GCN Circular 3109

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
Not found
 
 
).

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
Not found
 
 
), 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
Not found
 
 
) as well as with the preliminary XMM-Newton
position by Gonzalez-Riestra et al. (GCN 3060
Not found
 
 
).

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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