TITLE: GCN CIRCULAR NUMBER: 6360 SUBJECT: GRB 070429B: Swift XRT refined analysis DATE: 07/04/29 13:19:19 GMT FROM: Andy Beardmore at U Leicester A.P. Beardmore, K.L. Page, P.A. Evans, R.L.C. Starling (U. of Leicester), C. Guidorzi (INAF-OAB, Univ. Bicocca), and C. Markwardt (GSFC/UMD) report on behalf of the Swift XRT team : We have analysed the first three orbits of Swift XRT data from GRB 070429B (trigger number 277582, Markwardt et al. GCN circular 6358), which has a total photon counting mode exposure of 5.8ks. Due to a delayed slew because of an Earth-limb constraint, the XRT arrived on the source 244s after the BAT trigger. The tentative X-ray source identified in GCN 6358 is clearly detected in the first orbit of data, and is absent thereafter. The refined XRT position from 2.4ks of data in the first orbit is RA, DEC (J2000) = 328.0159, -38.8284 which is RA(J2000) = 21d 52m 03.82s DEC(J2000) = -38d 49' 42.2" with an uncertainty of 5.1 arcsec (radius, 90% containment). This is 61 arcsec from the inital BAT position and 6.2 arcsec from the initial X-ray position reported in GCN 6358. An XRT light curve created from the first orbit of data, binned with a minimum of 10 counts/bin, reveals a decaying source. A powerlaw fit yields a poorly constrained decay slope of 0.94+/-0.47. The X-ray spectrum from the first orbit (covering 254 to 2691 seconds after the BAT trigger), modelled with an absorbed powerlaw and fit using Cash statistics, gives a photon index of 2.5+1.3-1.2. An upper limit of 5.9e21 cm^-2 on the column density was found, compared with the Galactic value of 1.8E+20 cm^-2 in this direction. The absorbed (unabsorbed) 0.3-10.0keV flux for this spectrum was 5.9E-13 (1.1E-12) ergs cm^-2 s^-1. Assuming the source continues to decay at the same rate, we predict an XRT count rate of 3.0e-4 count/s at T+24 hours, which corresponds to an observed (unabsorbed) flux of 1.5E-14 (2.8E-14) ergs cm^-2 s^-1. This circular is an official product of the Swift XRT team.