GCN Circular 10992
V. Mangano (INAF IASF-PA), B. Sbarufatti (INAF OAB/INAF IASF-PA) and W. H. Baumgartner (GSFC/UMBC) report on behalf of the Swift-XRT team: We have analysed 10 ks of XRT data for GRB 100725B (Baumgartner et al. GCN Circ. 10983), from 68 s to 58.8 ks after the BAT trigger. The data comprise 288 s in Windowed Timing (WT) mode (the first 8 s were taken while Swift was slewing) with the remainder in Photon Counting (PC) mode. The light curve shows flaring activity for the initial 300 s (i.e. the WT observation), then can be modeled with a series of power-law decays and a small Gaussian flare. The initial decay index is alpha=4.66 (+0.4 -0.2). The Gaussian flare peaks at T+780 (+/-37) s with a sigma of 145 (+/- 30) s. At T+770 s the decay change in a possible slow rise with an alpha of 0.30 (+/- 0.27) before breaking again at T+8660 s to a final decay with index alpha=1.3 (+/- 0.2). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.75 (+/-0.06). The best-fitting absorption column is 4.7 (+0.4, -0.3) x 10^21 cm^-2, in excess of the Galactic value of 5.4 x 10^20 cm^-2 (Kalberla et al. 2005). The PC mode spectrum (extracted from T+359 s to T+6761 s, 2.9 ks exposure) has a photon index of 2.5 (+/-0.2) and a best-fitting absorption column of 5.1 (+0.7, -0.6) x 10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 4.1 x 10^-11 (1.1 x 10^-10) erg cm^-2 count^-1. If the light curve continues to decay with a power-law decay index of 1.3, the count rate at T+48 hours will be 4.3 x 10^-3 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.8 x 10-13 (4.7 x 10-13) erg cm^-2 s^-1. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00429969. This circular is an official product of the Swift-XRT team.