TITLE: GCN CIRCULAR NUMBER: 41496 SUBJECT: EP250821a: Swift/XRT counterpart fading DATE: 25/08/22 14:41:37 GMT FROM: Simone Dichiara at Pennsylvania State University <sbd5667@psu.edu> 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 further follow-up observations of the EP-WXT transient EP250821a (Hu et al., GCN 41459), collecting a total of 3.0 ks of Photon Counting (PC) mode data between T0+61.0 ks and T0+72.9 ks. The source detected in XRT in the first 1.9 ks of data previously reported as “Source 1” (Dichiara et al., GCN 41469), consistent with the optical counterpart (Li et al., GCN 41470, Yao et al., GCN 41485, An et al., GCN 41490), has faded more than 3 sigma in the latest X-ray observation. The count rate derived from the follow-up observation is 2.2 (+/-0.3) x 10^-2 cts/s. The light curve can be modelled with a power-law decay with an index of alpha=1.03 +/- 0.07. A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.0 +/- 0.2. The best-fitting absorption column is 1.4 (+0.7, -0.6) x 10^21 cm^-2 , in excess of the Galactic value of 7.26 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 3.51 x 10^-11 (4.48 x 10^-11) erg cm^-2 count^-1. The observed (unabsorbed) flux obtained from the follow-up observations from T0+61.0 ks to T0+72.9 ks is thus 7.7 x 10^-13 (9.9 x 10^-12) erg cm^-2 s^-1. The results of the XRT-team automatic analysis are available at https://www.swift.ac.uk/EP/EP_FIELD00059/ This circular is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/41496. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...