TITLE: GCN CIRCULAR NUMBER: 28300 SUBJECT: GRB 200826A: Swift-XRT afterglow detection DATE: 20/08/28 15:55:28 GMT FROM: Boris Sbarufatti at PSU A. D'Ai (INAF-IASFPA), B. Sbarufatti (PSU), S. R. Oates (U. Birmingham), D.N. Burrows (PSU), J. D. Gropp (PSU), J.P. Osborne (U. Leicester), K.L. Page (U. Leicester), A.P. Beardmore (U. Leicester), A. Melandri (INAF-OAB), T. Sbarrato (INAF-OAB) and P.A. Evans (U. Leicester) report on behalf of the Swift team: Swift-XRT has performed follow-up observations of theFermi/GBM and IPN detected burst GRB 200826A (Gupta et al. GCN Circ. 28288, Hurley et al., GCN Circ. 28291) in a series of observations tiled on the sky. The total exposure time is 7.4 ks, distributed over 4 tiles; the maximum exposure at a single sky location was 5.2 ks. The data were collected between T0+59.9 ks and T0+169.6 ks, and are entirely in Photon Counting (PC) mode. Seven uncatalogued X-ray sources are detected, of which one ("Source 3") is consistent with ZTF20abwysqy, reported by ZTF (Ahumada et al. GCN Circ. 28293 and 28295). Source 3 is fading with 3-sigma significance, and is therefore likely the GRB afterglow. Using 1504 s of PC mode data and 1 UVOT image, we find an enhanced XRT position (using the XRT-UVOT alignment and matching UVOT field sources to the USNO-B1 catalogue): RA, Dec = 6.78466, +34.02670 which is equivalent to: RA (J2000): 00h 27m 08.32s Dec(J2000): +34d 01' 36.1" with an uncertainty of 3.3 arcsec (radius, 90% confidence). This position is 104 arcsec from the Fermi/GBM position. The light curve can be modelled with a power-law decay with a decay index of alpha=1.3 (+0.7, -0.5). A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 1.8 (+0.6, -0.5). The best-fitting absorption column is 2.2 (+2.5, -1.6) x 10^21 cm^-2, consistent with the Galactic value of 6.0 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 (4.6 x 10^-11) erg cm^-2 count^-1. A summary of the PC-mode spectrum is thus: Total column: 2.2 (+2.5, -1.6) x 10^21 cm^-2 Galactic foreground: 6.0 x 10^20 cm^-2 Excess significance: <1.6 sigma Photon index: 1.8 (+0.6, -0.5) The results of the XRT-team automatic analysis of the likely afterglow are at https://www.swift.ac.uk/xrt_products/TILED_GRB00093/Source3.php. The results of the full analysis of the tiled XRT observations are available at https://www.swift.ac.uk/xrt_products/TILED_GRB00093. The Swift/UVOT began observations of the optical counterpart ZTF20abwysqy, 140ks after the Fermi/GBM trigger (Fermi GBM Team, GCN Circ. 28284). A source consistent with ZTF20abwysqy is detected in the UVOT summed exposures. ZTF20abwysqy is consistent with XRT source 3, however no earlier UVOT photometry is avaliable since XRT source 3 is outside of the field of view for all earlier tiles. The UVOT detection is possibly explained by the underlying galaxy emission. The preliminary detection and 3-sigma upper limits using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the early exposures are: Filter T_start(s) T_stop(s) Exp(s) Mag white 140782 169578 5032 21.86 +/- 0.13 The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of E(B-V) = 0.07 in the direction of the burst (Schlegel et al. 1998). This circular is an official product of the Swift-XRT and Swift-UVOT teams.