TITLE: GCN CIRCULAR NUMBER: 43984 SUBJECT: GRB 260310A / AT2026fgk: OSIRIS+/GTC spectroscopy confirms redshift z = 0.153 DATE: 26/03/13 11:16:28 GMT FROM: Antonio de Ugarte Postigo at LAM, CNRS <adeugartepostigo@gmail.com> A. de Ugarte Postigo (LAM), L. Izzo (INAF/OACN and DARK/NBI), A. Martin-Carrillo (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), D. A. Perley (LJMU), F. M. Pérez Toledo (GTC), D. Pérez Valladares (GTC) report: We observed AT2026fgk (a.k.a. GOTO26buh, O’Neill et al. 2026, TNS Discovery Report 294132; Hinds et al., AstroNote 2026-65), proposed as the optical counterpart (Konno et al., GCN 43974; Hinds et al., GCN 43977; Lipunov et al., GCN 43978; Martin-Carrillo et al., GCN 43979; Moreno Méndez et al., GCN 43979) of GRB 260310A (Hamburg & Meegan, GCN 43975; Salunke et al., GCN 43958) using the Gran Telescopio Canarias (GTC) equipped with the OSIRIS+ instrument. The seeing conditions were very poor (3.5" measured in the acquisition image). In the 30-s acquisition image (beginning on 2026-03-13 at 04:23:43 UT, that is 2.98 days after trigger), the optical afterglow is well detected with a magnitude r = 18.05 + 0.05 (AB), calibrated against nearby Pan-STARRS objects, and not corrected for Galactic extinction. We warn that, given the poor seeing, some contamination from the nearby galaxy cannot be excluded. A total of 3x600 s spectra were secured in each of grism R1000B (3600-7800 AA) and R1000R (5100-10000 AA), with mean epoch 2026-03-13 at 04:58:13 UT (3.00 days after trigger). The slit covers both the optical transient and the core of the nearby galaxy. Continuum is visible over the entire range 3600-10000 AA. We detect a number of emission features that extend from the galaxy core to the location of the transient, with a typical drift due to galaxy rotation. These features include [O II], [O III], H-beta, H-alpha, [N II] and [SII] at a redshift of z = 0.153, confirming, and consolidating, the measurement by Hinds et al. (GCN 43977). No absorption features are detected either at the emission redshift (no Ca II, Ca I or Na I) nor in any other part of the spectrum. The lack of Mg II detection (typically the strongest feature in GRB afterglow spectra) in our spectral range would discard a redshift larger than 0.3 for a long GRB sight line with typical line strengths (using as reference the sample of de Ugarte Postigo et al. 2012, doi:10.1051/0004-6361/201219894). In particular at a redshift of 0.3, where the SNR would be worse, Mg II would have to be fainter than 95% of the GRB sight lines. At z = 0.7, this line of sight would need to be weaker than any in the sample. Furthermore, the chance probability of a transient appearing within 5" from the galaxy without being related is just ~1% (Moreno Méndez et al., GCN 43979). All of this indicates that the transient is indeed at the redshift of z = 0.153. The GRB afterglow is located in the outskirts of a galaxy, at a projected distance of ~13.5 kpc from the galaxy core. This offset is very large for a GRB from a collapsar origin and more in line with the expectations for mergers (e.g. Fong et al. 2022, doi: 10.3847/1538-4357/ac91d0), although not unprecedented even for collapsars (see e.g. Thoene et al. 2024, doi:10.1051/0004-6361/202348141). Together with the location in the merger region of the Epeak - Eiso diagram (Atteia et al., GCN 43981), this opens the possibility that GRB 260310A had a merger progenitor. Further monitoring to constrain the emergence of an associated SN will clarify the nature of the source and is strongly encouraged. 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/43984. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...