TITLE: GCN CIRCULAR NUMBER: 41469 SUBJECT: EP250821a: Swift XRT and UVOT counterpart detection DATE: 25/08/21 16:30:52 GMT FROM: Phil Evans at U of Leicester <pae9@star.le.ac.uk> S. Dichiara (PSU), S. R. Oates (Lancaster U.), J.A. Kennea (PSU), K.L. Page (U. Leicester) report on behalf of the Swift-XRT and Swift-UVOT team: Swift-XRT has performed follow-up observations of the Einstein Probe/WXT-detected source EP250821a (Hu et al., GCN 41459), collecting 1.6 ks of Photon Counting (PC) mode data between T0+4.9 ks and T0+10 ks after the trigger. A likely counterpart has been found. The details of this source are: Source 1 (SWIFT J191932.6-432250): ================================== RA (J2000.0): 289.8861 = 19 19 32.66 Dec (J2000.0): -43.3807 = -43 22 50.5 Error: 3.6 (arcsec, radius, 90% confidence). Detect flag: GOOD Distance: 2.2 arcmin from the Einstein Probe/WXT position. Mean rate: 0.263 +/- 0.015 ct s^-1 Mean flux: (9.44 +/- 0.53)e-12 erg cm^-2 s^-1 Peak rate: 0.440 +/- 0.099 ct s^-1 Peak flux: (1.58 +/- 0.35)e-11 erg cm^-2 s^-1 ECF: 3.58e-11 erg cm^-2 ct^-1 assuming NH=1.38e+21 cm^-2, gamma=1.93 determined from a spectral fit. XMM UL: 5.5e-12 erg cm^-2 s^-1, (0.3-10 keV) so the source is 2.9-sigma above this 3-sigma upper limit. The source may be fading, at the 1.1-sigma level. All fluxes are 0.3-10 keV, observed. For all flux conversions and comparisons with catalogues and upper limits from other missions, we assumed a power-law spectrum with NH=3x10^20 cm^-2 and photon index (Gamma)=1.7 unless otherwise stated. The results of the XRT-team automatic analysis of the XRT observations, including a position-specific upper limit calculator, are available at https://www.swift.ac.uk/EP/EP_FIELD00059. Source 1 was also detected in the UVOT u-band image. The preliminary UVOT position is: RA (J2000.0): 289.883 = 19 19 31.86 Dec (J2000.0): -43.3816 = -43 22 53.8 Preliminary detection u-band magnitude in 1897 s exposure is 18.88 +/- 0.08, using the UVOT photometric system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) We note that the source is also listed in the Guide Star Catalog (Lasker et al, 2008) and it has clearly brightened from its catalogued brightness (Bmag = 22.3). This circular is an official product of the Swift XRT and UVOT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/41469. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...