ratio and from the power law photon index.
We extract the XRT spectra for each of the first two orbits, separately.
We then fit the two spectra within 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 7.9x10^20 cm-2 (Kalberla et al.
2005, A&A 440 775) and we allow for a 30% uncertainty in the fit. We
binned the spectra to 50 counts per energy bin and used the latest (v12)
response matrix. The detection in the UVW2 filter by UVOT (Siegel &
Racusin 2010, GCN 11499) constrains the redshift to be <1.4.
The fit with the cutoff power-law is relatively good with a reduced
chi^2 of 1.13 (508 dof).
Motivated by the similarities with GRB 060218 (extremely long duration,
soft prompt-emission spectrum, bright smoothly decaying X-ray afterglow
light curve, UV detection with optical non-detection), we fit the X-ray
spectrum including a blackbody component.
The fit improves considerably (and the cutoff energy goes well above the
XRT energy band, becoming unimportant for both spectra).
The new fit provides a reduced chi^2 of 1.07 (504 dof), based on an F-test
the addition of the blackbody component improves the fit by 4.2 sigma.
In the intrinsic column density vs. redshift plane there is just one
deep minimum hinting for a redshift z=0.07^+0.13_-0.04 (90% confidence
level) and an intrinsic column density N_H(z)=(1.5^+0.5_-0.2)x10^21 cm^-2.
The power law component shows a small softening from the first to the
second orbit, with a photon index around 2.2-2.4.
The blackbody component is hot with a temperature of ~1 keV, consistent
among the two observations. The blackbody radius decreases from the first
observation to the second from ~2x10^10 cm to ~0.6x10^10 cm.

Ground-based optical follow-up observations are strongly encouraged in order
to get spectroscopic confirmation for the redshift estimate and, eventually,
to monitor the coming out of a supernova.