TITLE: GCN CIRCULAR NUMBER: 20025 SUBJECT: GRB 161010A: Swift-XRT afterglow detection DATE: 16/10/11 07:31:55 GMT FROM: Antonino D'Ai at IASF-PA L.M. McCauley (PSU), J.A. Kennea (PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), B. Mingo (U. Leicester), A. Melandri (INAF-OAB), P. D'Avanzo (INAF-OAB), V. D'Elia (ASDC), B. Sbarufatti (INAF-OAB/PSU) and P.A. Evans (U. Leicester) report on behalf of the Swift-XRT team: Swift-XRT has performed follow-up observations of the INTEGRAL-detected burst GRB 161010A (Mereghetti et al. GCN Circ. 20021), collecting 5.0 ks of Photon Counting (PC) mode data between T0+12.0 ks and T0+30.3 ks. An uncatalogued X-ray source is detected consistent with being within 148 arcsec of the INTEGRAL position and is above the RASS limit and fading with 3-sigma significance, and is therefore likely the GRB afterglow. Using 123 s of PC mode data and 1 UVOT image, we find an enhanced XRT position (using the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue): RA, Dec = 275.21435, -28.78515 which is equivalent to: RA (J2000): 18h 20m 51.44s Dec(J2000): -28d 47' 06.6" with an uncertainty of 2.9 arcsec (radius, 90% confidence). This position is 12 arcsec from the INTEGRAL position. The light curve can be modelled with a power-law decay with a decay index of alpha=1.36 (+0.22, -0.21). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.78 (+0.18, -0.17). The best-fitting absorption column is 3.1 (+0.9, -0.6) x 10^21 cm^-2, consistent with the Galactic value of 2.5 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 4.2 x 10^-11 (5.7 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 3.1 (+0.9, -0.6) x 10^21 cm^-2 Galactic foreground: 2.5 x 10^21 cm^-2 Excess significance: <1.6 sigma Photon index: 1.78 (+0.18, -0.17) If the light curve continues to decay with a power-law decay index of 1.36, the count rate at T+24 hours will be 0.023 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 9.9 x 10^-13 (1.3 x 10^-12) 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/00020702/index_18.php. The results of the full analysis of the XRT observations are available at http://www.swift.ac.uk/ToO_GRBs/00020702. This circular is an official product of the Swift-XRT team.