TITLE: GCN CIRCULAR NUMBER: 7459 SUBJECT: GRB 080319B: Swift-XRT Team Refined Analysis DATE: 08/03/19 20:06:16 GMT FROM: Judith Racusin at PSU J. Racusin (PSU), D. N. Burrows (PSU), P. A. Evans (U Leicester), C. Pagani (PSU), report on behalf of the Swift-XRT Team: We have analysed the first four orbits of Swift-XRT data obtained for GRB 080319B (Racusin et al. GCN Circ. 7427), totaling 1.1 ks of Windowed Timing (WT) data beginning 64 s after the BAT trigger, and 4 ks of Photon Counting (PC) data beginning 5 ks after the BAT trigger. The UVOT-enhanced XRT position has been given by Evans et al. in GCN Circ 7449. The bright X-ray light-curve can be fit by a broken power-law, with an initial decay index of 1.46 +/- 0.01 followed, after a break at 6100 +/- 440 seconds, by a steeper decay index of 2.48 +/- 0.10. Both the WT and PC spectra are strongly affected by photon pile-up, which can alter the spectral fits. To eliminate these effects, we exclude the central 8 pixel radius region of the WT data, and the central 3 pixel radius region of the PC data when creating the spectra. Preliminary fits to the WT spectrum (64-4943 seconds), modeled with an absorbed power-law, result in a photon index of 1.65 +/- 0.02 and an absorbing column at z=0.937 (Vreeswijk et al., GCN Circ. 7444) of NH = (5.28 +/- 1.03)e20 cm^-2, in addition to Galactic absorption of 1.12e20 cm^-2 in the direction of the burst, with a reduced Chi^2=1.05. The PC spectrum (5-19 ks) can be modeled as an absorbed power-law, with photon index of 1.92 +/- 0.07 and an absorbing column at z=0.937 of NH = (23.5 +/- 5.6)e20 cm^-2, in addition to the Galactic absorption, with a reduced Chi^2=1.06. If we freeze the NH in the PC model to the WT fitted value, we fit a photon index of 1.73 +/ 0.04, with a reduced Chi^2=1.4, which excludes this model as a viable fit. As another attempt to model the spectra without NH evolution, we freeze the WT NH to the value from the PC spectral fits and add an additional low energy thermal component. The resulting fits yield a photon index of 1.73 +/- 0.02, kT=0.06 +/- 0.01 keV, and a reduced Chi^2=1.07. Therefore, the extra thermal component is a possible explanation for the apparent spectral evolution. Assuming the source continues to decay with the same decay index of 2.5, we predict an XRT count rate of 8.8e-4 counts/s at T+24 hours, which corresponds to an 0.3-10.0 keV observed (unabsorbed) flux of 6.1e-14 (6.2e-14) ergs cm^-2 s^-1. This circular is an official product of the Swift-XRT team.