TITLE: GCN CIRCULAR NUMBER: 29413 SUBJECT: GRB 210204A / ZTF21aagwbjr: Swift-XRT observations DATE: 21/02/06 16:22:25 GMT FROM: Phil Evans at U of Leicester J.A. Kennea (PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), E. Ambrosi (INAF-IASFPA) , M. Capalbi (INAF-IASFPA), M. Perri (SSDC & INAF-OAR), A. Tohuvavohu (U. Toronto), B. Sbarufatti (PSU), D.N. Burrows (PSU), A.P. Beardmore (U. Leicester) and P.A. Evans (U. Leicester) report on behalf of the Swift-XRT team: Swift-XRT has performed follow-up observations of ZTF21aagwbjr (Kool et al., 29405), which may be related to the Fermi/GBM / GECAM / IPN detected burst GRB 210204A (GCN Circ., 29390, GCN Circ. 29392, GCN Circ. 29408), collecting 2.9 ks of Photon Counting (PC) mode data between T0+160.9 ks and T0+173.2 ks. An uncatalogued X-ray source is detected consistent with being within 1 arcsec of the ZTF position and is believed to be the afterglow, or at least, the X-ray counterpart of the ZTF21aagwbjr. Using 2886 s of PC mode data and 6 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 = 117.08071, +11.40951 which is equivalent to: RA (J2000): 07h 48m 19.37s Dec(J2000): +11d 24' 34.2" with an uncertainty of 1.7 arcsec (radius, 90% confidence). This position is 0.7 arcsec from the ZTF position. The light curve is consistent with a constant source of mean count rate 9.1e-02 ct/sec. A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.73 (+0.31, -0.29). The best-fitting absorption column is 1.7 (+1.3, -1.1) x 10^21 cm^-2, in excess of the Galactic value of 3.5 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 4.0 x 10^-11 (4.9 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 1.7 (+1.3, -1.1) x 10^21 cm^-2 Galactic foreground: 3.5 x 10^20 cm^-2 Excess significance: 2.0 sigma Photon index: 1.73 (+0.31, -0.29) The results of the XRT-team automatic analysis are available at http://www.swift.ac.uk/xrt_products/00014055. The results of the full analysis of the XRT observations are available at https://www.swift.ac.uk/ToO_GRBs/00014055. This circular is an official product of the Swift-XRT team.