TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  4005
SUBJECT: GRB050820B: analysis of the XMM-Newton observation
DATE:    05/09/20 16:59:46 GMT
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 GRB050820B, discovered by Swift (Page et al., GCN3839)
on 2005, August 20 at  23:50:27 UT.

The XMM-Newton observation started on 2005, August 21 at 
05:21:30 UT (~5h 30min after the GRB) and lasted for 58.5 ks.
We report here on the analysis of the data collected by
the EPIC instrument.

As reported by Rodriguez & Calderon (GCN3844),
the afterglow of GRB050820B is detected in all the EPIC cameras.
The background-subtracted, time-averaged count rate in the pn camera, 
estimated from a 25" radius extraction region (containing ~80% 
of the total counts), is 0.014+/-0.001 cts/s in the 0.2-8 keV
energy range.

We improved the astrometry of the XMM-Newton/EPIC images
by matching X-ray sources in the field to stars in the USNO-B1
catalogue. The refined position (J2000) for the X-ray afterglow is

RA: 09h 02m 25.03s Dec: -72d 38' 44.0" 

The 1 sigma error radius is 1.5 arcsec (including the rms error on
the cross-correlation as well as systematic uncertainties in the
optical catalogue). The position is consistent with the XRT
coordinates reported by Burrows (GCN3842).

The afterglow is clearly seen to fade along the XMM-Newton
observation, spanning the time range 20-79 ks after
the GRB. The background-subtracted light curve (0.3-3 keV)
decays as a power law with index delta=1.55+/-0.15 (90% c.l.)
(reduced chi2=1.3, 28 d.o.f.).

We extracted time-averaged spectra from the three EPIC cameras 
and we generated ad-hoc response files. 
We quote here errors at 90% confidence level for a single interesting
parameter.
A simultaneous fit with an absorbed power law model yields a good
description of the data (reduced chi2=0.90, 67 d.o.f.). The best fit value 
for the NH is 1.6+/-0.4x10^21 cm^-2, somewhat higher than the expected
Galactic value in the burst direction (NH~8x10^20 cm^-2, Dickey & Lockman, 
1990); the best fitting power law photon index is Gamma=2.3+/-0.2.
The observed flux in the 0.2-10 keV range is 4.1x10^-14 erg cm^-2 s^-1,
corresponding to an unabsorbed flux of 7.7x10^-14 erg cm^-2 s^-1.

No significant spectral variation as a function of the time is
found in the data.