TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3920 SUBJECT: GRB 050904: early Swift XRT analysis results DATE: 05/09/05 10:55:21 GMT FROM: Teresa Mineo at INAF T. Mineo(INAF-IASF), V. Mangano(INAF-IASF), V. La Parola(INAF-IASF), G. Cusumano (INAF-IASF), L. Angelini (GSFC), J. Cummings (GSFC/NRC), D. N. Burrows (PSU), F. Marshall (GSFC), P. Boyd (GSFC-UMBC) report on behalf of the Swift XRT team: We have analyzed the Swift XRT data from the first observation of GRB 050904 (Cummings et al. 2005, GCN 3910) consisting of four orbits (about 20 ks). The refined coordinates of the X-ray afterglow are: RA(J2000) = 0h 54m 50.6s Dec(J2000) = +14d 05' 04.5" with an estimated uncertainty is of 6 arcseconds radius (90% containment). This position is 37 arcsec from the revised BAT position given in GCN 3918 (Palmer et al. 2005), 4.5 arcseconds from the XRT position determined on board and 11 arcseconds from the SOAR position (Haislip et al., GCN 3913). The 0.2-10 keV light curve, that starts in Windowed Timing (WT) mode 169 seconds after the BAT trigger (T0), shows a fading behaviour. Moreover two flares are clearly detected in the first orbit: the first at T0+466 s in WT data and the second at at T0+1240 s in Photon Counting (PC) mode data. The light curve decay index, obtained excluding the flare time intervals, is -2.08+/-0.03. The remaining three orbits show irregular rate variations likely due to other flares. A preliminary spectral fit to WT and PC data of the first orbit, excluding the flare time intervals, shows an evidence of spectral evolution from hard to soft: Time interval(s) Photon Index NHx10^20(cm^-2) Flux(erg cm^-2 s^-1) T0+170-370 1.33+/-0.04 15+/-1 1.8x10^-9 T0+580-1760 1.70+/-0.08 5.6+/-1.0 4.6x10^-11 The Galactic absorption in the GRB direction is 5x10^20 cm^-2. I.A.S.F. INAF Mailing System Outgoing mail is certified Virus Free. Checked by McAfee anti-virus system