TITLE: GCN CIRCULAR NUMBER: 23474 SUBJECT: GRB 181201A: Swift-XRT afterglow detection DATE: 18/12/01 14:17:02 GMT FROM: Kim Page at U.of Leicester K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), V. D'Elia (ASDC), A. D'Ai (INAF-IASFPA), A. Melandri (INAF-OAB), S. J. LaPorte (PSU), J.A. Kennea (PSU), 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 181201A (Mereghetti et al. GCN Circ. 23469), collecting 5.0 ks of Photon Counting (PC) mode data between T0+14.3 ks and T0+26.4 ks. Five uncatalogued X-ray sources are detected, of which one ("Source 5") is above the RASS limit, and is therefore likely the GRB afterglow. Using 4968 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 = 319.29666, -12.63082 which is equivalent to: RA (J2000): 21h 17m 11.20s Dec(J2000): -12d 37' 51.0" with an uncertainty of 2.0 arcsec (radius, 90% confidence). This position is 28 arcsec from the INTEGRAL position. The light curve can be modelled with a power-law decay with a decay index of alpha=1.44 (+/-0.18). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.74 (+0.08, -0.07). The best-fitting absorption column is 6.1 (+2.0, -0.9) x 10^20 cm^-2, consistent with the Galactic value of 5.2 x 10^20 cm^-2 (Willingale et al. 2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.8 x 10^-11 (4.2 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 6.1 (+2.0, -0.9) x 10^20 cm^-2 Galactic foreground: 5.2 x 10^20 cm^-2 Excess significance: <1.6 sigma Photon index: 1.74 (+0.08, -0.07) If the light curve continues to decay with a power-law decay index of 1.44, the count rate at T+24 hours will be 0.30 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 1.2 x 10^-11 (1.3 x 10^-11) erg cm^-2 s^-1. The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00020848. The results of the full analysis of the XRT observations are available at http://www.swift.ac.uk/ToO_GRBs/00020848. This circular is an official product of the Swift-XRT team.