TITLE: GCN CIRCULAR NUMBER: 22174 SUBJECT: GRB 171124A: Swift-XRT afterglow detection DATE: 17/12/01 11:21:08 GMT FROM: Phil Evans at U of Leicester J.P. Osborne (U. Leicester), A. D'Ai (INAF-IASFPA), A. Melandri (INAF-OAB), P. D'Avanzo (INAF-OAB), D.N. Burrows (PSU), A. Tohuvavohu (PSU), S. J. LaPorte (PSU), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester) and P.A. Evans (U. Leicester) report on behalf of the Swift-XRT team: Swift-XRT has conducted further observations of the field of the Fermi/LAT-detected burst GRB 171124A (Longo et al. GCN Circ. 22156). The observations now extend from T0+21.8 ks to T0+579.1 ks. Of the sources reported by Osborne et al. (GCN Circ. 22165), "Source 2" is fading with 3-sigma significance, and is therefore likely the GRB afterglow. Using 4982 s of PC mode data and 3 UVOT images, we find an enhanced XRT position (using the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue): RA, Dec = 335.47598, +35.33213 which is equivalent to: RA (J2000): 22h 21m 54.24s Dec(J2000): +35d 19' 55.7" with an uncertainty of 2.3 arcsec (radius, 90% confidence). This position is 7.1 arcmin from the Fermi/LAT position. The source is fading with alpha >0.7. A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.8 (+0.8, -0.7). The best-fitting absorption column is 7.9 (+4.7, -3.5) x 10^21 cm^-2, in excess of the Galactic value of 1.3 x 10^21 cm^-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 2.9 x 10^-11 (1.0 x 10^-10) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 7.9 (+4.7, -3.5) x 10^21 cm^-2 Galactic foreground: 1.3 x 10^21 cm^-2 Excess significance: 3.1 sigma Photon index: 2.8 (+0.8, -0.7) The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00020784. The results of the full analysis of the XRT observations are available at http://www.swift.ac.uk/ToO_GRBs/00020784. This circular is an official product of the Swift-XRT team.