GCN Circular 3695

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
Not found
 
 
).

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
Not found
 
 
), 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
Not found
 
 
). 
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
Not found
 
 
) 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
Not found
 
 
), 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.