TITLE: GCN CIRCULAR NUMBER: 7608 SUBJECT: GRB 080413B: Swift XRT refined analysis DATE: 08/04/13 18:55:01 GMT FROM: Eleonora Troja at INAF-IASFPA E. Troja (U. Leicester/INAF-IASFPa) and M. Stamatikos (NASA/ORAU) report on behalf of the Swift-XRT team: We have analysed 9.9 ks of Swift XRT data for GRB 080413B (Stamatikos et al. GCN Circ. 7598), from T0+134 s to T0+20.1 ks. The data set consists of 376 s exposure in Windowed Timing (WT) mode followed by 9.5 ks exposure in Photon Counting (PC) mode. By using orbits 2-4 of PC data [from T0+6.0 ks to T0+20.1 ks] in order to avoid pile-up effects, we derive an XRT refined position RA (2000) = 21h 44m 34.43s = 326.1435 d Dec (2000) = -19d 58' 52.9" = -19.9814 d with an uncertainty of 3.7 arcsec (90% containment). This position is 3.6 arcsec from the UVOT candidate afterglow (GCN Circ. 7598), and 3.4 arcsec from the source reported in Kruehler et al. (GCN Circ. 7599). The X-ray light curve is well fitted by a simple power-law, with a decay slope of 0.88 +/- 0.06. A spectrum obtained from the WT mode data, from T0+135 s to T0+511 s, can be modelled with an absorbed power-law with a photon index of 2.05 +/- 0.10 , and a redshifted absorbing column of (0.32 +/- 0.13) x 10^22 cm^-2 at a redshift of z=1.1 (Vreeswijk et al. GCN Circ 7601) in addition to the Galactic column of 3.06 x 10^20 cm^-2 (Kalberla et al. 2005). The observed (unabsorbed) 0.3-10 keV flux in this spectrum is 1.79 x 10^-10 (2.13 x 10^-10) erg/cm^2/s. The PC spectrum of the first four orbits, from T0+512 s to T0+20.1 ks, can be fit by an absorbed power-law model with photon index 1.93 +/- 0.08 and a column density consistent with the WT spectrum. All the reported errors are at the 90% confidence level. If the source continues to decay at the present rate, we predict a count rate of 0.035 count/sec at T0+24 hours, which corresponds to a 0.3-10 keV observed (unabsorbed) flux of 1.6 x 10^-12 (1.8 x 10^-12) erg/cm^2/s. This circular is an official product of the Swift-XRT team.