GCN Circular 8470
V. Mangano, B. Sbarufatti, V. La Parola (INAF-IASF Pa), T. Ukwatta (GSFC/GWU) report on behalf of the Swift-XRT team: Swift-XRT began observing the field of GRB 081102 (trigger 333427, Ukwatta et al., GCN circ.8462) at 17:46:11.5 UT, 92.0 seconds after the BAT trigger. We analysed data from the first 10 orbits, consisting of 151 s in Windowed Timing mode, from T+98 s to T+251s and 14 ks in Photon Counting mode, from T+251s to T+42.1 ks. The lightcurve is best fit by a broken powerlaw model with slopes alpha_1 = -2.35 +/- 0.15 and alpha_2= -1.22 (-0.11 +0.22) and break time T_break = T+560 (-160 +500)s. After the break, we observe a strong flare at T+960 (-11+21) s with decay time 157 +/-15 s. The WT spectrum is best fit by a simple absorbed powerlaw with photon index 2.7 +/- 0.1 and an absorbing column consistent with the Galactic value of 4.9E21 cm^-2 (Kalberla et al. 2005). The observed (unabsorbed) flux in the 0.3 - 10 keV band is 4.0E-10 (1.3E-9) erg cm^-2 s^-1. The PC spectrum is best fit by a simple absorbed powerlaw with photon index 1.97 +/- 0.1 and an absorbing column consistent with the Galactic value. The observed (unabsorbed) flux in the 0.3 - 10 keV band is 1.1 (1.8)E-11 erg cm^-2 s^-1. The countrate to flux conversion factor is 7.1E-11. The predicted countrate for November 3, 17:45 UT (T+ 24h) is 1.3E-3 counts/s equivalent to a flux of 9.2E-14 erg cm^-2 s^-1. All quoted errors are at 90% confidence level. This circular is an official product of the Swift-XRT team.