GCN Circular 3695
Subject
GRB050713: analysis of the XMM-Newton observation
Date
2005-07-28T19:27:12Z (19 years ago)
From
Andrea De Luca at IASF-CNR,Milano <deluca@mi.iasf.cnr.it>
Andrea De Luca (IASF Mi) on behalf of a larger collaboraton
report:
We have analyzed the data from the XMM-Newton observation
of GRB050713A, discovered by Swift on 2005, July 13 at
04:29:02.39 UT (Falcone et al., GCN3581).
The XMM-Newton observation started on 2005, July 13 at
10:18 UT and lasted for 30.7 ks. We report here on the
analysis of data collected with the EPIC/pn detector,
which started observing the field at 10:54 UT (~6h 20min
after the GRB).
As reported by Loiseau et al. (GCN3594), the afterglow of
GRB050713A is clearly detected in the pn image, at a
position fully consistent with the refined Swift/XRT one
(Morris et al., GCN3606).
Extracting source events from a circle of 25 arcsec radius
(containing ~80% of the total counts), the time-averaged,
background-subtracted count rate in the 0.2-8 keV range
is 0.547+/-0.005 cts/s.
The afterglow is clearly seen to fade along the XMM-Newton
observation, spanning the time range 23.5-51.5 ks after
the GRB. The background-subtracted light curve (0.5-5 keV)
is well fitted (reduced chi2=0.9, 26 d.o.f.) by a power law
decay with index delta=1.45+/-0.07 (90% c.l.).
The afterglow decay has significantly steepened with
respect to the epoch of the earlier Swift observation:
Morris et al. (GCN 3606) observed an index delta=0.82+/-0.11
in the time range 5-10 ks after the burst using Swift/XRT data.
This implies the presence of a break in the afterglow X-ray
light curve between 10 ks and 23.5 ks from the GRB.
We extracted the time-averaged spectrum and generated ad-hoc
response and effective area files. We quote here errors at
90% level for a single interesting parameter, unless otherwise
specified.
A fit in the energy range 0.2-8 keV with an absorbed power
law model yields a reduced chi2 of 1.25 for 172 d.o.f.
The resulting NH=(3.25+/-0.15)x10^21 cm^-2 is higher than
the expected Galactic value in the burst direction (NH=1.1x10^21
cm^-2, Dickey & Lockman, 1990); the best fitting power law photon
index is Gamma=2.16+/-0.05.
Such results are consistent with the XRT ones (Morris et al.,
GCN 3606), which implies no significant spectral evolution with
respect to the earlier phase of the afterglow.
A better fit to the pn spectrum (reduced chi2=0.97, 171 dof)
may be obtained fixing the NH to the expected Galactic value
(NH=1.1x10^21 cm^-2) and adding a neutral, redshifted absorber
component to the spectral model. With a simple F-test we evaluate
the chance occurrence probability of the improvement to be
of 5x10^-11. The best fit value for the intrinsic NH is 4.0x10^21
cm^-2, while the best fit value for the redshift z is 0.55.
At 90% c.l. for 2 parameters, we obtain the following ranges:
intrinsic NH=(0.4-3.2)x10^22 cm^-2; redshift z=(0.4-2.6).
Using such model, the resulting power law photon index is
Gamma=2.04+/-0.05.
The observed flux is of 2.2x10^-12 erg cm^-2 s^-1 in 0.2-10 keV;
the corresponding unabsorbed flux is of 3.8x10^-12 erg cm^-2 s^-1.
As a last step, we divided the pn dataset into two time intervals
of ~9500 s and ~14600 s (each containing about half of the counts
from the afterglow) and we repeated the spectral analysis.
We found no significant spectral changes in the two considered
intervals.