TITLE: GCN CIRCULAR NUMBER: 30725 SUBJECT: GRB 210826A: Swift-XRT observations DATE: 21/08/27 10:16:23 GMT FROM: Paolo D'Avanzo at INAF - OAB paolo.davanzo@inaf.it

A. D'Ai (INAF-IASFPA), B. Sbarufatti (PSU), 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-XRT team:

Swift-XRT has performed follow-up observations of the Fermi/LAT-detected burst GRB 210826A (Ohno et al. GCN Circ. 30721), for a total exposure time of 4.9 ks. The data were collected between T0+36.5 ks and T0+49.7 ks, and are entirely in Photon Counting (PC) mode.

Eight uncatalogued X-ray sources have been detected, however none of them is above the RASS limit or shows definitive signs of fading. Therefore, at the present time we cannot identify which, if any, is the afterglow. We notice that one of the sources (Source 8) is consistent with a catalogued galaxy at redshift z = 0.05. Details of these sources are given below:

Source 1: RA (J2000.0): 45.3361 = 03:01:20.66 Dec (J2000.0): -71.7072 = -71:42:25.8 Error: 8.7 arcsec (radius, 90% conf.) Count-rate: (1.46 [+0.80, -0.61])e-3 ct s^-1 Distance: 48 arcsec from Fermi/LAT position.

Source 2: RA (J2000.0): 45.0955 = 03:00:22.93 Dec (J2000.0): -71.6531 = -71:39:11.1 Error: 5.0 arcsec (radius, 90% conf. [Enhanced position]) Count-rate: (3.06 [+1.16, -0.94])e-3 ct s^-1 Distance: 286 arcsec from Fermi/LAT position. Flux: (8.2 [+3.1, -2.5])e-14 erg cm^-2 s^-1 (observed, 0.3-10 keV)

Source 4: RA (J2000.0): 45.1820 = 03:00:43.67 Dec (J2000.0): -71.5711 = -71:34:15.8 Error: 9.7 arcsec (radius, 90% conf.) Count-rate: (1.36 [+0.80, -0.59])e-3 ct s^-1 Distance: 483 arcsec from Fermi/LAT position. Flux: (8.2 [+4.8, -3.6])e-14 erg cm^-2 s^-1 (observed, 0.3-10 keV) This source is 10" away from the AGN candidate WISEA J030045.23-713422.8 (Assef et al., 2018, ApJS, 234, 23)

Source 5: RA (J2000.0): 45.4777 = 03:01:54.65 Dec (J2000.0): -71.6539 = -71:39:13.9 Error: 8.4 arcsec (radius, 90% conf.) Count-rate: (3.7 [+1.3, -1.0])e-3 ct s^-1 Distance: 260 arcsec from Fermi/LAT position. Flux: (5.8 [+2.0, -1.6])e-14 erg cm^-2 s^-1 (observed, 0.3-10 keV)

Source 6: RA (J2000.0): 45.1850 = 03:00:44.41 Dec (J2000.0): -71.7836 = -71:47:00.9 Error: 10.0 arcsec (radius, 90% conf.) Count-rate: (1.60 [+0.86, -0.65])e-3 ct s^-1 Distance: 327 arcsec from Fermi/LAT position. Flux: (4.2 [+2.2, -1.7])e-14 erg cm^-2 s^-1 (observed, 0.3-10 keV)

Source 7: RA (J2000.0): 45.3683 = 03:01:28.39 Dec (J2000.0): -71.6617 = -71:39:42.1 Error: 10.3 arcsec (radius, 90% conf.) Count-rate: (1.54 [+0.84, -0.63])e-3 ct s^-1 Distance: 158 arcsec from Fermi/LAT position. This source is 6" away from the AGN candidate WISEA J030129.33-713938.6 (Assef et al., 2018, ApJS, 234, 23)

Source 8: RA (J2000.0): 44.7903 = 02:59:9.68 Dec (J2000.0): -71.6112 = -71:36:40.2 Error: 9.3 arcsec (radius, 90% conf.) Count-rate: (1.11 [+0.78, -0.56])e-3 ct s^-1 Distance: 660 arcsec from Fermi/LAT position. This source is 9" away from the catalogued galaxy 2MASX J02591150-7136430 at redshift z = 0.05

Source 9: RA (J2000.0): 45.6834 = 03:02:44.02 Dec (J2000.0): -71.6574 = -71:39:26.7 Error: 8.8 arcsec (radius, 90% conf.) Count-rate: (2.16 [+1.02, -0.79])e-3 ct s^-1 Distance: 460 arcsec from Fermi/LAT position. Flux: (8.1 [+3.8, -3.0])e-13 erg cm^-2 s^-1 (observed, 0.3-10 keV)

The results of the XRT-team automatic analysis of the tiled XRT observations, including a position-specific upper limit calculator, are available at https://www.swift.ac.uk/xrt_products/TILED_GRB00101.

This circular is an official product of the Swift-XRT team.