{
  "bibcode": "2021GCN.29413....1K",
  "body": "J.A. Kennea (PSU), J.P. Osborne (U. Leicester), K.L. Page (U.\nLeicester), E. Ambrosi\t  (INAF-IASFPA) , M. Capalbi (INAF-IASFPA), M.\nPerri (SSDC & INAF-OAR), A. Tohuvavohu (U. Toronto), B. Sbarufatti\n(PSU), D.N. Burrows (PSU), A.P. Beardmore (U. Leicester) and P.A. Evans\n(U. Leicester) report on behalf of the Swift-XRT team:\n\nSwift-XRT has performed follow-up observations of ZTF21aagwbjr (Kool et\nal., 29405), which may be related to the Fermi/GBM / GECAM / IPN\ndetected burst GRB 210204A (GCN Circ., 29390, GCN Circ. 29392, GCN\nCirc. 29408), collecting 2.9 ks of Photon Counting (PC) mode data\nbetween T0+160.9 ks and T0+173.2 ks. \n\nAn uncatalogued X-ray source is detected consistent with being within 1\narcsec of the ZTF position and is believed to be the afterglow, or at\nleast, the X-ray counterpart of the ZTF21aagwbjr. Using 2886 s of PC\nmode data and 6 UVOT images, we find an enhanced XRT position (using\nthe XRT-UVOT alignment and matching UVOT field sources to the USNO-B1\ncatalogue): RA, Dec = 117.08071, +11.40951 which is equivalent to:\nRA (J2000): 07h 48m 19.37s\nDec(J2000): +11d 24' 34.2\"\n\nwith an uncertainty of 1.7 arcsec (radius, 90% confidence). This\nposition is 0.7 arcsec from the ZTF position.  The light curve is\nconsistent with a constant source of mean count rate 9.1e-02 ct/sec. \n\nA spectrum formed from the PC mode data can be fitted with an absorbed\npower-law with a photon spectral index of 1.73 (+0.31, -0.29). The\nbest-fitting absorption column is  1.7 (+1.3, -1.1) x 10^21 cm^-2, in\nexcess of the Galactic value of 3.5 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 4.0 x 10^-11 (4.9 x 10^-11) erg\ncm^-2 count^-1. \n\nA summary of the PC-mode spectrum is thus:\nTotal column:\t     1.7 (+1.3, -1.1) x 10^21 cm^-2\nGalactic foreground: 3.5 x 10^20 cm^-2\nExcess significance: 2.0 sigma\nPhoton index:\t     1.73 (+0.31, -0.29)\n\nThe results of the XRT-team automatic analysis are available at\nhttp://www.swift.ac.uk/xrt_products/00014055. The results of the full\nanalysis of the XRT observations are available at\nhttps://www.swift.ac.uk/ToO_GRBs/00014055.\nThis circular is an official product of the Swift-XRT team.",
  "circularId": 29413,
  "createdOn": 1612628545000,
  "email": "pae9@leicester.ac.uk",
  "subject": "GRB 210204A / ZTF21aagwbjr: Swift-XRT observations",
  "submitter": "Phil Evans at U of Leicester  <pae9@leicester.ac.uk>",
  "eventId": "GRB 210204A"
}