TITLE: GCN GRB OBSERVATION REPORT NUMBER: 5018 SUBJECT: GRB060428A: Swift XRT Team refined analysis DATE: 06/04/28 10:45:33 GMT FROM: Vanessa Mangano at INAF-IASFPA V. Mangano (INAF-IASFPA), E. Troja (INAF-IASFPA), D. N. Burrows (PSU) report on behalf of the Swift XRT Team: We have analyzed the first orbit data of GRB060428A (Mangano et al 2006, GCN 5014). The XRT data set consists of 39 s exposure in Windowed Timing (WT) mode followed by 1670 s exposure in Photon Counting (PC) mode. The refined position of the source is RA (2000) = 08h 14m 10.98s Dec (2000) = -37d 10' 10.3" with an uncertainty of 3.6 arcsec (90% confinement). This position is only 1.5 arcsec from the onboard detected position (Mangano et al 2006, GCN 5014). The 0.2-10 keV X-ray light curve shows an initial very fast decay with slope -6.3 +/- 0.4, a break at 144 +/- 6 s from the trigger and a flattening with a decay slope of -0.11 +/- 0.03. A small flare with a gaussian profile is present at 676 +/- 56 s after the trigger. The WT and PC spectra were fitted separately with an absorbed power law model and showed photon indices of 3.2 +/- 0.4 and 1.9 +/- 0.2, respectively (90% confidence level). The WT spectrum, covering the time interval 77-116 s, mostly accounts for the pre-break phase of the light curve. Both spectra are highly absorbed, but do not show evidence of an absorption column in excess with respect to the Galactic value of 8.5 x 10^21 cm^-2. The 0.2-10 keV unabsorbed fluxes of the two spectra are 2.9 x 10^-9 and 7.6 x 10^-11 erg cm^-2 s^-1, respectively. If decaying at the present rate the source will be at the flux level of 2.7 x 10^-11 erg cm^-2 s^-1 (corresponding to a count rate level of 0.18 counts s^-1) after one day. This Circular is an official product of the Swift XRT Team.