TITLE: GCN CIRCULAR NUMBER: 23106 SUBJECT: GRB 180809A: Swift-XRT afterglow detection DATE: 18/08/09 20:48:31 GMT FROM: Valerio D'Elia at ASDC V. D'Elia (SSDC), S.L. Gibson (U. Leicester), Z. Liu (NAOC / 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), J.P. Osborne (U. Leicester) and P.A. Evans (U. Leicester) report on behalf of the Swift-XRT team: Swift-XRT has performed follow-up observations of the MAXI-detected burst GRB 180809A (Yatabe et al. GCN Circ. 23103) in a series of observations tiled on the sky. The total exposure time is 777 s, distributed over 6 tiles; the maximum exposure at a single sky location was 898 s. The data were collected between T0+13.7 ks and T0+22.0 ks, and are entirely in Photon Counting (PC) mode. An uncatalogued X-ray source is detected and is above the RASS limit, and is therefore likely the GRB afterglow. Using 731 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 = 297.90271, -34.78546 which is equivalent to: RA (J2000): 19h 51m 36.65s Dec(J2000): -34d 47' 07.6" with an uncertainty of 2.3 arcsec (radius, 90% confidence). This position is 9.1 arcmin from the MAXI position. The light curve is currently consistent with a constant source of mean count rate 2.2e-01 ct/sec. A power-law fit gives an index of 0.3 (+/-0.9). More data are necessary to determine potential fading. A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.4 (+0.7, -0.6). The best-fitting absorption column is 8.4 (+5.4, -3.8) x 10^21 cm^-2, in excess of the Galactic value of 1.2 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 3.2 x 10^-11 (8.2 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 8.4 (+5.4, -3.8) x 10^21 cm^-2 Galactic foreground: 1.2 x 10^21 cm^-2 Excess significance: 3.1 sigma Photon index: 2.4 (+0.7, -0.6) The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00020814. The results of the full analysis of the tiled XRT observations are available at http://www.swift.ac.uk/xrt_products/TILED_GRB00072. This circular is an official product of the Swift-XRT team.