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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: NOT confirmation of the optical afterglow
GCN Circulars —

GCN Circular 45912
GRB 261010A: NOT confirmation of the optical afterglow
From: Daniele Bjørn Malesani at Cosmic Dawn Center, Niels Bohr Institute
Issued: 2026-10-11 22:16 UTC
Type: GRB, Afterglow, Optical Transient

G. Corcoran (UCD), D. B. Malesani (DAWN/NBI and Radboud), D. O’Neill (Birmingham), L. Izzo (INAF/OAC and DARK/NBI), B. P. Gompertz (Birmingham), A. Martin-Carrillo (UCD), A. Schenone-Zanuzzi (NOT), E. Georgiou (NOT and Turku Univ.) report on behalf of a larger collaboration:

We observed again the position of the Swift and Fermi GRB 261010A (Lien et al., GCN 45889; Palafox & Jameson, GCN 45911) using the Nordic Optical Telescope (NOT) equipped with the ALFOSC imager. A total of 6x300 s were secured in the SDSS r band, with a mean epoch 2026 Oct 11.85154 UT (25.152 hr after trigger).

The candidate optical counterpart (O’Neill et al., GCN 45890​; Aceituno et al., GCN 45891) is no longer detected in our images. Performing image subtraction using the Legacy survey as template, we measure a limiting magnitude r > 23.9 (AB; calibrated against nearby Pan-STARRS objects, and not corrected for Galactic extinction).

We also note that the refined Swift/XRT position (Osborne et al., GCN 45894) has slightly shifted compared to the promptly-released coordinates (Lien et al., GCN 45889), and is now fully consistent with the location of the optical counterpart candidate. We thus conclude that this object is indeed the afterglow of GRB 261010A.

The position of the afterglow is about 25" (34.5 kpc in projection) away from the nearby galaxy LEDA 1781464 at redshift z = 0.070. The chance association probability is of order 1% (comparable to that for the fainter, redder object 2" away from the afterglow; O’Neill et al., GCN 45890). We note that, if placed at z = 0.070, our limit implies a luminosity > 1.5 mag fainter than AT 2017gfo at a comparable phase (e.g. Villar et al. 2017, doi:10.3847/2041-8213/aa9c84).

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

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