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Transient Alerts » in reply to GRB 261010A

In plain words Notable

On 10 October 2026 several gamma-ray satellites (Fermi, Swift, Konus-Wind and others) recorded a burst of gamma rays in the constellation Hercules that lasted about a minute. Swift's X-ray telescope and ground telescopes quickly found a fading X-ray and optical afterglow, which confirms the position. The host galaxy is uncertain: it is either a nearby galaxy about 25 arcseconds away at redshift 0.070, or a fainter galaxy 2 arcseconds away at redshift 0.785, meaning the light left roughly 7 billion years ago. Several teams say the burst's flash pattern and energy look more like those of a merger of two compact stars (such as neutron stars) than the collapse of a massive star, even though the burst was long. No kilonova (the glow from freshly made heavy elements after a merger) has been seen, and radio searches found nothing; the deep, late near-infrared images are too shallow to rule one out.

How unusual is it? A long gamma-ray burst with an optical afterglow, a measured redshift, and several independent analyses pointing to a merger origin is out of the ordinary, but no kilonova has been seen and the host is unconfirmed. It would become exceptional if a kilonova were confirmed, and would drop back toward routine if the burst were shown to come from a collapsing massive star.

Written by an AI model (claude-sonnet-5-5) from the official reports in this thread, and rewritten as new reports arrived (version 9); last updated . It may contain mistakes: the reports themselves are the authority.

GRB 261010A: 10.4m GTC spectroscopy of the galaxy 2" from the afterglow
GCN Circulars —

GCN Circular 45928
GRB 261010A: 10.4m GTC spectroscopy of the galaxy 2" from the afterglow
From: Alberto Castro-Tirado at Inst.de Astro. de Andalucia
Issued: 2026-10-12 21:17 UTC

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.

Every one of the gamma-ray bursts GCN reports is posted here.
Relayed automatically from NASA's General Coordinates Network.

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