TITLE: GCN CIRCULAR NUMBER: 40166 SUBJECT: EP250416a: Swift/XRT counterpart fading DATE: 25/04/18 14:31:17 GMT FROM: Tullia Sbarrato at INAF-OAB <tullia.sbarrato@inaf.it> T. Sbarrato (INAF-OAB), R. Brivio (INAF-OAB), C. Salvaggio (INAF-OAB), M. Ferro (INAF-OAB), D. Malesani (DAWN/NBI and Radboud), K. Page (U. Leicester) and P.A. Evans (U. Leicester) report on behalf the Swift-XRT team: Swift-XRT has performed further follow-up observations of the EP-WXT transient EP250416a (Zhao et al., GCN 40154), collecting a total of 4.0 ks of Photon Counting (PC) mode data between T0+17.1 ks and T0+79.8 ks. The X-ray source detected in the first 2.0 ks of data reported as “Source 1”, consistent with the optical counterpart (Levan et al., GCN 40160), and presented as the possible transient counterpart (Sbarrato et al., GCN 40163) has faded more than 3 sigma in the latest observation, confirming the trend observed by EP-FXT about 12.3 hours after the trigger (Zhou et al., GCN 40165). The flux derived from the follow-up observation is 1.8 (+/-0.4) x 10^-2 cts/s, that corresponds to 6.5 (+/-1.3) x 10^-13 erg cm^-2 s^-1. The light curve can be modelled with a power-law decay with an index of alpha=1.18 A spectrum formed from the PC mode data can be fitted with an absorbed power-law with a photon spectral index of 2.0 (+0.5, -0.4). The best-fitting absorption column is 1.6 (+1.7, -1.3) x 10^21 cm^-2. The counts to observed (unabsorbed) 0.3-10 keV flux conversion factor deduced from this spectrum is 3.65 x 10^-11 (5.01 x 10^-11) erg cm^-2 count^-1. The observed (unabsorbed) flux obtained from all the PC mode data is thus 1.8 x 10^-12 (2.5 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_FIELD00034/ This circular is an official product of the Swift-XRT team. View this GCN Circular online at https://gcn.nasa.gov/circulars/40166. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...