TITLE: GCN CIRCULAR NUMBER: 13943 SUBJECT: GRB 121102A: Swift-XRT refined Analysis DATE: 12/11/02 14:46:05 GMT FROM: Phil Evans at U of Leicester C. Pagani (U. Leicester), V. Mangano (INAF-IASFPA), A. Melandri (INAF-OAB), B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows (PSU), J.A. Kennea (PSU), M.C. Stroh (PSU), A.P. Beardmore (U. Leicester), P.A. Evans (U. Leicester) and V. D'Elia report on behalf of the Swift-XRT team: We have analysed 9.0 ks of XRT data for GRB 121102A (D'Elia et al. GCN Circ. 13938), from 53 s to 18.8 ks after the BAT trigger. The data comprise 203 s in Windowed Timing (WT) mode with the remainder in Photon Counting (PC) mode. The best available XRT position (using the promptly downlinked event data, the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue) is RA, Dec = 270.9009, -16.9576 which is equivalent to: RA (J2000): 18 03 36.22 Dec(J2000): -16 57 27.5 with an uncertainty of 2.0 arcsec (radius, 90% confidence). The light curve can be modelled with a series of power-law decays. The initial decay index is alpha=3.33 (+0.22, -0.24). At T+111 s the decay flattens to an alpha of 1.59 (+0.18, -0.11) before breaking again at T+696 s to a final decay with index alpha=1.04 (+0.08, -0.07). A spectrum formed from the WT mode data can be fitted with an absorbed power-law with a photon spectral index of 1.95 (+/-0.13). The best-fitting absorption column is 8.9 (+/-1.0) x 10^21 cm^-2, in excess of the Galactic value of 5.3 x 10^21 cm^-2 (Kalberla et al. 2005). The PC mode spectrum has a photon index of 1.74 (+0.23, -0.22) and a best-fitting absorption column of 7.7 (+1.9, -1.6) x 10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 5.9 x 10^-11 (9.6 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 7.7 (+1.9, -1.6) x 10^21 cm^-2 Galactic foreground: 5.3 x 10^21 cm^-2 Excess significance: 2.4 sigma Photon index: 1.74 (+0.23, -0.22) If the light curve continues to decay with a power-law decay index of 1.04, the count rate at T+24 hours will be 2.8 x 10^-3 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.7 x 10^-13 (2.7 x 10^-13) erg cm^-2 s^-1. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00537266. This circular is an official product of the Swift-XRT team.