TITLE: GCN CIRCULAR NUMBER: 6172 SUBJECT: GRB 070306: XRT Team Refined Analysis DATE: 07/03/06 23:32:49 GMT FROM: Kim Page at U.of Leicester K.L. Page (U. Leicester) and S.B. Pandey (UCL-MSSL) report on behalf of the Swift-XRT team: We have analysed the first three orbits of Swift-XRT data obtained for GRB 070306 (Pandey et al., GCN Circ. 6169), which includes 156 s of Windowed Timing (WT) data and 3.3 ks of Photon Counting (PC) mode data. Using the PC data we derive a refined position of RA, Dec = 148.0974, 10.4820, which is equivalent to RA(J2000) = 09 52 23.39 Dec(J2000) = +10 28 55.3 with an estimated error radius of 3.6 arcsec (90% confidence). This lies 67 arcsec and 3.3 arcsec from the onboard BAT and XRT positions given in GCN Circ. 6169, respectively. The XRT light-curve initially decays between ~155 and 165 seconds after the trigger, followed by a small increase in count rate until 180 seconds; after this time, the curve shows a steep decay, with alpha_1 = 6.3 +/- 0.2 until 411 +/- 17 seconds post-trigger, at which point the decay flattens to alpha_2 = 0.22 +/- 0.08. All error bars are at the 90% confidence level. Note that Swift triggered on a precursor; if T0 for the light-curve fits were to be referenced to the main peak of the burst, around 100 seconds later, this initial decay would not be so steep. The WT spectrum can be modelled by a power-law of Gamma = 2.29 +/- 0.05, with a total absorbing column of 3.9x10^21 cm^-2, compared to the Galactic column of 2.94x10^20 cm^-2 in this direction. This value of NH was calculated from the later PC data (Gamma = 2.14 +/- 0.17), when the spectral evolution apparent during the initial steep decay has ceased. The mean observed (unabsorbed) flux over 0.3-10 keV for this WT spectrum is 1.73e-9 (3.48e-9) erg cm^-2 s^-1. If the decay continues with a slope of alpha = 0.22, the count rate is predicted to be 0.21 count s^-1 at 24 hours. This corresponds to an observed flux of 1.0e-11 erg cm^-2 s^-1 (over 0.3-10 keV), with the unabsorbed value being 1.9e-11 erg cm^-2 s^-1, using the spectral fit to the PC data. This circular is an official product of the XRT team.