TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3109 SUBJECT: GRB050223: analysis of the XMM-Newton observation DATE: 05/03/17 18:25:00 GMT FROM: Andrea De Luca at IASF-CNR,Milano 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. This message may be cited.