TITLE: GCN CIRCULAR NUMBER: 6646 SUBJECT: GRB 070721B: Swift-XRT refined analysis DATE: 07/07/21 18:32:41 GMT FROM: Andy Beardmore at U Leicester A.P. Beardmore, K.L. Page, P.A. Evans (U. Leicester) and H. Ziaeepour (MSSL-UCL) report on behalf of the Swift-XRT team : We have analysed the first three orbits of Swift-XRT data on GRB 070721B (Ziaeepour et al., GCN 6640). Using 5.78ks of Photon Counting mode data, we find a refined XRT position of RA, Dec = 33.13710, -2.19462 which is RA(J2000): 02:12:32.90 Dec(J2000): -02:11:40.6 with an uncertainty of 3.5 arcseconds (90% containment radius). This is 35 arcseconds from the refined BAT position reported by Palmer et al. (GCN 6643), 8.9 arcsec from the original XRT position reported in Ziaeepour et al. (GCN 6640), and 0.7 arcseconds from the UVOT position reported by Schady (GCN 6641). The X-ray light curve shows an initial steep decay from T+100s to T+144s, followed by a number of flares from T+255s to T+800s which reached a maximum count rate of 50 count/s at T+315s. The underlying decay is a powerlaw with a slope of approximately -0.9, though it is difficult to be precise with the data available so far. The X-ray spectrum from the Windowed Timing mode data obtained during the non-flare intervals from the 1st orbit (T+144s to T+220s and T+400s to T+475s) can be fit with an absorbed powerlaw to give a photon index of 1.48 +0.18-0.16 and a column density of 1.9 (+1.8-1.9) e20 cm^-2, consistent with the Galactic value of 2.3e20 cm^-2 in this direction (Kalberla et al. 2005). The observed 0.3-10 keV flux is 2.37+/-0.25 e-10 ergs cm^-2 s^-1, which corresponds to an unabsorbed flux of 2.45+/-0.25 e-10 ergs cm^-2 s^-1. If the underlying decay continues at the same rate, we predict an XRT count rate of 0.025 count/s at T+24hr, which corresponds to an observed 0.3-10keV flux of 1.4e-12 ergs cm^-2 s^-1. This circular is an official product of the Swift-XRT team.