S. Campana (INAF-OAB), S. Covino (INAF-OAB), A. Cucchiara (UCB/LBNL/UCSC),
P. A. Evans (U Leicester), D. Grupe (PSU), D. Malesani (DARK/NBI),
T. Sakamoto (NASA/UMBC) and R. L. C. Starling (U Leicester):
Swift BAT triggered on the early activity of GRB 101213A (Sakamoto et
al. 2010, GCN 11448). Swift XRT started observing 106 s after the
trigger observing the main event. We selected the time interval between
112-154 s during which the hardness ratio (as judged from the Swift/XRT
spectrum repository at http://www.swift.ac.uk/xrt_spectra/00440285/ )
and the power law photon index (as judged from the Swift/XRT burst analyser
http://www.swift.ac.uk/burst_analyser/00440285/ ) are consistent
with a constant. We bin the spectrum to 30 counts per energy bin and
used the latest response matrices v012. The spectrum is in Windowed Timing
mode (WT) and contains about 6,000 counts. We fit the WT spectrum with
using the model tbabs*ztbabs*(cutoff), (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 4.6x10^20 cm^-2 (Kalberla et al.
2005, A&A 440 775). In the intrinsic column density vs.
redshift plane there is just one deep minimum hinting for a redshift
z=2.9^+0.2_-0.8 (90% confidence level) and an intrinsic column density
N_H(z)=(1.5^+0.7_-0.3)x10^23 cm^-2. The X-ray spectrum is soft with
Gamma~2.5 at the end of the prompt phase and no signs of spectral curvature
(cut-off energy > 30 keV).
The absorption pattern towards GRB 101213A is complicated by the galaxy
reported by Malesani & de Ugarte Postigo (2010, GCN 11449). We tried to fit
an extra absorption component fixed either to z=0.1 or z=0.5 (i.e. the two
possible photometric redshifts of the putative host galaxy observed in the
SDSS, z=0.10+/-0.07 and z=0.47+/-0.22, respectively), with basically no
effects on the overall results. This low-redshift component is not able
alone to account for the observed absorption pattern.
Looking at the burst analyser page (see above), we also note that there is
a mismatch in the BAT and XRT power law photon indices during the short
interval during which there are contemporanous data. Motivated by this we
fit the XRT spectrum with a broken power law.
The fit is as good as the power law fit. In this case we obtain a very soft
low-energy photon index of Gamma1=3.1+/-0.2 and a high-energy photon index
Gamma2=2.0+/-0.2 (consistent with the BAT extrapolation). The transition
energy is around 3 keV. In this case, the redshift is z=0.3^+0.4_-0.1
intrinsic column density is (1.1^+0.3_-0.2)x10^22 cm^-2. In this case the
redshift determination is consistent with the putative host galaxy but the
resulting X-ray spectrum is peculiar, with a spectral variation Delta
and a spectrum that remains (almost) constant for ~150 s, becoming even
softer later on.
Alternatively, the X-ray spectrum could be fit (equally well) with a black
body plus a power law component. In this case the redshift is
and the intrinsic column density NH=(1.1^+0.3_-0.2)x10^22 cm^-2. The power
law is very soft with Gamma~3.3 and the black body (rest-frame) temperature
is high kT=2.3^+1.3_-0.5 keV. The black body radius is ~2x10^11 cm.
We note that the black body temperature is much higher than what is observed
in, e.g., GRB 060218 (kT~0.2 keV, Campana et al. 2006, Nat 442 1008) or
GRB 100316D (kT~0.1 keV, Starling et al. 2010, MNRAS in press).
We can conclude that in all cases, the X-ray afterglow flux is heavily
absorbed and this is in line with the non-detection with UVOT.
Given the presence of a candidate host galaxy within the XRT refined error
circle (Malesani & de Ugarte Postigo 2010), we cannot rule out a low
redshift solution, but in this case a peculiar X-ray spectrum is observed.