[vsnet-grb-info 3635] GRB060729: analysis of XMM-Newton observation

[GCN Circulars]

TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  5469
SUBJECT: GRB060729: analysis of XMM-Newton observation
DATE:    06/08/21 17:48:27 GMT
FROM:    Sergio Campana at INAF-OAB  <sergio.campana at brera.inaf.it>

Sergio Campana (OAB) & Andrea De Luca (IASF Mi)
on behalf of a larger collaboration report:

We have analyzed the data from the XMM-Newton observation of
GRB060729, discovered by Swift on 2006, Jul 29, 19:12:29 UT
(Grupe et al., GCN5365).

The XMM-Newton observation started on 2006, July 30 07:59 UT
and lasted for ~61 ks. A high particle background level affects
only mildly the whole observation and only the highest flares
have been cut out, in consideration also of the source strength.

The background subtracted pn light curve (0.3-10 keV) shows a
clear decay from about 5 c/s at the beginning of the observation
down to 2 c/s. The light curve can be fit with a power law decay
with index delta=1.04+/-0.02 but with a large chi2=1.8 (58 dof).
This is due to a large bump in the middle of the exposure. Adding
a Gaussian we obtain a better chi2=0.7 (55 dof). The central time
is 69+/-1 ks (90% confidence) from the burst start and the
(Gaussian) width over the time is about 7+/-2%. We note also that
the time of the XMM-Newton observation is close to the break time
observed by Swift XRT in the GRB060729 light curve (Grupe, GCN5432).
Fitting a broken power law to we obtain a first decay index
delta1=0.85+/-0.05, which is inconsistent with the early slope of
0.34 reported by Grupe.

We extracted time-averaged spectra from the EPIC pn camera (since
in the initial part of the MOS observation the GRB spectrum should
be mildly piled-up) and the two RGS spectrometers (first and second
order) and we generated ad-hoc response files.
A simultaneous fit with an absorbed power law model with Galactic
and intrinsic (at a redshift of z=0.54, Thoene et al. GCN5373)
yields a reduced chi2 of 1.05 (1824 dof, without any systematic
error). The resulting NH=(3.9+/-0.9)x1020 cm-2 (consistent with
the Galactic value of NH=4.8x1020 cm-2, Dickey & Lockman 1990)
and a relatively low NH_host=(9.2+/-0.4)x1020 cm-2. The best
fitting power law photon index is Gamma=2.09+/-0.01.
The RGS spectrometers do now show any clear evidence of absorption
or emission lines in the 0.3-2 keV energy range.
The observed flux is of 9.5x10-12 erg cm-2 s-1 in 0.3-10 keV;
the corresponding unabsorbed flux is of 1.2x10-11 erg cm-2 s-1.

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