TITLE: GCN CIRCULAR NUMBER: 30172 SUBJECT: GRB 210606A: Swift-XRT afterglow detection DATE: 21/06/10 20:13:23 GMT FROM: Aaron Tohuvavohu at U Toronto B. Sbarufatti (PSU), D.N. Burrows (PSU), J. D. Gropp (PSU), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), A. Melandri (INAF-OAB), T. Sbarrato (INAF-OAB), P. D'Avanzo (INAF-OAB) and P.A. Evans (U. Leicester) report on behalf of the Swift-XRT team: Swift-XRT has performed follow-up observations of the Swift/BAT-GUANO detected burst GRB 210606A (James DeLaunay et al. GCN Circ. 30134), collecting 7.3 ks of Photon Counting (PC) mode data between T0+62.3 ks and T0+385.7 ks. Four uncatalogued X-ray sources are detected consistent with being within 394 arcsec of the Swift/BAT position, of which one ("Source 1") is fading with 4.6-sigma significance, and is therefore likely the GRB afterglow. Using 4521 s of PC mode data and 5 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 = 170.94110, +0.81293 which is equivalent to: RA (J2000): 11h 23m 45.86s Dec(J2000): +00d 48' 46.5" with an uncertainty of 2.3 arcsec (radius, 90% confidence). This position is 6.1 arcmin from the Swift/BAT position. The light curve can be modelled with a power-law decay with a decay index of alpha=1.1 (+0.9, -0.6). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.8 (+0.5, -0.4). The best-fitting absorption column is 1.4 (+1.6, -1.0) x 10^21 cm^-2, consistent with the Galactic value of 3.8 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 3.5 x 10^-11 (4.3 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 1.4 (+1.6, -1.0) x 10^21 cm^-2 Galactic foreground: 3.8 x 10^20 cm^-2 Excess significance: <1.6 sigma Photon index: 1.8 (+0.5, -0.4) The results of the XRT-team automatic analysis of the likely afterglow are at https://www.swift.ac.uk/ToO_GRBs/00021451/Source1.php. The results of the full analysis of the XRT observations are available at https://www.swift.ac.uk/ToO_GRBs/00021451. This circular is an official product of the Swift-XRT team.