TITLE: GCN CIRCULAR NUMBER: 6192 SUBJECT: GRB 070311: Swift-XRT refined analysis DATE: 07/03/11 12:41:53 GMT FROM: Cristiano Guidorzi at INAF-OAB C. Guidorzi, P. Romano, S. Vergani and A. Moretti (Uni-Bicocca and OAB) report on behalf of the Swift-XRT team: We have analysed the first two orbits of Swift-XRT data obtained for the GRB 070311 detected by INTEGRAL (Mereghetti et al., GCN Circ. 6189), which includes 4.2 ks of Photon Counting (PC) mode data, beginning 7000 s after the burst trigger time. Using the PC data we found a fading source at the position of RA, Dec = 87.5351, 3.3750, which is equivalent to RA(J2000) = 05 50 08.43 Dec(J2000) = +03 22 30.0 with an estimated error radius of 3.8 arcsec (90% confidence). This lies 3.3 arcsec from the optical afterglow discovered by REM (Covino et al., GCN Circ. 6190) and confirmed by PAIRITEL (Bloom et al., GCN Circ. 6191). The XRT light-curve can be fit with a single power law with index of 1.9 ± 0.4 from 7 ks to 14 ks after the burst. The PC photon spectrum can be modelled by a power-law of Gamma=1.6 +/- 0.2, with a total absorbing column fixed to the Galactic value of 2.6x10^^21 cm^-2 in this direction. Alternatively, if the absorbing column is left free to vary, we find Gamma=1.9 +/- 0.4 and a column density of (3.8 +/- 1.6)x1021 cm^-2 (90% confidence). The mean observed (unabsorbed) flux over 0.3-10 keV over this period is 3.9e-12 (4.9e-12) erg cm^-2 s^-1. If the decay continues with the same rate as above, the count rate is predicted to be 1.1e-3 count s^-1 at 24 hours. This corresponds to an observed flux of 6.5e-14 erg cm^-2 s^-1 (over 0.3-10 keV), with the unabsorbed value being 7.9e-14 erg cm^-2 s^-1, using the spectral fit to the PC data. This circular is an official product of the XRT team.