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GCN Circular 3109

Subject
GRB050223: analysis of the XMM-Newton observation
Date
2005-03-17T18:25:00Z (19 years ago)
From
Andrea De Luca at IASF-CNR,Milano <deluca@mi.iasf.cnr.it>
A. De Luca (IASF-Mi), S. Campana (OAB) on behalf of a larger
collaboration report:

We have analyzed the data from the XMM-Newton observation of
GRB050223, discovered by Swift on 2005, Feb 23, 03:09:06 UTC
(Giommi et al., GCN3054).

The XMM-Newton observation started on 2005, Feb. 23 at 13:04 UT
and ended on 2005, Feb 24 at 06:14:20, with a gap between
Feb. 23 16:54 UT and Feb. 23 18:56 UT  due to a ground station
outage. A first account of results, based on the analysis of
preliminary data, have been presented by the XMM-Newton SOC team
(http://xmm.vilspa.esa.es/external/xmm_news/items/grb050223/index.shtml).

Observation Data Files have been produced and released only for
the part of the observation starting from Feb. 23 18:55:57 UT
(after the gap) and lasting for 41 ks.

The whole observation is badly affected by low-energy particle
background, dramatically reducing the signal to noise, especially
in the data from the back-illuminated pn detector, which is
particularly sensitive to such background component.
This hampers a detailed study of the spectral and temporal
phenomenology of the faint source XMMU J180532.5-622821
(Gonzalez-Riestra et al., GCN 3060), the likely afterglow
of GRB050223.

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

RA: 18h 05m 32.49s  Dec: -62d 28' 21.07"

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 fully consistent with the XRT coordinates
(Giommi et al., GCN 3054) as well as with the preliminary XMM-Newton
position by Gonzalez-Riestra et al. (GCN 3060).

The EPIC light curve in the 0.5-2 keV range (where the signal-to-noise
is maximum) shows hints that the source is fading, following
a power law decay with index in the range 0.5-3 (90% confidence level).

We extracted time-averaged spectra from the three EPIC cameras and we
generated ad-hoc response and effective area files. A slightly absorbed
(NH<10^21 cm^-2, consistent with the Galactic column density in this
direction, 7x10^20  cm^-2) power law model with photon index
1.6(+0.6,-0.4) gives an acceptable description of the data in the 0.3-4
keV range, with a reduced chi2 of 1.5 (32 d.o.f). The quoted errors are
at 90% conf. level for a single interesting parameter.
The not optimal quality of the fit is most likely due to the heavy
background affecting the observation.
The observed flux (0.2-10 keV) is of ~3.7x10^-14 ergs cm^-2 s^-1;
the corresponding unabsorbed flux is ~4x10^-14 ergs cm^-2 s^-1.

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