TITLE: GCN CIRCULAR NUMBER: 27561 SUBJECT: GRB 200412B: Swift-XRT afterglow detection DATE: 20/04/12 23:35:46 GMT FROM: Boris Sbarufatti at PSU K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), E. Ambrosi (INAF-IASFPA) , M. Capalbi (INAF-IASFPA), M. Perri (ASDC), A. Tohuvavohu (U. Toronto), B. Sbarufatti (PSU), D.N. Burrows (PSU) and P.A. Evans (U. Leicester) report on behalf of the Swift-XRT team: Swift-XRT has performed follow-up observations of the Fermi/LAT-detected burst GRB 200412B (Longo et al. GCN Circ. 27557) in a series of observations tiled on the sky. The total exposure time is 4.9 ks, distributed over 7 tiles; the maximum exposure at a single sky location was 1.5 ks. The data were collected between T0+20.1 ks and T0+31.7 ks, and are entirely in Photon Counting (PC) mode. Two uncatalogued X-ray sources are detected, of which one ("Source 2") is above the RASS limit, and is therefore likely the GRB afterglow. Using 671 s of PC mode data and 2 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 = 278.31339, +62.53253 which is equivalent to: RA (J2000): 18h 33m 15.21s Dec(J2000): +62d 31' 57.1" with an uncertainty of 2.5 arcsec (radius, 90% confidence). This position is 30.1 arcmin from the Fermi/LAT position. The light curve can be modelled with a power-law decay with a decay index of alpha=1.9 (+1.3, -1.2). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.8 (+0.6, -0.5). The best-fitting absorption column is 1.6 (+1.8, -1.0) x 10^21 cm^-2, consistent with the Galactic value of 5.1 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 3.1 x 10^-11 (3.8 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 1.6 (+1.8, -1.0) x 10^21 cm^-2 Galactic foreground: 5.1 x 10^20 cm^-2 Excess significance: <1.6 sigma Photon index: 1.8 (+0.6, -0.5) If the light curve continues to decay with a power-law decay index of 1.9, the count rate at T+24 hours will be 0.015 count s^-1, corresponding to an observed (unabsorbed) 0.3-10 keV flux of 4.7 x 10^-13 (5.8 x 10^-13) erg cm^-2 s^-1. The results of the XRT-team automatic analysis of the likely afterglow are at https://www.swift.ac.uk/xrt_products/TILED_GRB00089/Source2.php. The results of the full analysis of the tiled XRT observations are available at https://www.swift.ac.uk/xrt_products/TILED_GRB00089. This circular is an official product of the Swift-XRT team.