TITLE: GCN CIRCULAR NUMBER: 44749 SUBJECT: AT2026nik: EP-FXT X-ray counterpart DATE: 26/05/31 21:01:15 GMT FROM: Jonathan Quirola at Radboud University <jaquirola1990@gmail.com> J. Quirola-Vasquez (Radboud), P. G. Jonker (Radboud), A. A. Chrimes (ESA/ESTEC), A. J. Levan (Radboud & Warwick), A Inkenhaag (Bath), D. Coppejans (Warwick), P. Groot (Radboud/UCT/SAAO): We observed the field of AT2026nik (AstroNotes: 2026-151, 2026-152) with the Follow-up X-ray Telescope (FXT) aboard the Einstein Probe mission. The observation was carried out from 2026 May 29 18:45:25.9 to May 30 00:22:29.8 UT, with a total exposure time of 7.2 ks using Modules A and B. After stacking the data from both modules, we detected an X-ray source with a significance of S/N ≈ 5.5 at the position (with an uncertainty of 10"): RA = 11:22:52.1 Dec = -06:13:21.7 No X-ray source is reported at this position in the ROSAT, eROSITA, Swift-XRT, XMM-Newton, or Chandra catalogs, according to the STONKS Multi-Mission X-ray Catalog (Quintin et al. 2024). The position of the detected X-ray source is consistent with that of AT2026nik, suggesting that it is the X-ray counterpart of the transient. The source has a net count rate of 2.58e-3 counts/s. Assuming a Galactic hydrogen column density of N_H = 4e20 cm-2 and a power-law spectrum with photon index Γ = 1.5, we estimate an absorbed X-ray flux of approximately 5.8e-14 erg/cm2/s. AT2026nik exhibited a blue, nearly featureless optical spectrum (P. Pessi and J. Wise; AstroNotes: 2026-151, 2026-152), with a tentative detection of the interstellar Mg II absorption doublet at z = 0.182. Assuming this redshift, the inferred X-ray luminosity is approximately L_X~5.4e42 erg/s, consistent with that observed in AT2018cow and other LFBOTs at a similar epoch (e.g., Margutti et al. 2019). The blue optical spectrum and the newly detected X-ray counterpart support the classification of AT2026nik as a luminous fast blue optical transient (LFBOT). We thank the Einstein Probe staff for their execution of these observations. View this GCN Circular online at https://gcn.nasa.gov/circulars/44749. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...