{
  "bibcode": "2018GCN.23106....1D",
  "body": "V. D'Elia (SSDC), S.L. Gibson (U. Leicester), Z. Liu (NAOC / U.\nLeicester), A. D'Ai (INAF-IASFPA), A. Melandri (INAF-OAB), P. D'Avanzo\n(INAF-OAB), D.N. Burrows (PSU), A. Tohuvavohu (PSU), S. J. LaPorte\n(PSU), J.P. Osborne (U. Leicester) and P.A. Evans (U. Leicester) report\non behalf of the Swift-XRT team:\n\nSwift-XRT has performed follow-up observations of the MAXI-detected\nburst GRB 180809A (Yatabe et al. GCN Circ. 23103) in a series of\nobservations tiled on the sky. The total exposure time is 777 s,\ndistributed over 6 tiles; the maximum exposure at a single sky location\nwas 898 s. The data were collected between T0+13.7 ks and T0+22.0 ks,\nand are entirely in Photon Counting (PC) mode. \n\nAn uncatalogued X-ray source is detected and is above the RASS limit,\nand is therefore likely the GRB afterglow. Using 731 s of PC mode data\nand 3 UVOT images, we find an enhanced XRT position (using the XRT-UVOT\nalignment and matching UVOT field sources to the USNO-B1 catalogue):\nRA, Dec = 297.90271, -34.78546 which is equivalent to:\n\nRA (J2000): 19h 51m 36.65s\nDec(J2000): -34d 47' 07.6\"\n\nwith an uncertainty of 2.3 arcsec (radius, 90% confidence). This\nposition is 9.1 arcmin from the MAXI position.\tThe light curve is\ncurrently consistent with a constant source of mean count rate 2.2e-01\nct/sec. A power-law fit gives an index of 0.3 (+/-0.9). More data are\nnecessary to determine potential fading.\n\nA spectrum formed from the PC mode data can be fitted with an absorbed\npower-law with a photon spectral index of 2.4 (+0.7, -0.6). The\nbest-fitting absorption column is  8.4 (+5.4, -3.8) x 10^21 cm^-2, in\nexcess of the Galactic value of 1.2 x 10^21 cm^-2 (Willingale et al.\n2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion\nfactor deduced from this spectrum  is 3.2 x 10^-11 (8.2 x 10^-11) erg\ncm^-2 count^-1. \n\nA summary of the PC-mode spectrum is thus:\nTotal column:\t     8.4 (+5.4, -3.8) x 10^21 cm^-2\nGalactic foreground: 1.2 x 10^21 cm^-2\nExcess significance: 3.1 sigma\nPhoton index:\t     2.4 (+0.7, -0.6)\n\nThe results of the XRT-team automatic analysis are available at\nhttp://www.swift.ac.uk/xrt_products/00020814.\nThe results of the full analysis of the tiled XRT observations are\navailable at http://www.swift.ac.uk/xrt_products/TILED_GRB00072.\n\nThis circular is an official product of the Swift-XRT team.",
  "circularId": 23106,
  "createdOn": 1533847711000,
  "email": "valerio.delia@ssdc.asi.it",
  "subject": "GRB 180809A: Swift-XRT afterglow detection",
  "submitter": "Valerio D'Elia at ASDC  <valerio.delia@ssdc.asi.it>",
  "eventId": "GRB 180809A"
}