TITLE: GCN CIRCULAR NUMBER: 45302 SUBJECT: EP260806a: OSIRIS+/GTC redshift z=0.089 DATE: 26/08/07 08:25:50 GMT FROM: Antonio de Ugarte Postigo at LAM, CNRS <adeugartepostigo@gmail.com> A. de Ugarte Postigo (LAM), A. Martin-Carrillo (UCD), L. Izzo (INAF/OACN and DARK/NBI), G. Corcoran (UCD), D. B. Malesani (DAWN/NBI and Radboud), S. Geier (GTC), C. C. Thoene (AbAO), M. A. Aloy (UV), J. P. U. Fynbo (DAWN/NBI), L. Galbany (IEEC-CSIC),G. Lombardi (GTC), N. A. Rakotondrainibe (LAM), B. Schneider (LAM), N. R. Tanvir (Univ. Leicester), G. Gómez Velarde (GTC) and A. Tejero Caro (GTC) report: We observed the optical counterpart candidate (He et al. GCN 45300) of EP260806a (Li et al. GCN 45299) using the Gran Telescopio Canarias (GTC) equipped with the OSIRIS+ instrument. The 30-s acquisition r-band image (beginning on 2026-08-07 at 03:21:44 UT, that is 4.32 hr after trigger), shows the underlying galaxy mentioned by He et al. We performed image subtraction and photometry with respect to a PanSTARRS reference image using STDWeb (Karpov 2025). The residuals show a clear excess at r = 19.05 +/- 0.01 mag consistent with the core of the galaxy. This implies a 0.27+/-0.04 mag brightening with respect to the observation by He et al. The location of the counterpart and the galaxy core are consistent with a catalogued LOTSS and VLASS radio source, which could be indicative of an active galactic nucleus. A total of 3 spectra of 900 s were secured, starting on 2026-08-07 at 03:26:22 UT (4.40 hr after trigger), using grism R1000B. Continuum is visible over the wavelength range 3620 - 7800 AA. We detect strong emission lines due to [OII], [OIII], HeI, HII, [NII] and [SII], as well as CaII absorptions, all at a common redshift z = 0.089, which we suggest to be the redshift of the EP260806a event. We also note that this galaxy is part of an interacting system with another, larger galaxy, located 22” at the West-southwest of this one. This second galaxy also falls in the slit and we confirm a consistent redshift. This work has used the GRBspec database at http://grbspec.eu (de Ugarte Postigo et al. 2014, doi:10.1117/12.2055774). View this GCN Circular online at https://gcn.nasa.gov/circulars/45302. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...