TITLE: GCN CIRCULAR NUMBER: 44264 SUBJECT: EP260409a: Gemini/FLAMINGOS2 J-band counterpart DATE: 26/04/11 09:24:03 GMT FROM: P.G. Jonker at Radboud University <p.jonker@astro.ru.nl> J. Quirola-Vasquez (Radboud), J. Sanchez-Sierras (Radboud), A. J. Levan (Radboud & Warwick), P. G. Jonker (Radboud), A. Martin-Carrillo (UCD), D. B. Malesani (DAWN/NBI and Radboud), G. Corcoran (UCD), R. A. J. Eyles-Ferris (Leicester), F. E. Bauer (SSI and UTA), J. Chácon (PUC), B. Lemaux (Gemini) report on behalf of a larger collaboration: We observed the field of the fast X-ray transient EP260409a (Jiang et al. 2026, GCN 44242) using the FLAMINGOS2 instrument mounted on the Gemini-South telescope. Two J-band observations were obtained, one on 2026-04-09 06:52:40 (~0.27 d after the EP trigger, T0, 8x40 s exposures) and one on 2026-04-11 02:13:36 UT (i.e., ~2.08 d after T0, 16x40 s exposures). In our first epoch stacked images, we detect a candidate counterpart at coordinates (J2000): RA: 11:31:37.985 DEC: -9:20:43.00 Photometry yields J = 21.06 +/- 0.10 (Vega) calibrated against nearby stars from the 2MASS catalog (not corrected for Galactic extinction). During the second epoch, the target is no longer detected, with a 3sigma limit of J>22.65 (Vega), implying it faded significantly. No source is detected at this location in the Legacy Survey in any band, nor in archival J-, H-, and K-band images from the VISTA Hemisphere Survey. Given the lack of optical detection (Globus et al., GCN 44243; Li et al., GCN 44244) and from our Las Cumbres observations we report a 3 sigma limit of r>23.5 and z>21.5 AB mag (obtained at ~1.37 and 3.26 hours, respectively, after T0 with an exposure time of 6x300 s), this candidate counterpart is significantly red, implying EP260409a could be a high-redshift or high extinction Fast X-ray Transient. We thank the Gemini-South staff, for their rapid execution of these observations. View this GCN Circular online at https://gcn.nasa.gov/circulars/44264. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...