GCN Circular 10024
R. Margutti (INAF-OAB) reports on behalf of the Swift-XRT team: The Swift ToO observations of the SuperAGILE GRB 091010 (Donnarumma et al., GCN Circ. 10004; Feroci, GCN Circ. 10005) started on 2009 Oct 10th at 14:48:26 UT. The XRT data set consists of 9 ks exposure in PC mode. The UVOT-enhanced XRT position was given by Evans et al. in GCN Circ. 10012. The X-ray source reported by Margutti & Guidorzi, GCN Circ. 10006 has faded to a level of (2.4 +/- 1.0)e-3 count/sec, with a best fit power-law decay index of alpha=-1.4 +/- 0.6 . This source is thus confirmed as the afterglow of GRB 091010. The spectrum extracted in the time interval 43-50 ks contains about 170 photons and can be fitted using an absorbed power law model. Spectral channels have been grouped so as to have 5 counts per bin and then weighted using the Churazov method (Churazov et al. 1996). The best fit photon index is found to be 2.3 +/- 0.4. The spectrum shows evidence of absorption at the level of (1.9 +/- 0.1)e21 cm-2, in excess of the Galactic value in the direction of the burst which is 8.1e20 cm-2, Kalberla et al. 2005. The observed (unabsorbed) flux over this time interval is 1.5e-12 (2.8e-12) erg cm^-2 s^-1. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.6 x 10^-11 (6.6 x 10^-11) erg cm^-2 count^-1. All quoted errors are at 90% confidence level. The results of the xrt automatic analysis are available at http://www.swift.ac.uk/xrt_products/00020116. This is an official product of the Swift-XRT team.