TITLE: GCN GRB OBSERVATION REPORT NUMBER: 5215 SUBJECT: GRB 060604: Swift-XRT Team refined analysis DATE: 06/06/05 07:35:45 GMT FROM: Kim Page at U.of Leicester K.L. Page, E. Rol (U. Leicester) and M.J. Page (UCL-MSSL) report on behalf of the Swift-XRT team: We have analysed the first 3 orbits of X-ray data obtained for GRB 060604 (BAT trigger 213486). Using 5.1 ks of Photon Counting (PC) mode data, we find a refined position of: RA(J2000) = 22 28 54.97 Dec(J2000) = -10 54 59.9 with an uncertainty of 3.7 arcsec (90% containment). This is 3.3 and 4.1 arcsec from the on-board XRT and UVOT positions respectively (GCN 5212; Page et al.) and 74.6 arcsec from the ground-calculated BAT position given in GCN 5214 by Parsons et al. The source was at a high enough count-rate to remain in Windowed Timing (WT) mode for the whole of the (short) first orbit (117-255 seconds after the trigger). During this time, the light-curve shows two strong flares, peaking at ~137 and ~172 seconds after the burst. Underlying these flares is a steep decay of alpha_1 = 3.07 +/- 0.20. Between the end of the WT data and the start of the second orbit of (PC) data, there is a break to a much shallower slope of alpha_2 = 0.48 +/- 0.20. The WT flares show clear spectral evolution. The later PC data are well fitted by a single power-law with Gamma = 1.89 +/- 0.14, with no evidence for excess NH above the Galactic value of 4.57e20 cm^-2. (The 90% upper limit for any excess is 7e20 cm^-2.) Assuming the shallow decay contines, at 24 hours the count rate is predicted to be 0.030 count s^-1, corresponding to an observed (unabsorbed) flux of 1.40e-12 (1.57e-12) erg cm^-2 s^-1. This circular is an official product of the Swift-XRT team.