GCN Circular 45928
Subject
GRB 261010A: 10.4m GTC spectroscopy of the galaxy 2" from the afterglow
Event
Date
2026-10-12T21:17:07Z (20 hours ago)
From
Alberto Castro-Tirado at Inst.de Astro. de Andalucia <ajct@iaa.es>
Via
email
A. J. Castro-Tirado, I. Pérez-García (IAA-CSIC), Y.-D. Hu (GXU), F. J. Aceituno, M.D. Caballero-Garcia, E. Fernandez-Garcia, M. Gritsevich, S. Guziy (IAA-CSIC), on behalf of a larger collaboration, report:
We observed the field of GRB 261010A, detected by Fermi/GBM (the Fermi team, GCN 45888), Swift (Lien et al. GCN 45889), and Konus-Wind (Frederiks et al. GCN 45925), with the OSIRIS+ spectrograph at the 10.4m Gran Telescopio Canarias in La Palma (Spain). Long-slit spectra with the R1000R and R1000B grisms (600 s each) were taken on Oct 11, 2026, starting at 20:28 UT, i.e. 25.2 h post burst. The slit covered both the position of the optical afterglow (O’Neill et al., GCN 45890; Aceituno et al., GCN 45891) and the red object located 2.2 arcsec NE of it (Corcoran et al., GCN 45912).
The spectrum of the redder object confirms the source to be a galaxy, displaying the [O II] 3727 A emission line, the Ca II H&K absorption doublet and the 4000 A break, all consistent at a redshift z = 0.785 +/- 0.001.
At this redshift the projected offset of the afterglow from the galaxy centre is 16 kpc.
If this galaxy at 2” is the host, GRB 261010A would have an isotropic-equivalent energy of E_iso ~ 3-4 x 10^52 erg and a rest-frame peak energy of ~0.8-1.5 MeV (using the Fermi/GBM and Konus-Wind spectral parameters, GCN 45911 and 45925), well above the Amati relation of long GRBs, thus placing this event in the same parameter space as GRB 211211A and GRB 230307A (both long GRBs which originated from a compact binary merger rather than a collapsar).
Further observations are planned.
We thank the GTC staff for the rapid execution of these observations.