GCN Circular 8791
G. Stratta (ASDC), M. Perri (ASDC) and J. R. Cummings (GSFC/UMBC) report on behalf of the Swift-XRT team: The Swift ToO observations of GRB 090107B started on 2009 Jan 7th at 17:30:50 UT. XRT data set consists of 5.7 ks exposure in PC mode. The single, uncatalogued X-ray source observed in the field of INTEGRAL, previously reported by Perri et al. (GCN Circ. 8789), shows fading flux, confirming its afterglow nature. The 0.3-10 keV light curve from T + 4.2 ks to T + 96.4 ks possibly indicates evidence for a plateau phase followed by a steeper decay, but the available data prevented us from constraining the parameters (i.e. the temporal break and the two decay indexes). Assuming a simple power law model, the best fit decay index is alpha = 1.3 +/- 0.1. The average spectrum accumulated in the same time interval was fitted with an absorbed power law. The best fit photon index is 1.8 +/- 0.4. The spectrum shows evidence of absorption at the level of (9 +/- 1)e20 cm-2, in excess of the estimated Galactic value (5e20 cm-2, Kalberla et al. 2005). The average observed[unabsorbed] 0.3-10 keV average flux is about 2.8[3.6]e-12 ergs cm-2 s-1. The predicted count rate 48 hours after the trigger is 0.002 cts/s The count-rate-to-flux conversion factor is 5.2e-11 ergs cm-2 cts-1 All quoted errors are at 90% confidence level. Burst Advocate for this burst is J. R. Cummings. This circular is an official product of the Swift-XRT team.