GCN Circular 6227
P.A. Evans, A.P. Beardmore, M.R. Goad, K.L. Page, R.L.C. Starling, L.G. Tyler (U. Leicester) and C.B. Markwardt (GSFC/UMD) report on behalf of the Swift/XRT team: We have analysed the first two orbits of Swift-XRT data obtained for GRB 070328 (Markwardt et al., GCN Circ 6224) which includes 1.3 ks of Windowed Timing (WT) mode data and 2.2 ks of Photon Counting (PC) mode data. Using the PC mode data we find the following astrometrically corrected XRT refined position (by matching the UVOT images with the USNO-B1 catalogue): RA, Dec = 65.1150, -34.0668 deg, which is equivalent to: RA(J2000) = 04 20 27.60 Dec(J2000) = -34 04 00.4 with an estimated error radius of 2.31 arcsec (90% confidence). This lies 39.7 and 8.4 arcsec from the refined BAT and onboard XRT positions given in GCN Circs. 6225 and 6224 respectively. The XRT light curve begins 95 s after the trigger, and decays as a power-law with alpha1=1.33 +0.25/-0.12 until 184 +10/-20 s after the trigger. At this point the decay shallows to alpha2=0.28 +/-0.06. This shallow decay continues until 629 +33/-27 s after the trigger, when the lightcurve breaks again to alpha3=1.34 +/-0.03. All errors are quoted at the 90% confidence level. The WT spectrum can be modelled with a power-law of Gamma=2.22 +/-0.03 with a total absorbing column of 2.6 +/-0.4x10^21 cm-2, compared to the Galactic value of 2.1x10^20 cm^-2. The PC mode data give almost identical values, however with larger errors as there are less counts in the spectrum. If the decay continues with a slope of alpha=1.34 the count-rate at 24 hours post-trigger is predicted to be 0.029 counts s^-1. This corresponds to an observed (unabsorbed) 0.3-10 keV flux of 2.74e-12 (4.96e-12) erg cm^-2 s^-1 . This circular is an official product of the Swift/XRT team.