TITLE: GCN CIRCULAR NUMBER: 23334 SUBJECT: GRB 181013A: Swift-XRT afterglow detection DATE: 18/10/13 07:02:02 GMT FROM: Paolo D'Avanzo at INAF-OAB J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), P. D'Avanzo (INAF-OAB), V. D'Elia (ASDC), A. D'Ai (INAF-IASFPA), J.A. Kennea (PSU), B. Sbarufatti (INAF-OAB/PSU), D.N. Burrows (PSU), A.P. Beardmore (U. Leicester) and P. D'Avanzo (INAF-OAB) report on behalf of the Swift-XRT team: We have analysed 190 s of Photon Counting (PC) mode XRT data for the Swift/BAT-detected burst GRB 181013A (D'Avanzo et al. GCN Circ. 23332), collected between T0+115 s and T0+306 s. An uncatalogued X-ray source is detected consistent with being within 296 arcsec of the Swift/BAT position and is above the RASS limit and fading with 3-sigma significance, and is therefore likely the GRB afterglow. The position of this source is RA, Dec=207.6518, +40.1082 which is equivalent to: RA (J2000): 13:50:36.43 Dec(J2000): +40:06:29.5 with an uncertainty of 3.9 arcsec (radius, 90% confidence). This position is 4.8 arcmin from the Swift/BAT position. The light curve can be modelled with a power-law decay with a decay index of alpha=7.6 (+0.4, -1.0). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 0.98 (+0.30, -0.19). The best-fitting absorption column is consistent with the Galactic value of 9.4 x 10^19 cm^-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 6.2 x 10^-11 (6.2 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 9 (+/-90) x 10^19 cm^-2 Galactic foreground: 9.4 x 10^19 cm^-2 Excess significance: <1.6 sigma Photon index: 0.98 (+0.30, -0.19) If the light curve continues to decay with a power-law decay index of 7.6, the count rate at T+24 hours will be 5.9 x 10^-21 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 3.6 x 10^-31 (3.6 x 10^-31) erg cm^-2 s^-1. The results of the XRT-team automatic analysis of the likely afterglow are at http://www.swift.ac.uk/ToO_GRBs/00866783/Source1.php. The results of the full analysis of the XRT observations are available at http://www.swift.ac.uk/ToO_GRBs/00866783. This circular is an official product of the Swift-XRT team.