TITLE: GCN CIRCULAR NUMBER: 8429 SUBJECT: GRB 081028: Swift-XRT refined analysis DATE: 08/10/28 12:05:18 GMT FROM: Cristiano Guidorzi at INAF-OAB C. Guidorzi, R. Margutti, J. Mao (INAF-OAB) on behalf of the Swift-XRT team report: We have analysed the first 6 orbits of Swift-XRT data obtained for GRB 081028 (Guidorzi et al., GCN Circ. 8421), covering 650 s of Windowed Timing and 12.5 ks of Photon Counting (PC) mode data, between 197 s and 29 ks after the trigger. The UVOT-enhanced XRT position was given by Evans et al. in GCN Circ. 8427. The light-curve shows a very complex evolution. In the first orbit data, covered entirely in WT mode, from 200 to 300 s it can be fit with a power law decay with an index of ~0.5. This temporally coincides with the decay of the second peak observed by BAT, which peaked at 220 s (Barthelmy et al. GCN Circ. 8428). Afterwards, there is flaring activity up to 400 s. From 400 to 700 s the afterglow undergoes a steep decay with a power-law index of 3.4, followed by a large flare with a peak rate of ~30 counts/s and lasting longer than 150 s. First orbit data stopped during the peak of this flare. From 4.1 to 8 ks the curve exhibits a steep decay with a power-law index of 3.8 +- 1.2 and the count rate drops to a value of ~5e-2 counts/s, after which it experiences a long rebrightening (at least up to 29 ks) with flaring activity, reaching a peak count rate of ~0.3 counts/s. During the first orbit, the WT data spectrum undergoes a significant hard-to-soft evolution, with Gamma changing from 1 to 3. A WT spectrum extracted during the steep decay from 400 to 700 s can be modelled with an absorbed power-law, with Gamma = 2.51 +- 0.07 and NH = (8.0 +- 1.1)e20 cm^-2, in excess of the Galactic column in this direction of 4.0e20 cm^-2. The observed (unabsorbed) flux over this time interval is 7.8e-10 (1.1e-9) erg cm^-2 s^-1. A PC spectrum extracted from 5.0 to 23.2 ks can be modelled with an absorbed power-law, with Gamma = 2.25 +- 0.16 and NH = (9.3 +- 3.4)e20 cm^-2. The observed (unabsorbed) flux over this time interval is 7.3e-12 (1.0e-11) erg cm^-2 s^-1. Presently, given the ongoing flaring behaviour, we cannot provide a reliable projection of the count rate over the next hours. The results of the xrt automatic analysis are available at http://www.swift.ac.uk/xrt_products/00332851. This is an official product of the Swift-XRT team.