TITLE: GCN GRB OBSERVATION REPORT NUMBER: 5031 SUBJECT: GRB060428B: Swift XRT Team refined analysis DATE: 06/04/28 21:03:14 GMT FROM: Eleonora Troja at INAF-IASFPA E. Troja (INAF-IASFPA), V. Mangano (INAF-IASFPA), P. Romano (INAF-OAB), D. N. Burrows (PSU) report on behalf of the Swift XRT Team: We have analyzed the XRT data of the first four orbits of GRB060428B (Campana et al 2006, GCN 5017). The data set consists of 205 s exposure in Windowed Timing (WT) mode followed by 8.4 ks exposure in Photon Counting (PC) mode. The refined position of the source is RA (2000) = 15h 41m 25.57s Dec (2000) = +62d 01' 28.94" with an uncertainty of 3.8 arcsec (90% confinement). This position was evaluated from PC data of the orbits 2-4, that are not affected by pile-up. This position is 3.35 arcsec from the onboard detected position (Campana et al 2006, GCN 5017) and 1.4 arcsec from the optical afterglow candidate found by Price et al 2006 (GCN 5019). To evaluete this offset we used the KAIT coordinates from Li et al.(GCN 5027). The 0.2-10 keV X-ray light curve shows an initial very fast decay with slope -4.4 +/- 0.07, a break at 689 +/- 28 s from the trigger and then a decay slope of -0.89 +/- 0.06. The WT spectrum from the first orbit was fitted with an absorbed power law model and showed a photon index of 2.82 +/- 0.08 (90% confidence level). The WT spectrum covers the 212-418 s time interval. The spectrum shows evidence of an absorption column at the level of (6.7 +/- 0.7) x 10^20 cm^-2 in excess with respect to the Galactic value of 1.7 x 10^20 cm^-2. The 0.2-10 keV unabsorbed flux is 9.8 x 10^-10 ergs cm^-2 s^-1. The PC spectrum of the orbits 2-4 (time interval 4.4-18.5 ks) is well fit by an absorbed powerlaw model with photon index 2.08 +/- 0.16. The 0.2-10 keV unabsorbed flux is 1.6^-12 ergs cm^-2 s^-1. If decaying at the present rate the source will reach the flux level of 2.6^-13 erg cm^-2 s^-1 (corresponding to a count rate level of 5 x 10^-3 counts s^-1) after one day. This Circular is an official product of the Swift XRT Team.