{
  "bibcode": "2019GCN.24706....1D",
  "body": "P. D'Avanzo (INAF-OAB), S. J. LaPorte (PSU), J.A. Kennea (PSU), B.\nSbarufatti (INAF-OAB/PSU), K.L. Page (U. Leicester), A.P. Beardmore (U.\nLeicester), M.G. Bernadini (INAF-OAB), E. Ambrosi  (INAF-IASFPA) , \nA. Tohuvavohu (PSU) and P.A. Evans (U. Leicester) report on behalf of\nthe Swift-XRT team:\n\nSwift-XRT has performed follow-up observations of the\nFermi/LAT-detected burst GRB 190531B (Axelsson et al. GCN Circ. 24701),\ncollecting 5.0 ks of Photon Counting (PC) mode data between T0+18.2 ks\nand T0+29.7 ks. \n\nEleven uncatalogued X-ray sources are detected, of which one (\"Source\n8\") is above the RASS limit, and is therefore likely the GRB afterglow.\nUsing 1291 s of PC mode data and 1 UVOT image, we find an enhanced XRT\nposition (using the XRT-UVOT alignment and matching UVOT field sources\nto the USNO-B1 catalogue): RA, Dec = 24.29620, -41.95841 which is\nequivalent to:\n\nRA (J2000): 01h 37m 11.09s\nDec(J2000): -41d 57' 30.3\"\n\nwith an uncertainty of 1.6 arcsec (radius, 90% confidence). This\nposition is 3.4 arcmin from the Fermi/LAT position. \n\nThe light curve can be modelled with a power-law decay with a decay\nindex of alpha=1.0 (+/-0.3).\n\nA spectrum formed from the PC mode data can be fitted with an absorbed\npower-law with a photon spectral index of 2.20 (+0.15, -0.14). The\nbest-fitting absorption column is  9.1 (+1.2, -1.1) x 10^21 cm^-2, in\nexcess of the Galactic value of 1.8 x 10^20 cm^-2 (Willingale et al.\n2013). The counts to observed (unabsorbed) 0.3-10 keV flux conversion\nfactor deduced from this spectrum  is 4.2 x 10^-11 (9.0 x 10^-11) erg\ncm^-2 count^-1. \n\nA summary of the PC-mode spectrum is thus:\nTotal column:\t     9.1 (+1.2, -1.1) x 10^21 cm^-2\nGalactic foreground: 1.8 x 10^20 cm^-2\nExcess significance: 12.8 sigma\nPhoton index:\t     2.20 (+0.15, -0.14)\n\nIf the light curve continues to decay with a power-law decay index of\n1.0, the count rate at T+24 hours will be 0.14 count s^-1,\ncorresponding to an observed (unabsorbed) 0.3-10 keV flux of 5.9 x\n10^-12 (1.3 x 10^-11) erg cm^-2 s^-1.\n\nThe results of the XRT-team automatic analysis of the likely afterglow\nare at http://www.swift.ac.uk/ToO_GRBs/00020895/Source8.php.\nThe results of the full analysis of the XRT observations are available\nat http://www.swift.ac.uk/ToO_GRBs/00020895.\n\nThis circular is an official product of the Swift-XRT team.",
  "circularId": 24706,
  "createdOn": 1559375210000,
  "email": "auc444@psu.edu",
  "subject": "GRB 190531B: Swift-XRT afterglow detection",
  "submitter": "Aaron Tohuvavohu at PSU/Swift  <auc444@psu.edu>",
  "eventId": "GRB 190531B"
}